Pageys ca Optitha moptics .........................•.....•...................................1

Chapter 1: Light : 1 )Chapter 2: Properties of light f? ••••••••••••••••••••••••••••••••••••••••• .4

Secti 0n II: Geometric Ophthalmoptics ~.............................•.....10Chapter 3: Reflection of light.. I. 10

Chapter 4: Refraction of light 5 14fihapter 5: Refraction by prisms ::t- 19

~hapter 6: Refraction by lenses .I.Q. 26

~hapter 7: Refraction by the eye :J 36

Section III: Clinical 0 hthalmoptics 46/Chapter 8: Aberrations (Optical defects) ~ 46

Chapter 9: Ametropia (Errors of refraction) :: 53IChapter 10: Accommodation and its disturbances f 71lChapter 11: l3inocular muscular coordination E: 79Chapter 12: Binocular muscular anomalies ~.: 84

v Chapter 13: Visual functions : ?r:- ••••••..••••..••••.••...... 97

v Chapter 14: Retinoscopy (Skiascopy) :.:: +. 109v Chapter 15: Ophthalmoscopy ~ 126V Chapter 16: The verification of the refraction of theeye r. 145

Secti 0n IV: Ophthalmoptic Appliances ; 153v Chapter 17: Spectacles ;..~.':L 153

V Chapter 18: Contact lenses : 177 "',

Chapter 19: Intraocular lenses ~ 200 J

vChapter 20: Low vision aids 208

almoptic Instru ments 212Chapter 21: Telescopes 212

Chapter 22: Microscopes :.' 214vChapter 23: Slit-lamp ~ 218

c/ Chapter 24: Gonioscopy (Goniolenses) 229Chapter 25: Fundus camera 231

VChapter 26: Laser and photo therapeutics ?~•........................234Ghapter 27: Refractometers j 255

1/./ Chapter 28: Operating instruments and devices 259

vChapter 29: Keratometer 265

v ./Chapter 30: Focimeter (Lensmeter) 270

~hapter 31: Orthoptic instruments ~ 273

Chapter 32: Refraction surgery, : j 276

In d ex ........................................................................•.•.•.•.•.•.•.•.•.. :•.•...•..•.•.•.•.•........•................... 283

CHAPTERlLIGHT

DEFINITION: Light is the radiant energy to which the human eye is sensitive.NATURE OF LIGHT: Light has a dual nature:

(1)A wave model (the electromagnetic wave): When light propagates in space.(2)A particle model (the photon):When light is created or destroyed (as by

photoelectric lamp).ELECTROMAGNETIC SPECTRUM(ENERGY SPECTRUM): Electromagnetic

spectrum is the electromagnetic radiations of waves which extend from the cosmicrays with the shortest wavelength to the gamma, X, ultraviolet, infrared, radar,television and radio waves with the longest wavelength (Fig. 1. 1).

Wavelength in.nm10} lOG

Rad<lr10'TV

Fig.l.l:Electromagnetic spectrum. Fig.1.2:Electromagnetic wave.ELECTROMAGNETIC FIELDS:

Definition:Physically electric and magnetic fields are force fields that act uponcharged particles.

Importance: These two force fields Electric field E and magnetic field B have amagnitude and a direction and are perpendicular to each other and to thedirection of the wave v (Fig. 1.2), a property which is important in thediscussion of polarization (with electric fields in the same direction).

VISIBLE SPECTRUM: The visible spectrum is the visible part of electromagneticspectrum which is only a tiny portion near the centre and has a wavelengthbetween 400-760 nm (nanometre) Le. between 4000-7600 AO (Angstrom unit).

WAVE LENGTH (WL)UNITS: 1 nanometre (run) === 10 Angstrom (AO).1 mlcrometre (JJ.tn) !% 1000 nanometre.1 mlllitneter (mm) - 1000 mlcrometre.

WAVE LENGTH DISCRIl\1INATION tCObOUR SPECTRUM):(1)The normal eye is able to discriminate between light of shorter and longer

wavelength within the visible spectrum by means of colour sense (Fig. 1.1):l)Light of shortest to the longest wavelength.-Is violet, indigo, blue, blue-green,

green, yellow, orange and red (VIBBGIOR as in rainbow ,colour pattern).2)Slln/ight:Contains light of different wavelengths.3) White light: is n mixture of the wavelengths of the visible ~;pectrum.4) The piglJlenl epithelium qlthe.1Jris:.Absorbs light of mos~ wave lenghts.

(2) Radiation of shorter or longer wavelengths than t~e ,yisible spectrum: Isdetectable to man in other ways e.g. infrared rays(in heat), ultraviolet rays (inaphakic eyes) and X-rays. .

(3) Ultraviolet radiations in the sunlight (A, B and C):1) UV-A (3 J 5-380 nm): Can enter the eye and are absorbed by the crystalline

lens and so the retina is nut sensitive to these rays in the normal human eye(bul in aphakic patients ,these rays can reach the retina and absorbed be thepigment epithelium with retinal damage).

2) UV-B (280-315 nm): Cannot enter the eye but may cause skin burn, blisteringand photophthalmia (due to disruption oflhe corneal epithelium).

3) UV-C (100-280 11m): Cannot reach the earth surface.rJ:P;,T\lIlCwlv aphakic paticnts:Evervtlling looks biller due to sellsitivitv oftll retitla to

These shorter )v(lj·e/el1gtlls which give rise to the sensatioll of blue or violet colollr.(4) Infrared radiations in sunlight: Rarely exceeds the biologically acceptable

level except in eclipse blindness which may lead to macular burn.LIG HT VELOCITY: Light velocity = 186,000 miles/sec. = 300,000 kIn/sec.

oMs ravs. (MAs waves: (ellts 1I'1H'e frollts.Fig. 1.3: Light I 'lIving a point SOUl'CC:

'V AVE THEOI Y OF LIGHT:(1) Path of light as waves through space: Although the path of light is otten

represented diagranunatically as a straight an'owed line or parallel raystravelling from left to right on the page (Fig. 1.3a), it really travels (passes)through space as waves (r"ig. I.3b).

(2)Wave fronts: Are the combined elTect of many rays which is drown asconcentric circles through the crests of the waves (Fig. 1.3c).

(3) Wave motion (Fig. 1.4 ):A disturbance or energy passing through a medium:1) Wavelength (A):Distance between two symmetrical parts of wave motion.2) Cycle: One complete oscillation and occupies one wavelength.3) Amplitude (A): Is the maximum displacement of an imaginary particle on the

wave from the base line.4) Phase: Any portion of the cycle is called a phase.

(4) Phase difference: If two waves of equal wavelength are travelling in the samedirection but are out of step with each other, the fraction of a cycle orwavelength by which one leads the other is known as the phase difference (Fig.1.5): 1)Light waves that are out of phase are called incoherent.

2)Light waves that are exactly in phase are called coherent.

meyethe

--)000

Direction of propoQolion

r.ble

tftenraysises)

CHAPTER 2PROPERTIES OF LIGHT

(1) INDEX OF REFRACTIONDEFINITION: It is the optical density of a medium i.e. when light propagates

through a transparent medium such as glass or air, it induces oscillations of theconstituent particles of that medium, which in turn decreases the velocity of light.

TYPES OF THE REFRACTIVE INDEX:(1)Absolute index of refraction of a medium (n):

Definition:l) It is a unitless number that describes the new velocity relative to the

velocity oflight in air.2) Absolute index of refraction of a medium n (air nmedium)

Velocity of light in air._- Velocity of light in medium

Examples: 1) Air = 1.002) Water (aqueous) = 1.333) Cornea = 1.374) Crystalline lens = 1.38-1.425) Crown glass = 1.52-1.606) Flint glass = 1.70-1.90

(2) Relative index of refraction (n2 / n1):Definition: It is the comparison of velocity of light in two media other than air.Example: water 11 glass = n glass I n water = n2/ n 1.

(2) DISPERSIONDEFINITION: The refractive. index of any medium differs slightly for light of

different wavelengths as light of shorter wavelengths is deviated more than light oflonger wavelengths i.e. blue light is deviated more than red (Fig. 2.1).

RodYollowSlUG

Fig. 2.1: Dispersion of light through a medium.ANGLE OF DISPERSION: Is the angle formed between the wavelengths of

different colours dispersed through the medium.DISPERSIVE POWER OF A MEDIUM: Is indicated by the angle of dispersion

(not related to the refractive index of the medium).

CLINICAL APPLICATIONS:(1) Dispersion through lenses:

1) Chromatic aberration: Chapter 8 & Fig. 8.1.2) Achromatic lenses: Chapter 8.

(2) Dispersion through prisms (line spectrum):1)The image of a light source as the sun is seen through a prism placed between

an aclu'omatic lens and a screen, as a displaced elongated blurred image withthe red nearest the original image and the blue furthest away~this is theprinciple of the line spectrum.

2) If monochromatic light is used, a single colour of the line spectrum is made.(3) INTERFERENCE

TYPES: When two waves of light travel along the same path, an interference effect isproduced which depends upon whether or not the waves are in phase:(l)Constructive interference: If the waves are in phase, the resultant wave

will be summation of the two (Fig. 2.2a).(2)Destructive interference: If the two waves are of equal amplitude and

out of phase by a half cycle, they will cancel each other out (Fig.2.2b).(3)lntermediate interference: Phase differences of less than half a cycle

result in a wave of intermediate amplitude and phase (Fig. 2.2c).

(a)Col1structive. (b)Destructive. (c)Intermediate.Fig.2.2:Interference of two waves.

CLINICAL APPLICATIONS:(1)Collagen bundles of the corneal stroma: Are so spaced that any light deviatec

by them is eliminated by destructive interference.

(2)Antireflection coating of lens sUliaces:ls a thin layer of transparent material ofappropriate thickness-71ight reflected from the superficial surface of the layerand the light reflected from the deep layer eliminate each other by destructiveintelference.

(3) Laser interferometer: To estimate the visual acuity of media as cataract by theuse of":0herent laser light with the phenomenon of interference (Chapter 25),

(4) Laser reflector: To determine the refractive error by the use of coherent laserlight with the phenomenon of interference (Chapter 25).

(4) SCATTERINGDEFINITION: Whcn light travels through a medium ·with many small particles in it

as a dusty room or dusty air, the light that happens to hit any of the particles isreflected off the particles in a new direction.

APPLICATIONS:(l)OcuJar scattering:

1) Normal ocular sc:aflering:1- Cornea: Corneal stroma scatters 10% of light incident upon it (that is why

we see corneal structures with the slit-lamp).2- Lens: Due to the presence of the faint yellow pigment in the lens.3- Retina: Due to the presence of Muller's fibres,

NB1:llIuc iris colour:ls due to scatteri,,!! offi!!"t bv stromal fibres.NB2:SclcI'al whitcncss:ls due to scattering of light by collagenous bundles which are

surrounded by ground substance of a different refractive index;.2) Abnormal ocular scattering:

1- Cornca:(a)Stromal oedema.(b )Endothelial or epithelial oedema,(c )Corneal scars.

2- Lens: Cataract.NJ3:Scnsitivitv to £larc:/s increased in presence ofincreased corneal or

lenticular scaltering(cataract test).(2)Sunlight blue scatter:

1)The sky looks blue.2)The sun seams red during the day.3 )The sun seams more red at night fall (specially with air pollution) as

the sun rays travel a longer distance.(5) DIFFRACTION

DEFINITION: Diffraction is the spread of light when a wave front of lightencounters the edge of an obstruction or a narrow opening in which the sides of thewave front tend to deviate (tend to fall away) from the main wave front.

DIFFRACTION THROUGH A CIRCULAR APERATURE: When light passesthrough a circular aperture as the small pupil, a circular d(flraction pattern isproduced after passing through the converging lens which consists of (Fig. 2.3):l)Bright central aily disc:I-The airy disc is 0.01 mm when the pupil is 2 mm in diameter (less with

larger pupil and more with smaller pupil).2- 83% of energy of the point source will be focussed on the airy disc and so the

smaller the airy disc the more accurate the image with greater resolving power.2)Alternate dark and light rings: Around the central airy disc.

CLINICAL APPLICATIONS: '(1)The most efficientdiameter of the pupil is 2.5 mm:

1)Below that: Diffraction effects limit the resolution.2) Above that: Spherical and peripheral aberrations limit the resolution.

(2)The pinhole test used in refraction to estimate visual acuity: The pinhole testwill improve the visual acuity of an eye with refractive errors (not with eyedisease) to 6/9 only because of the diffraction effects which limit resolution.

(6) RESOLUTIONRESOLVlNGPOWER (ANGLE OF RESOLUTION): It is the smallest angle of

spectrum, W, between two points which allows the formation of two discriminableimages by an optical system (Fig. 2.4).

, \( \

I \I \( \

\\\\\

Intensity ofillumination

~ III

(a)Noll-polarize(L (b )Polart7-sJ. 'Fig.2.4:Anglc of resolution. Fig.2.S:Light beam.

THE LIMIT FOR RESOLUTION(ANGLE OF RESOLUTION): The limit ofresolution is reached when two airy discs are separated so that the centre of onefalls on the first dark ring of the other (Fig. 2.4).

THE ANGULAR RESOLVING POWER OF THE NORMAL EYE: Is about Iminute of an arc which is nearly similar to the theoretical angular resolvingpower(O.78 minutes =47 seconds).

CLINICAL APPLICATION: 711etest types for visual acuity (resolving power) ofthe eye to recognize a shape which subtends a known angle at the eye when viewedfrom the appropriate distance (Chapter 13).

(7) POLARIZATIONDEFINITON:

(I) If a number of light waves are travelling in the same direction, their electricfields (wave motion) mayor may not be parallel to each other:I) If the electric field directions are randomly related to each other, the light is

unpolarized.2) If all the electric fields are in the same direction(parallel), the light is linearly

polarized( electric fields are perpendicular on magnetic fields).(2) Therefore the orientation of the plane of the wave motion of rays comprising a

beam of light is seen in cross section of a beam of light on a page to be:1)The unpolarized light beam is randomly perpendicular to the page (Fig.2.5a).2)The polarized light beam have the individual wave motions lying parallel to

each other (Fig. 2,5b).POLAIUZED LIGHT: Is a light beam with parallel electromagnetic wave motion

which is produced from ordinary light by intersecting it with a polarizing substance(as polarized plastic) with a polarizing angle (Brewster angle) of the substance.

J\1ECHANISMS OF CREATION OF POLARIZED LICIIT:(1)Transmission:From ordinary light by an encounter with a polarizing substance.(2)Reflection:

1) From a planc surface such as watcr with the direction of polarization parallelwith the surface.

2) Polarized sun glasses can be prepared to exclude selectively the reflcctedhorizontal polarized light and so useful in reducing glare from the sea or w troads.

(3)Scattering: Of light molecules in the atmosphere.NB:lf there arc clouds in the atmosoltcrc: LigJltrellClwJ fJH~9'(! litter mllhirla.5c:rt'tCI \;1,)5

ill tlte clouds which destroy tlte pQlllrization. '.CLINICAL APPLICATIONS OF POLARIZED LIGHT:

(1) Polarized glasses: To reuuce glare from thc sea or wet roads.(2) Polarized light In ophthalmoptlc instruments: As the ophthalmoscopes and

slit-lamps to reduce the rellected glare n'om the cornea.(3)To produce Haidinger's brushes in pleoptics: An entoptic phenomenon used

in foveal training.(4)To dissociate eyes in assessment of binocular vision:By polarizing glasses.(5)To examine lenses for stress: In optical lens making.

(8) PHOTOMI~TRYDEFINITION:It is the quantitative measurement of light and its effects upon the

visual sensation.MEASURABLE UNITS OF LICHT:

(1) How much light is being emitted which is measured by:

tedw t

l)Luminous flux: Which is the total flow of light in all directions from a sourceand is measured in lumens.

2)Luminous intensity.'Light emitted in a given direction and its unit is candela.(2)How much light is falling on a surface which is measured by illumination:

So surface illumination is measured in lumen per square foot (lumen/ft2).(3)How much light is reflected from the surface which is measured as the

luminance by the foot lambert: A luminance of one foot lambert of a surfaceemitting or reflecting one lumen/ft2.

(9) FLUORESCENCEDEFINITION: Light may be absorbed by an electron in ground state raising the

electron into an excited state-7 if the excited electron decays to a state higher thanthe ground state, i~ will emit a photon that is less energetic and so of longerwavelength-7 fluorescence.

CLINICAL APPLICATIONS:Fluorescein angiography:ln which fluoresceinabsorbs light at 490 nm in the blue and re-radiates it at 530 nm in the green.

(10) ABSORBANCEDEFINITION: When light falls upon an object it may be absorbed (or reflected,

transmitted or undergo som7 combination of the above) by the object.CLINICAL APPLICATIONS:

(1)Optical devices: As light filters and sun glasses.(2)Re-radiation: Absorbed light is usually converted into heat by the absorbing

electrons but it may be used to excite an elec~ron into a higher level and be re-raqiated (as in the case of fluorescence) .

. (11) LASERS (CHAPTER 26).

3REFLECTION OF LIGHT

LIGHT BEHAVIOR AT INTERFACE: When light meets an interface betweentwo media, its behavior depends on the nature of the two media involved and soone of the following events may happen:(1) Absorption: Of light by the new medium which is called an opaque medium.(2) Reflection: Of light back into the first medium.(3)Transmission: Of light onward through the new medium.(4) Some combination of the above: This occurs to some degree at all interfaces.

REFLECTION: It is the sent back of light at an interface between two media intothe first medium e.g. light reflection by mirrors.

Fig. 3.1: Reflection at a plane surface.LOWS OF REFLECTION (Fig.3.t):1)These are the two laws which govern reflection of light at any interface:

1)The incident ray, the reflected ray and the normal to the reflecting surface alllie in the. ame plane.

2) The angle of incidence i equals the angle of reflection r.(2)The normal: Is a line perpendicular to the surface at the point of reflection.(3)The angle e of incidence: Is the angle between the normal and the incident ray.(4)The angle of reflection: Is the angle between the normai and the reflected ray.

AU •• OlllnelAiD' - Im"llll

Fig. 3.2: Refection of a•• ;.:,·tcndedobject at a plane surface.REFLECTION BY REELECTING SURFACES (MIRRORS):

(1) Reflection at a plane reflecting surface (plane mirror):The image formed by a plane mirror of an extended object will be (Fig. 3.2):

1) Upright (erect) laterally inverted and virtual. .2)It lies along a line perp ndicular to the reflecting surface.3)It is as far behind the su 1ace as the object is in front of it.NB:Virtual and real images:

(1) The image is called virtual if the observer cannot point it and socannot be captured on a screen.

(2) The image is called real if the observer can point it and so can becaptured on a screen.

(2) Reflection at a spherical surface (spherical mirror):A spherical mirror: s a reflecting surface having the form of a portion of a

sphere.Types of spherical mirrors'

1) Concave mirror: f the reflecting surface lies on the inside of the curve.2) Convex mirror:/lfthe reflecting surface lies on the outside of the curve.

Optical properties of spherical mirrors (Fig. 3.3):1)The centre of curvature C:/Is the centre of the sphere of which the mirror

is a part.2) The pole of the mirror P:/Is the centre of the reflecting surface.3) The radius of curvature r: Js the distance CPo4) T e axis, Is any line passing through the centre of curvature and striking

the mirror:1- The principal axis: Is the axis passing through the pole of the mirror.2- The subsidiary axis: Is any axis not passing through the pole of the

mIrror.5)Tfie principal focus F: (Is a point in front of the concave mirror or behindthe convex mirror at which rays parallel to the principal axis are reflected.6) The focal length f: Is the distance FP which is equal to half CP or 1/2 r.

~~......•.••..•" ...•-" --•••••F .•.••.••

,," c,,'"

(a)Concave mirror. (b)Convex mirror.Fig. 3.3: Spherical mirrors:Rejlection of parallel rays:

Construction of the image by spherical mirrors:1)Diagrammatic construction of image using two rays (Fig. 3.4):

1-A ray parallel to the principal axis and reflected to the principal focus.2- A ray from the top of the object passing through the centre of curvature

and reflected back on its own path.

Fig. 3.4: Image for,mation by the concave minor:(a) Object at 00; (b) Object outside C (outside 2F); (c) Object at C

(at 2 F);(d) Object between 2F and F; (e) Object at F; (f) Object inside F.2) The image formation by the concave mirror·: he characteristics of the

image depend on the distance of the object, situated on the optical axis,from the mirror as seen in Table 1 and Fig. 3.4.

Fig. 3.5: Image formation by the convex mirror at any finite distance.3) Image ormation by the convex mirro :

I-If the object is at 00, the image is at F.2-Ifthe object is at any finite distance on the principal axis of the mirror

the image is virtual, erect and diminished (Fig. 3.5) and inside F.Calculation of the position and size of image formed by spherical mirrors:

1) Calculation of the position of image using the following formula:1 /v+ 1 /u= 1 /f=2/r

Where: u = The distance of the object from the mirror, v = The distance ofthe image from the mirror, f = The focal length of the mirror and r = Theradius of curvature of the mirror.

2) Calculation of the size of the image (magnification) using the .following formula: I/O = v / uWhere: I = Image size, 0 = Object size, v = The distance of image fromthe mirror and u = The distance of object from the mirror.Therefore magnification is defined as the ratio of image to object size.

SIGN CONVENTION: When using the previous two formulae we must adhere tothe sign convention (Fig. 3.6):

Mirrorllens

i+

J

Fig. 3.6: Sign convention.(1)Distances:Are measured from the pole of the mirror or the vertex of the lens:

l)Positive'Distances measured in the same direction as the incident light(light is assumed to travel from left to right) are +ve.

2)Negativ :(a)Distances measured against the direction of incident light are -ve.(b)Values of f and r in convex mirrors and f in concave lenses are -ve.

(2)Magnification:l)+ve: For erect images above the principal axis.2)-ve: For inverted images below the principal axis.

CLINICAL APPLICATIONS OF THE REFLECTING SURFACES:(1)Reflecting surfaces of the eye:

l)Keratometers: Using the principle that the anterior surface of the cornea actsas a convex mirror to measure the radius of curvature of the cornea.

2) Catoptric images (Purkinje's images): Are the images fonned by thereflecting surfaces of the eye.

(2)Reflecting mirrors:1) Concave mirrors are of use in 1- Distant direct method at 22 em.

2- Direct ophthalmoscopy.3- Indirect ophthalmoscopy.

2)Plane mirrors are of use in:l- Retinoscopy.2- Stereoscopes and synoptophores.

~REFRACTION OF LIGHT

REFRACTION: It is the change in the direction of light when it passes from onetransparent medium into another of different optical density (the incident ray, therefracted ray and the normal all lie in the same plane).

FACTOR~ AFF CTING THE AMOUNT OF REFI~ACTION (BENDING OFLIGHT RAYS):

(1) The "~nsityof the medium (refractive index).(2)The obliquity of falling of light rays (angle of incidence).(3) The wavelength oflight (dispersion).

REFRACTION OF A LIGHT RAY ENTERING AN OPTICALLY DENSERMEDIUM THAN AIR: On entering an optically dense medium from a less densemedium, the velocity of light becomes slower and so light is deviated towards thenonnal (Fig. 4.1):

Fig"'.1: Refraction of light: Light entcr"jng Fig. 4.2: Rcfraction of light through anoptically densc medium from air. parallel-sided platc of glass.(l)The normal: Is a line perpendicular to the interface at the point of refraction.(2) The angle of incidence i: Is the angle between the incident ray and the normal.(3) The angle of refraction r: Is the angle between the refracted ray and the normal.

SNELL'S LAW: It states that (Fig. 4. 1):(l)The incident ray, refracted ray and the normal all lie in the same plane.(2) The angles of incidence i and refraction r are related to the refractive illdex n of

the media concerned by the equations:

SIll I1)11=-.-smr

Where n is the refractive index of the second medium when first medium is air.

n2 SIn]2) ~=~

Where n2 is the refractive index of the second medium and n 1 is the refractiveindex of the first medium.

(3)T71e incident and refracted rays are on opposite sides of the normal.

REFRACTION OF LIGHT THROUGH PARALLEL-SIDED PLAT OFGLASS(GLASS BLOCK):Principle(FIG.4.2):(1)Light passing obliquely through a plate of glass is deviated laterally and the

emerging ray is parallel to the incident ray(i.e. the angle of incidence equals theangle of emergence) and so the direction of light is unchanged but is laterallydisplaced.

(2)Deviation of light is more with greater thichness of the glass plate(block) but itsintensity is less.

Clinical applications:(1) A sheet of glass can be used as an image splitter- As in the teaching mirror of

the indirect ophthalmoscope in which:1)Most of light is refracted across the glass sheet to the examiner's eye.2)A small portion of light is reflected at the anterior surface of the glass sheet, and enables an observer to see the same view as the examiner (Fig. 4.3).

(2) Helmholtz ophthalmometer. Chapter 29.

Fig. 4.3: Parallel sided glass Fig.4.4:Refraction of light at a convexsheet used as an image splitter. spherical refracting surface(as cornea).

REFRACTION OF LIGHT AT A CURVED INTERFACE(CONVEXSPHERICAL SURFACE AS THE CORNEA):

(1)The fundamental formula of a convex spherical surface (Fig. 4.4):Construction:

APB=The convex spherical surface.n=The refractive index of the medium bounded by APB surface.C=The centre of curvature of the surface.O=A luminous point on the axis.I=The image of O.

81=The angle of incidence.82=The angle of refraction.

CLN=The normal to the surface.LD=Perpendicular from L to cut axis at D.

u=Dis tance of 0 from APB.v=Distance of I from·APB.r=Radius of curvature of APB.

Calculation.:SIn 1

l-::'l = ~- and so, sin 1= n sin r.SUir

2- The SInes of the angle of incidence and of refraction equal to theirnumencal values as both are s111a11angles (when PD is consideredas a s111allregion).

3-81 = a + c and 82 = c - b and so a + c = n (c - b).4- As the angles a, band c are small and can be replaced by their

tangents (ren1embering that v and r are negative):LD LD LD LD LD LD

:. a = PO = --;- , b = PI = -v and c = PC = -r .

5-Substituling in the above a + c = n (c - b):

LD + LD = n (LD _ LDJu -f -f-V

6- Dividing by LD to get the fundamental formula of a convex sphericalsUlface:

.1 + 1- = n i.e. .1 _1 = 11. (_1 + 1) i.e. 1 _.1 = _ 11. + nu -r u r r v u r r vn 1 n 1

i e - - - = --; - - So, 0 / v-I / u = 0-1 / r.. v u r rWhere, 11 = The refractive index of the 111edium.

u = The distance of object fro1l1 the convex surface.v = The distance of lInage fr01n the convex surface.r = The radius of curvature of the convex surface.

7- If u is at 00, v will be at principal focus F (i.e. at focal distance f):So, f = n I" / n-r

8- When light is refracted fr0111a 111edilllTIof n1 to another of n2, nnl n2

beC01nes n2 and nl So, n= III r /112 - oJ and f2= il2 I" / n2 - nl

Where h = The anterior focal distance.f2 = The posterior focal distance.

(2)The surface refracting power of a convex surface:l)It is given by the formula: Surface refracting power = n2 - nl/ r.

Where r = The radius of curvature of the surface in Ineh"es.n2 = Refr"1ctive index of the secondmedlum.

nl = Refractive index of the first medium.2)The surface refracting power is measured in dioptres which is +ve

for converging surfaces and negative for diverging surfaces.3-The anterior surface of the cornea is an exanlple of such a refracting

surface and its power accounts for most of refracting power the eye.REAL AND APPARENT DEPTH:Principle:(l)Objects situated in an optically dense medium appear displaced when viewed

from a less dense medium (Fig. 4.5)due to the refraction of the emerging rayswhich now appear to come from a point, I, the virtual image of object, O.

. . _ Velocity of light in air _(2)RefractlVe mdex of water - V· 1 't fl' ht·· t - 4/3.- e OCI y 0 19 m wa er(3)Pr~ctically it is not necessary to find the 2 velocities directly as both can bereplaced by the real and apparent depth which are easily found and so,the real depth!Apparent depth=Velocity oflight in air/Velocity oflight in water.(4) Objects in water seem less deep than they are e.g. one's toes in the bath.

Fig. 4.5: Real and apparent depth. Fig. 4.6: Graefe knife in ACClinical Application: This principle applies to surgical instruments in the anterior

chamber: For example, when making Graefe section, the knife in the anteriorchamber appears to be more superficial than it really is (therefore the point ofthe knife is aimed at the opposite limbus to emerge 1 mm behind the limbus,Fig. 4.6).

TOTAL INTERNAL REFLECTION:Principle: Rays emerging from a denser medium to a less dense medium (as from

glass to air) suffer a variety of facts depending on the angle of incidence i.e theangle at which they strike the interface (Fig. 4.7):

(1)Ray A, strikes at 90° t the interface and is undeviated.(2)Ray B, emerges after refractio.(3)As the rays meet the interface more obliquely,a stage is reached where the

refracted ray, ray C, runs parallel with the interface:1)The angle between the incident ray C, and the normal( angle of incidence)

is called the critical angle c.

2)The refracting angle at the critical angle is 90 .(4)Ray D, striking more obliquely fails to emerge from the denser medium and

is reflected back into it (as from a mirror):1)This is called total internal reflection.2)The angle of incidence d is larger than the critical angle c.

Fig. 4.7: Total internal reflection. 4.9: Fibroptic tubule.Clinical applications:(l)The total internal reflection occurs at surfaces within the eye (notably the

cornea-air interface) and prevents visualization of parts of the eye as:I) The angle of the anterior chamber (Fig. 4.8).2) The periphery of the retina.NB:This problem is overcome by applying a contact Icns made of a material with a

higher refractive index than the eye and filling space betwecn the eye and Icnswith saline to destroy cornea-air refracting surface and allowing visualization of:(l)The anterior chamber angle by gonioscopy (Chapter 24).(2) The retinal periphery by a 3-mirror contact lens (Chapter 23).

(2)Forms of prisms used in ophthalmic instruments: Chapter 5.(3)To get the index of refraction n of a medium by measuring the critical angle c:.. .

S1111 S111Cn=-- =sin r sin 90°

(4)Fibreoptics (Fig.4.9):I)Optical fibre consists of a core of transparent solid material (as glass or

plastic) with a high refractive index surrounded by a caddling with a lowerrefractive index.

2)The high-index to low-index interface between the core and the glass tube isthe cause of repeated total internal reflection of a ray.

3)Parallel bundles of these fibers are called a coherent fibreoptic bundle whichtransposes the entire incidence face to the emergence face as in severalelectro-optical devices including computer output tern1inals.

(5)infernal reflection explqins the secondary rainbow formation: But cannotexplain antireflection of lenses which is explained by destructive interference.

1and REFRACTION BY PRISMSDEFINITIONS(FIG.S.l):(I)Aprism: Is a portion of a refracting medium bordered by two plane refracting/ surfaces which are inclined at a finite angle at the apex of the prism called the

refracting (apical) angle.

EmergentI"ClV

Fig. 5.1: Prism(refracting angle a). Fig.5.2:Light pass through a prism.2)Theedge:ls line of intersection of the 2 refracting surfaces at the apex of the prism.(3)Theaxis: Is a line bisecting the apical angle.(4)Thebase: Is the surface opposite to the apical angle.(S)Theorientation of prisms: The orientation is indicated by the position of the base

as base.:up, base-down, base-out or base-in.(6)Theangle of deviation (d):

1)The ray is deviated towards the base of the prism (Fig. 5.2).2)The total amount of deviation between tnemcident ray and the emergent ray is

called the angle of deviation d.3)For a prism in air, the angle of deviation is determined by 3 factors:

I-The refractive index of the prism material.2-The apical angle of the prism.3-The angle of incidence of the ray considered (its obliquity).

NB: These three factors also determine the apical refracting angle.(7)Theangle of minimum deviation:

1)Least angle of deviation when angle of incidence equals angle of emergence.2) Refraction is called symmetrical under these conditions.

RELATION OF THE ANGLE OF MINIMUM DEVIATION TO THE APICALANGLE(FIG.S.3):Prismsused in ophthalmoptics are made of thin glass with n=1.5 and therefore:1)Prisms are symmetrical:' In which the angle of incidence equals the angle of

emergence with the least angle of deviation and so light passes symmetrically.2)Prisms have an apical angle of less than 100 in which:

1- Sine the angle is the same value as the angle.

2- d=1/2 a(d = angle of minimum deviation and a = apical angle):Therefore, the angle of minimum deviation d (i.e. The deviationproduced by symmetrical prisms with small apical angles) is equal tohalf the apical angle a.

IMAGE FORMATION BY THE PRISM(FIG.5.4):(1)Erect.(2)Virtual.(3)Displaced towards the apex of the prism (deviation is reduced to minimum when

ligh~passes through prism symmetrically). .

I -_ ••q- _

-~~~-o

Fig.5.3:Rclation between angle d and a. Fig.5.4:lmage formation by prism.DISPERSION OF LIGHT THROUGH PRISMS(FIG.5.5):(1) Spreading the white light into its component wavelengths by the different

refractive indices of the prism occurs by dispersion.(2) Light of shorter WLs is deviated more than light of longer(3) Dispersive power of a prism is not related to refractive power or index of prism.(4) The angle of dispersion is not the angle of refraction and not the angle of

polarization (Browester angle). ~

IIII

I

Equivalent -----..,/solid prism I

II

IIIIl- _

VioletFig.5.5:Dispcrsion of light. Fig.5.6:Fresnel prism.

DISTORTIONS OF PRlSMS (FIG. 5.7):(1) Horizontal magnification.(2) Vertical magnification.(3) Curvature of vertical lines.(4) Asymmetrical horizontal magnification.(5) Change in vertical magnification with horizontal angle.

iationual to

NO HORIZONTALDISTORTION MAGNIFICATION

VERTICALMAGNIFICATION

CURVATURE ASYMMETRICof HORIZONTAL

VERTICAL LINES MAGNIFICATION

CHANGE IN VERTICALI MAGNIFICATIONI withI HORIZONTAL ANGLE

_ 5.7: Distortion of prisms.NOTATION OF PIUSMS:(1)Theprism dioptre (1\):

l)A prism of one dioptre power (1 A) produces a linear apparent displacement of1 cm of a tangent (of an object, 0) situated at 1 m (Fig. 5.8).

2) 10 t1 is not 10 times 1 t1 because the tangent of 10 f1 is not 10 times that of 1 t1.(2)Angle of minimum (apparent) deviation (d'):

1)The apparent displacement of the object 0, can also be measured in terms ofthe angle of apparent deviati on (9) (Fig, 5.7).

2) In Ophthalmoptics, a prism of 1 t1 produces an angle of minimum (apparent)rdeviation of 1/2° and So 1 t1 = 2" . ~,

(3)The centrad V: It is the strength of a prism which produces an apparentdisplacement of 1 em of an arc (of the object, 0) situated at 1 III (Fig. 5.9).

(4)The apical refracting angle:The refractive index of the material must be knownto get the apical r<.;fi·acting angle a (Fig. 5.l~t is not in use now.Nlll:Thc centrad has the advantage HUl.t: 10 V are 10 times V (ufllike tlte prism

diopfre hl which 10 A (Ire /lot /0 times I 1\.N132:Ccntr'ad is slightly more than pdsm d'optre:So produces a slightly grulter allgle

of deviatioll tltall tlte prism dioptre bllt tl e difference ilt practice is Ilegligible.

{t-------1 em . ------- _ ~.8---------------- ---- ,-----o. , m-(----------~.

fI -------- .. -lcrll -- _ - ... --------------- .-.,0_ 1m

INDICATIONS (USES) OF PRISMS:(1)Diagnostic:

1)Measurement of the angle of deviation in heterophoria (Chapter 2):1-Maddox rod and prism test.2- Maddox hand frame (Maddox rod and rotatory prism) test.3- Prism (or prism bar) test.

2) Prism vergence test to measure fusional reserve in heterophoria: hapter 12.3) Four deportee prism test for small degree of isotropic. Chapter 12.4) Estimation of the amplitude of convergence (Chapter 11).

1-Positive convergence: By the strongest prism base-out which is toleratedby the patient without diplopic.

2- Negative convergence: By the strongest prism base-in which is toleratedby the patient without diplopic.

5)Assessment of the probability of occurrence of diplopic. After a proposedstrabismus surgery in adults.

6)Assessment of simulated blindness: If a prism is placed in front of a seeingeye, the eye will move to regain fixation (in malingerers).

(2)Therapeutic: ,1)Exercising prisms to exercise the weak muscles in:

1- Convergence insufficiency:' The prisms are used base-out during thepatient exercise periods for building up the fissional reserve (they are notworn constantly). ,

Z- Heterophoria (prisms are commonly used in horizontal heterophoria)wi ease 0 e is towards the deviation:(a) n exo horia: With prisms base-out.(b) In esophoria With prisms base-in. )

2) Relieving prisms: 0 relieve diplopia and eye strain as in:1- Vertica e erophoria with the base of the prism against the deviation:

The eye with hyperphoria with a prism base-down, and the other eye withhypophoria with a prism base-up.

2- Older presbyopes (for convergence is necessary in spite of weakaccommodation) by cither:(a) Prism base-in in the lens (is rarely used),or

(b) Decentring of the lens.3) Nystagmus: Prisms can be prescribed binocularly with their bases left to

reduce nystagmus in patients whose nystagmus is least in right gaze(improving visual acuity and minimizing head turn).

(3) In ophthalmoptic instruments:1) Slit lamp microscope and applanation tonometer.2) Keratometer.3) Indicrect ophthalmoscope.4) Telescopes (as Galilean telescope with prism binocular).5) Cameras.

FREDe

g there not

(4)In ophthalmoptic appliances:1) Spectacle glasses for special uses: 1- Hemianopic' glasses.

2- Recumbent glasses.2) Contact lenses: - Prism ballast.

2- Truncated lens.FRESNEL PRISM:

Definition: A plastic thin flexible membrane of parallel tiny prisms of identicaloptical (reft'acting) angles and is made of clear polyvinyl chloride (PVC).

Optical principles:(l)Fresnel principle IS based 0 : Removal of the non-refracting portions of a

conventional prism to get a light weight, larger diameter optical element.(2) Reducing a prism size to 4 mm: Will result in a base thickness of 0.8 mm.(3)Fresnel prism can be imagined to be a series of small prisms(with the same

apical angle), adjacent to e vh other to form a thin membrane (Fig. 5.6).(4)Fresnel membrane prisms are designed for in-office application and removal:

1) App tcation 0 t e membrane' It can be cut with scissors to the appropriateshape and can be stuck simply with water to the back surface of the glassor plastic spectacle lenses.

NB: The membrane adheres to the lens for the same reason that twoflat pieces of glass bound together when air layer between them hasbeen removed.

2) Removal of the membrane: 'Can be done by peeling off the membrane,starting at the edge.

t\dvnntages(1) Cosmetically superior to a conventional prism: ~hin (0.8 mm)and light weight.

(2)Large diameter (64 mm) So can he cut to conform 10 the shape of mostspectacle carrier lenses. J

(3)A ranee of powers (0. 5-30 prism deports) are available:Fot treatment ofstrabismus.

(4)Refractive index is 1.525: hich is nearly similar to that of crown glass',(5)In-ollce applicatfon: Wi h easy removal, modification and succeSSive

applications.(6)Localized use on spectacles: an be pressed onto a portion or bifocal

segment of spectacle lenses.Disadvu tages:

(1) Moderate 1ass 0f contrast.(2) Slight loss of visual acuity.(3) RefJections and scattering of light from the prism facets.(4) Visibility of the grooves.

Indications (uses):(l)As a substitute of conventional prisms:

1) Diagnostic uses.

2) Therapeutic uses.3) Special glasses (hemianopic. and recumbent glasses).

(2)Localized use on spectacles:1)Pressed onto a bifocal segment of a high plus reading lenses as a low

vision aid to allow the addition of base-in prism without decentration(Chapter 20).

2) Cut and applied to the appropriate portion of the spectacle lens in certaindirections of gaze in paralytic strabismus.

FORMS OF PRISMS:(1) Forms of prisms used in diagnosis:

1)Single unmounted prisms.2) The prisms from the trial lens set.3) Prism bars.4) Rotatory (rotary or double) prism.5) Fresnel prism.

(2)Forms of therapeutic prisms:1) Temporary wean

1-Clip-on spectacle prisms for trial wear.2- Fresnel prism.

2) Permanent weW1"Permanent incorporation of a prism into spectacles can beachieved by:1-Decentring of the spherical lens already present.2- Mounted prisms in spectacles.

::fZ~(d)

INTdi1

"iig.5.10:Forms of prisms used in ophthalmic instruments as reflectors of light (with total rinternal reflection within the prism):(a)Right angled prism with deviation 90o.(b)Rightangled prism with deviation 180o(Porro prism). (c) Two right angled prisms. (d)Dove prism.(3) Forms of prisms used in optical instruments: Prisms are used as reflectors of

light( as mirrors) in which the prism is designed and oriented so that totalinternal reflection can occur within it as follows (Fig. 5.10):

a lowtration

·sofotal

1)Right angle prism with 90 ° deviation1-The angle of incidence is 45 ° (greater than the critical angle of glass which

is 42°) and so total internal reflection occurs.2-The incident parallel rays strike the surface on the side of the right angle of

the prism and emerge from the surface on the other side of the same anglewith 90° deviation ane lateral transposition of image i.e. the image istransposed left to right (later inversion of image).

3- The best example is the 2 reflecting prisms in the eyepieces of the indirectophthalmoscope (Chapter 15).

2)Right angled prism with 180 °deviation (Porro prism):1-The incident parallel rays strike the surface opposite the right angle of the

prism (hypotenuse) with 180° deviation and so emerge from the samesurface (hypotenuse).

2- The image is invelted bu~ not transposed left to right.3) Two right angled prisms (2 Porro prisms) with their edges at right angle to

each other:1-The first prism with its edge in a vertical position: ·

(a) The incident parallel rays strike the base of the prism and so the rays arereflected from one refracting surface of the prism to the other.

(b) The emergent rays are parallel to their original course with 180°deviation but are transposed left to right (lateral transposition or lateralinversion).

2- The second prism with its edge in a horizontal position: The emergent raysare inverted (image inve sion) with 180° deviation.NB: Two right angled prisms (2 Porro prisms) are placed between each,

eyepiece (concave lens) and objective (convex lens) of the Galileantelescopic system in binocular telescope (Chapter 21), slit-lamp(Chapter 23), fundus camera (Chapter 25) and operating microscope(Chapter 28). ,

4)pave prism (with no deviation): The image is inverted but not laterallytransposed.NB:Prisms give greater flexibility in dealing with an image than do mirrors:So many

possible systems of prism forms are available.INTERPRETATION OF PR 8M REPORTS: The refracting angle, aO

, the prismdioptre, A , and the centrad, "V , are equal while the angle of apparent deviation, d,is half each and so any of these methods can be adopted: So, A prism of loarefracting angle deviates light through 5" and has a power of 10 A or IU\?

A TER(iREFRACTION BY LENSES

DEFINITION: A lens is a portion of a refracting medium bordered by two surfaceswhich have a common axis and at least one of these two surfaces is curved.

FORMS OF LENSES:(1)Sphericallenses.(2)Astigmatic lenses: 1) Cylindrical lenses.

2) Toric lenses.(1) SPHERICAL LENSES

DEFINITION: A spherical lens is a lens in which each spherical surface forms partof a sphere and so all meridians of each surface have the same curvature and therefraction is symmetrical about the principal axis.

FORMS OF SPHERICAL LENSES:(1)Convex lenses: A convex lens may be considered as a collection of prisms base

to base i.e. it is built of prisms of gradually increasing angles i.e. biconvex lens,plano-convex lens or convex meniscus(with both surfaces convex).

(2) Concave lenses: A concave lens may be considered as a collection of prismsapex to apex i.e. it is built of gradually decreasing angles i.e. biconcave lens,plano-concave lens or concave meniscus(with both surfaces concane).

NBI :Convex or concave lens meniscus acts as a concave lens:.(i ille Dower 01 co~-:.surface is less with longer radius of curvature and acts as a convex lens if the reverse.

NB2:The power of convex or concave meniscus:ls the sum of the power of the two surfaces.REFRACTION BY SPHERICAL LENSES: A convex lens causes convergence of

incident light while a concave lens cauSes divergence of incident light(Fig.6.2),and the total vergence power of a spherical lens depends on:

~

OJ>(1)1(2)

12

Tolal vergonco +4 +4 -4 -4power

~)

~) (

Surface vergenco +2 +2 -2 +6 -2 -2 -6 +2 (••power

Fig.6.1 : Vergence power of a thin lens.(1)The vergence power of each surface.(2)The thickness of the lens:

1) Thin lenses: he thickness factor may be ignored and the total power of athin lens is the sum of the two surface powers (Fig. 6.1).

2) Thick lenses: Refraction b thick lenses is more com licated Cha ter 7 .NB:Refraction can he thought of as occurring at the principal plane 0

the lens and in the following lens diagrams in this hook, only theprincipal plane is shown.

A~s

PrincipJI••. 1. F

Il

(a)COllVCX lens. (b)Concavc lCl1~

Fig. 6.2: Light passing thrO'ilgh a spherical lens.OPTICAL PROPERTIES OF SPHERICAL LENSES:(1)Theprincipal axis: Is the axis on which the lens is centered (Fig. 6.2).(2)Asecondary axis(Fig. 6.3):

l)ls the axis on which the emergent ray is parallel to the incident ray.2)In thin lenses, rays passing through it may be considered undeviated).

a :; i

Fig. 6.3: A sccomhu-y axis. Fig.6.4:0ptic~ll ccnttc of lens.(3)The principal pl"l 10: Is a lil1e perpendicular to the principal' xis and intersects

with it at tl e prim:ipnl poi It.(4)Theprincipal point or nodai point (N): s the point at which the principal plane

and the principal axis intersect and through which light rays pass undeviated andits site coincides v"ith the site of the ptical centre.

(5)TI10 geon ...tt ica centf~: 1 tbe point in th' middle of' the lel1s and is a relation ofthe placement of the lens in its fl'ame.

(6)Theoptical centro '0 :l)Deflniliol: It is a,point which Jcmns the centre of the optical system oftlle lens,

where all secondary axes meet the principal axis and through which all rays maybe considc cd unde 'iuted (as we deal with thin lenses in ophthalmoptics).

2)The site 0 t: rp . ( C I e 1 ens is Fif,!. 6.4 .I-Inside the lens in bieollvex and bie ncavc lenses.2-0n the curved side in plano-convex and plano-concave lenses.3-0uts' de the more curved side in convex and concave meniscus lenses.

3) The 0 tical centre of a (hick lens is calcu atedfrom:i-The curvature of the lens surfaces.2-The lens thickness.3-Tlie refractive index of the lens.

(7)The principal focus (F): Is a point on the principal axis where parallel light raysto the principal axis are converged to or diverged from it

(8)The principal foci (F1 and F2): As the medium on both sides of the lens is thesame (air), parallel light incident on the lens from the opposite direction (i.e. fromthe right direction) will be refracted in an identical way and so there is aprincipal focus on each side of the lens, equidistant from the principal point(Fig.6.5):

~

----- ~-'-' N-- - (_--- F, F,--., __ . -. +". .. -11 • r::

o I

(a)Con vex lens. (b)Concave lens.Fig.6.5:The principal foci of thin spherical lenses.

1)Thefirst principal focus (F1). Is the point of origin of rays which afterrefraction by the lens are parallel to the principal axis.

2)The second principaTfocus (F2J Is the point which incident light parallelto the principal axis is converged to or diverged from it.

(9)The focal lengths (f1 and f2):1) 1eJirstfocallength (ff): s the distance between the first principal focus FI

and the principal point N.2)Tlie secondJocal ength (f2): s the distance between the second principal focus

F2 and principal point N.NBl The secondfocallength (f2) by sign convention has:

(1)A positive signfor the convex Lens.(2A negative sign for the concave lens.

B2 !Lenses are designated by their secondfocallength (f2):(l)Convex(converging) lenses are called plus len~es and are marked with +.

(2)Concave (diverging) lenses are called minus lenses and are marked with -.NB Distancesfl andf2 may be equal or not equal according to:

~ (1)f1 =f2 if the medium Oil either side of the lens is the same e.g. air.(2)f1 is not equal to f2 if second medium differs from first e.g. as in a contact lens.

CONSTRUCTION OF THE IMAGE BY SPHERICAL LENSES:(1) Diagrammatic construction of image using two rays (Fig. 6.6):

1)A ray parallel to the principal axis which after refraction passes either-1- Through F2 of a convex lens; or

2- Away from F2 of a concave lens.2)A ray from the top of the object: Which passes through the principal point

undeviated.

~~---"'''-.,..~ ..•..,.. ..•. _ •.•_t ...•- .••_ ••.- .•.....- -- . L:..ol:t..::-_- --.•.. ... ::::--- .' - ...v,o

Fig. 6.6: Image formation by a thin convex lens; (a)Object at oo;(b)Object outside 2F;(c)Object at 2F;(d) Object between 2F and F;(e) Object at F;(j)Object inside F.

Table II: Ima e formation b a convex lens.Ima e

SiteSite of object 011 theprincipal axis

(a) At 00:

(b) Outside 2 F:(c)At2F:(d) Between 2F and F:(e) At F:( Inside F: Virtual.

N.B:2F in the lens =C in mirrors.(2)The image formation by a convex lens: The characteristics of the image

depend on the distance of the object situated on the principal axis, from the lensas seen in table II and Fig. 6.6.

(3)Image formation by a concave lens:1) If the object is at 00 The image is at F2)If the a iect is at any finite distance on the principal axis of the lens: he

image is virtual, erect, diminished and inside F2 (Fig. 6.7a).THIN LENS FORMULA: Fig. 6.7b shows a concave lens with similar triangles

ABN and CDN and similar triangles ENF and CDF: 1 I v- 1 I u = 1 I fWhere, v = Distance of image from principal point N.

u = Distance of object from N.f= Focal length.

Real.Real.Real.

Inverted.Inverted.Inverted.

AtF.Between 2F and F.At2F.Outside 2F.At 00

Diminished.Same size.Enlarged.

~ <-::.:- - - -..•... ,...~ ....l"'- ••.•..••J - .-- .•.- --- 1::::..ollt..;:: __

•..• .... - •..... •..•: .•.•._- .' ..F,O

Fig. 6.6: Image formation by a thin convex lens; (a)Object at oo;(b)Object outside 2F;(c)Object at 2F;(d) Object between 2F alld F;(e) Object at F;(f)Object inside F.

Table II: Ima e formation b a convex lens.fma e

SiteSite of object 011 theprincipal axis

(a) At 00:

(b) Outside 2 F:(c)At2F:(d) Between 2F and F:(e) At F:( Inside F: Virtual.

N.B:2F in tlte lens =C in mirrors.(2)The image formation by a convex lens: The characteristics of the image

depend on the distance of the object situated on the principal axis, from the lensas seen in table II and Fig. 6.6.

(3)Image formation by a concave lens:1) If the object is at 00 The image is attJ2)If the 6bject is at any finite distance on the principal axis of the lens: he

image is virtual, erect, diminished and inside F2 (Fig. 6.7a).THIN LENS FORMULA: Fig. 6.7b shows a concave lens with similar triangles

ABN and CDN and similar triangles ENF and CDF: 1 I v- 1 I u = 1 I fWhere, v = Distance of image from principal point N.

u = Distance of object from N.f= Focal length.

Real.Real.Real.

Inverted.Inverted.Inverted.

AtF.Between 2F and F.At2F.Outside 2F.At <X)

Diminished.Same size.Enlarged.

NBl:ln a concave lells tllefocallellgtlz fand all distallces beliind me lellS are lle!!tE.NB2.:Tlleformula can be applied to a convex lells also.NB3:The position and nature of image formed by a sphericallells depelld on the position of

The object as with spherical mirrors and the formula applied is similar (chapter 3).

(a)Imageformatioll. (b)Thin lens formula.Fig. 6.7: A thin concave lens:

~ DIOPTRIC POWER OF SPHERICAL LENSES:(1) Lenses of shorter focal length are more powerful than lenses of longer focal

length.(2)Therefore the unit of lens power, the dioptre (D) is based on the reciprocal of

the second focal length in metres which gives the vergence power of thespherical lens in dioptres, thus: t = I / f2

Where, F = The vergence power of the lens.f2 = The second focal length in metres.

(3)A converging (convex) lens of second focal length +10 cm has a power of+1/0.1=+10D.

(4) Likewise,a diverging (concave) lens of second focal length -5cm has a power of- 1 / 0.05 = - 20 D

Fig. 6.8: Linear magnification. Fig. 6.9: Angular (apparent) size.M~GNIFICATION OF A CONVEX LENS:

(1) Linear magnification:Definitio : It is the ratio of the image height to the object height with

measurements perpendicular to the optic axis (it is equivalent to the ratio ofimage distance to object distance).

Application Linear magnification is applicable to lens and lens systems used inimage formation as in cameras, projectors and other instruments not useddirectly in conjunction with the eye.

Ca culation Linear magnification is calculated from the formula(Fig.6.8):I vM-- ---0 -u

Where, M = The linear magnification.I = The image size.

o = The object size(object within the focal length of convex lens).v =The distance of the image from the principal plane.u = The distance of the object from the principal plane.

(2)Angular magnification:Application Angular magnification is considered when a lens or an optical

system (as the magnifying lens) is used in conjunction with the eye becausein ophthalmoptics, the angle subtended at the eye is more important than theactual image and object size (because the angle subtended governs the retinalimage size).

Principlesl)1\:n u ar or apparent size of objects:Fi'g.6.9 shows that objects A, B, C

and D all subtend a same visual angle e at the eye and produce a retinalimage xy: (a)They are all therefore of identical apparent size, as angle is

the e same in all.(b)Apparent size is given by the ratio of object or image

size deviated by its distance from the eye i.e. tan e.

e·Fig.6.l0:Imagc formation of a convex lens. Fig.6.11:Simple magnifYing glass.

2)An ular magnification of a convex lens (magnifying glass or loupe):/1- When the object is at the first principal focus of a convex lens, the image

will be at infinity(Fig.6.1 0).2- The object and its infinitely distant image suotend the same angle 8 at

the convex lens and also at the eye if the eye is brought very close to theI, lens.

3- The angular magnification is therefore unity i.e. apparent object size andapparent image size are the same.

4- The use of a convex lens enables the eye to view the object at a muchshorter distance (than infinity) unaided and to retain a distinct image.

5- As the object approaches the eye it subtends a greater angle at the eyeand the retinal image size increases.

3) The magnifying power (effective angular magnification) of a convex lens:/

1-T e magmrying power can be calculated from: CApparent size ofimaf{e F

(a)Magnifying power (M) = At' if b" t = 4 at 25 cm frompparen Size 0 0 '.lecthe eye (Fig.6.1l) , Where, F = Power of the lens in D.

M=Effective angular magnification.25 (:(b) I'vfaf!nifyingpower =. . .

o vlsual acUlty (of the better eye) as percentage--2- ..fie rlagni ing power is increased by:(a) lmum an u ar ma ni lcation Occurs if the object lies directly at

the anterior focal point F1 of the lens.(b) ncrease ang~lar magniflcation: ccurs if the object moves closer

to the eye.(c) ItlOna angular magni lcation: Occurs if a second convex lens is F

placed between the object (which is brought closer to til~ eye)and theeye.

(d) t I an ular magniJicatLOn: Equals the first magnificationmultiplied by the second additional ,magnification (M = M 1 M2).

At\1\

= : tF =0==== --==

(a)COilvex lens (b)collcave lensFig. 6. 12:Prismatic deviation by sphericallellses.

SPHERICAL LENS DECENTRATION AND PRISM POWER:(1)The prismatic effect of a spherical lens:

1)Rays of light incident upon a lens outside its axial zone are deviated eithertowards a convex lens axis or away from a concave lens axis.

2) Therefore the peripheral portion of the lens acts as a prism.3) The refracting angle between the lens surfaces grows larger as the edge of the

lens is approached (Fig. 6.12).4J-17hereforethe prismatic effect increases towards the periphery of the lens.

100 em- ---- -- -- - - ---------_._---- ----_._-----,IpII

By similar triangles: ! ,..!QQ..h 100o

P 'h X 0prism diopters em diopters Prentice's rule

(2)Decentration of a spherical lens: It is the use of a non-axial portion of a lensto gain a prismatic effect as in:1)Spectacles when a prism is to be incorporated.2)'Poor centration of spectacle lenses especially high power lenses (needed for

correction of aphakia or high myopia) leading to spectacle intolerance.(3)Theprismatic power gained by decentration of a spheri':'~! !ens(Prentice,s

rule, Fig.6.13):p 100 (0 oJ 0 1h = lOOID = SlmJ ar tnang es)o So, P = D X, h

Where, P =The prismatic power in prism dioptres.D =The lens power in dioptres.h =The decentration (distance from the optical centre) in centimetres.

FRESNEL LENS:Definition: A plastic thin flexible membrane with concentric ridges on its

surface fornLing a series of tiny priSlTISof increasing power from the axisto the periphery and is made of polyvinyl chloride (PVC).

Optical principles:,(1)Fresnel principle is based on: Removal of the non-refracting portions of a

conventional lens (as in Fresnel prism principle).(2)The angle etween the swfaces: Is considered as a series of tiny prisms of

increasing apical (refracting) angle as one moves from the optical centre ofthe surface to the periphery (not the same as in Fresnel prism).

(3)Fresnel ens are commonly made of arrow flat surface prismgrooves(prismlets) of increasing apicl angle.

(4)Frensel lens can be imagined to be a series of flat ridges: In the form ofconcentric rings on a surface to [ann a thin membrane.

(5) <resnellenses are designedfor in-office application and removal: he sameas in Fresnel prisms (chapter 5).

Advantages: As in Fresnel prism but Fresnel lens powers are up to 20 D.Disadvantages: As in Fresnel prism.Indications (uses):) (1) Correction of spherical refractive errors:

IJ ermanent use: For high spherical errors.2)Temporary use: For postoperative aphakia, transient ametropia ana. .progreSSIvemyopia.

(2)Underwater diving masks and gas masks: To which the spherical equivalentof the refractive error is applied in a Fresnel lens.

(3)Addea lenses in bifocal segments: In presbyopes and in low vision aids.(2) ASTIGMATIC LENSES

DEFINITION: An astigmatic lens is a lens in which all meridians do not have thesamecurvature and a point image of a point object can not be formed.

FORMS OF ASTIGMATIC LENSES:(1)Cylindricallenses(Fig.6.14):

Surfaces of a cylindrical lens . These lenses have one plane surface and the otherforms part of a cylinder:1 he convex cy In er: s a part of a cylinder of glass cut parallel to its axis.2) . e oncave c r er: s taken from a mould of a cylinder.

The axis 0 the cylinder: It is the meridian in which the lens has no vergencepower (actc)as a parallel plate of glass) and is parallel to that of the lens fromwhich it is taken.

Formation of a line image (focal line) for a point object·1 In the meridian at right angles }o the axis, the cylinder acts as a spherical

lens while there is no vergence power in the direction of the axis.2)Thus a focal line (line image)' the image of a point object formed by a

cylindrical lens and it is parallel to the axis of the cylinder and as a resultno distinct image is formed.

3)Maddox rod is a high powered cylindrical lens (Chapter 12).

Clrclo ot 10051 contu,lon

1

(a)Collve.;'(cylinder. (b)Collcave cylillde:--Fig.6.14:cylindricallcnses. Ii'ig.6.15:Image formcd by toric astigmatic Icns.

(2)Toric lenses:1) Toric surfac .

Definition: t is a surface formed by bending of a cylindrical surface so thatthe axis becomes an arc of a circle and the resulted surface will be curved

. in both horizontal and vertical meridians, but not to the same extent..Principal meridians:} These are the meridians of maximum and minimum

curvature which are at 90° to each other in ophthalmoptic lenses.The base curve: JIs the principal meridian of minimum curvature and so of

mInImUm power.2) Toric lens (sphero-cylindricallens)

Definition· It is a lens with one toric surface.lma e formed by a toric lens(6.15):

l-rA toric lens aoes not produce a single defined image: Because theprincipal meridians form separate line foci at right angles to each other.

yamlt

2 Sturm's conoid: Is the figure fanned by the i'ays of light between the twoline foci.

3- nterva OjSturm: Is the distance between the two line foci.4- 'he circle of east ifJusion- Is the plane lying between FH and FV

where the two pencils of light lillcrsect (it is also called the circle of leastcrmfusion).

pherc-c 'lmdrica ractio. A toric lens (sphero-cylinder lens) can bethought of as a spherical lens with a cylindrical lens superimposed lipan itand so can be defined numerically as a fxz,stion:

Cross cylinder Is a sphero-cy lindricallens (i.e. a type of a toric lens) with:1-Th power of the cylinder is twice the power at the sphere

and of the opposite sign (Chapter 16).2-AddiHon of a cross cylhtder can increase the conoid of Sturm

and so decrease the visual acuity (or the reverse).

~ncerom

TER7REFRACTION BY THE EYE

THE OPTICAL CONSTANTS OF THE EYEDEFINITION: The optical constants of the eye are the indices of refraction, the radii

of cu~ature and the distances between the refractive surfaces of the eye.MEASUREMENTS: These optical constants of the eye have been determined

experimentally by several observers and following values are of Gullstrand's work:(1)The index of refraction of different refractive media of the eye: Is measured

by finding the critical angle using a special refractometer:-1) Cornea = 1.3762) Aqueous humour = 1.3363) Lens (Cortex-nucleus) = 1.386--1.4064) Vitreous humour = 1.336

(2)The radii of curvature of refracting surfaces of the eye (in mm): Aremeasured by the keratometer:-,)) Cornea, anterior surface = 7.7.2) Cornea, posterior surface = 6.83) Lens, anterior surface = 10.04) Lens, posterior surface = 6.05) Lens nucleus, anterior surface = 7.96) Lens nucleus, posterior surface = 5.8

NB:Crystalline lens cannot be considered as 2 spherical surfaces with a constantrefractive index as the cornea but it is made up of (Fig. 7.1):(1)Central biconvex nucleus: With a higher refractive index and a greater

curvature than the periphery.(2)Peripheral 2 menisci: Act as concave lenses as their curvature is less thall

that of the central nucleus.(3)The position of the refracting surfaces of the eye (in mm behind the

anterior corneal surface): Is measured by the pachometer:-2) Cornea, posterior surface = 0.53) Lens, anterior surface = 3.64) Lens, posterior surface = 7.25) Lens nucleus, anterior surf ace = 4.26) Lens nucleus, posterior surface = 6.6

NB: The measurements of the optical constants of the eye are used to calculate thecardinal points of the dioptric system of the eye.

GAUSS AND LISTING THEORY OF THICK LENSESGAUSS AND LISTING THEORY: It suggested the theoretical construction of the

cardinal points and planes· which are designed to permit the use of ray tracingprocedures in thick lenses:(l)At first, Gauss suggested the presence of two pairs of cardinal points (two

principal points and two focal points).(2) Then, Listing added another pair of cardinal points (two nodal points).

Fig.7.1:Structure of crystalline lens. Fig.7.2:Cardinal points of thick lens.APPLICATIONS OF GAUSS AND LISTING THEORY:

(1)A thick lens: With greater separation of the two refracting surfaces by the lenssubstance (Fig. 7.2):-

u-e (2)A compound homocentric system of lenses: With relatively long distancesbetween the lenses (Fig. 7.3).

(3)The dioptric system of the eye: With a number of coaxial spherical refractingsurfaces separated by relatively long distances (Fig. 7.4).NB:The thin lens formula and the formula of combination of two lenses are

inadequate to deal with the eye except in basic problems as these formulaeignore the lens thickness and the distance between the two lenses.

o :I__ 1,-.l-F,~

- f, ID

Fig.7.3:Cardinal points of a Fig.7.4:Cardinal points of eye dioptric systemcompound homocentric system. (Cornea C,aqueous A,lells L, Vitreous V alld retilla R).THE COMPOUND HOMOCENTRIC SYSTEM OF LENSES:

(1)Diagrammatic illustration of the system (Fig. 7.3):1)Cardinal points of the system,

1-Two prinCipal foci FI and F2t:(a)Anterior principal focus F I: Is the meeting of parallel rays emerging

from the system(b)Posterior principal focus F2: Is the meeting of parallel rays entering the

system.2- Two principal points (first principal point PI and second principal .

point P2):·Both PI and P2 correspond to conjugate foci of a simple lens

and are such that an incident ray passing through the first principal plane ofPI will pass through the second principal plane of P2 at the same verticaldistance from the optic axis( but the incident and emergent rays are notnecessarily parallel).

3- wo no al points (first nodal paint Nl and second nodal point N2):Both are the image of one another and are such that an incident ray passingthrough the first nodal point emerges through the second nodal pointparallel to its original course i.e. undeviated.NB:Whcn the medium on both sides of the thick lens is the samc:Tlre nodal DoillU.

coincide with principal points.2) Planes of the system:

1-Two ocal planes through Fl and F2:'(a) Anterior focal plan.@. s a perpendicular plane on the optic axis at Fl.(b) Posterior focal plane-: s a perpendicular plane on the optic axis at F2.

2-Two pri lcipal planes through PI and P2~(a) First principal lane: Is a perpendicular plane on the optic axis at Pl.(b) Secona pnnci- al lane: Is a perpendicular plane on the optic axis at P2.

3) Foca aistances of the system1-Xnterior ocal istance fl: Jts the interval which separates F I and P 1.2-Posterior focal distance f2: I~ the interval which separates F2 and P2.

4) onjugate distances of the system.I-Correlation between conjugate distances 11 and 12 and focal distances

f and f2): lll2 = fl f2 (Newoton' low).Where: 11= OFI (the distance between the object AO and the first

principal focus F).12= IF2 (the distance between the image IB and the second

principal focus F2).2-Correlation between conjugate distances (PI and P and foca)

istances (fl and f2): n I pI + 1'2I p2 = 1Where: PI = OP1 (the distance between the object AO and the first

principal point PI).P2 = IP2 (the distance between the image IB and the second

principal point P2).5) Magnification in conjugate planes referred to the focal points.

M = I / 0 So, M = 1'2112 = 11 I fl(2)Construction of the image formed by the system (Fig. 7.3):1f AO is an

object in front of the system far from Fl and perpendicular to the axis:l)Intersect two of the following rays: .

I-Ray AFID passing through Fl will be refracted parallel to the optic axis.

2- The second ray ACE is parallel to the optic axis and will emerge as EF2Bafter refraction to meet the first ray at B forming the image B of the pointA of the object.

3-The third ray AN passing through N will emerge through N as NB which isparallel to AN. I

2)Therefore: BI is the image of the object AO.THE DIOPTRIC SYSTEM OF THE EYE:

(1)The. dioptric system of the eye is a refracting system formed of a number ofco;axial spherical refracting surfaces separated by relatively long distances.

(2)The analysis of the dioptric system of the eye is based on Gauss and Listingtheory with its cardinal points (Fig. 7.4).

THE SCHEMATIC EYE:Defiliition: A scheinatic eye is a mathematical construct including the cardinal

points of the dioptric system of the eye in which the dimensions and opticalproperties approximate those of the living human eye (Fig. 7.5).

Ophthalmoptic properties:(1) The scnematic eye is useful in solving many of the problems 0

o hthalmoptics:I) Various schematic eyes have been suggested by such investigators as

Helmholtz, Gullstrand, Tschering, Listing and Donders.2)The schematic eye based on Gullstrand's work is probably the best

approximati on.(2) The schematic eyes differ slightly due to impelfections of the human eye as:

I) The refracting surfaces are slightly aspherical.2) The crystalline lens is usually decentred from the visual axis of the cornea.3) The crystalline lens consists of a non-homogeneous material.

(3) There are three major refracting interfaces to be considered in the eye.I) The anterior surface of the cornea.2)The anterior surface of the lens.3)The posterior surface of the lens.

NB:The effect of the Dostcdor corneal su~ace is verY small comnarea wnn (;-three interfaces:As the difference in the refractive index between comealstroma and aqueous is /lot large.

(4) The values of the cardinal points of the schematic eye. The cardinal points asdescribed by Gullstrand for the schematic eye (measured in mm behind theanterior corneal surface) are:I)First principal point PI = 1.35.2) Second principal point P2 = 1.6.3) First nodal point N 1= 7.4) Second nodal point N2 = 7.3.5) First focal po int F 1= -15.7.6) Second focal point F2 = 24.4.

(5) Refractive power. The calculated refractive (dioptric) power = +58.6 D.(6) The focal lengths measured from the principal points arC?:

l)Firstfocallength fl =-17.05 mm [-15.70+(-1.35)].2) Second focal length f2 = 23.05 mm [24.40 + (-1.35)].

Fig. 7.5:The schematic eye. Fig.7.6:The reduced eve.THE REDUCED EYE:

Definition: A reduced eye is a mathematical construct in which the eye is treatedas a single ideal curved refracting surface having:(1) one nodal point, one principal point and 2 focal points (Fig. 7.6).(2) An air medium in front (refractive index 1) and a water medium (aqueous

humour with refractive index 1.336) behind.Ophthalmoptic properties:

(1)GullustranCJ reduced eye·1) e va ues of the cardinal points of the Guliistrand reduced I

eye(measured in mm behind the anterior corneal surface):rI-Principal point P = 1.35.2-Nodal point N = 7.3':'First focal point Fl =- 15.7.4-Second focal point F2 = 24.1.

2)Radius of curvature of ideal spherical surface : 5.73 mm (7.08-1.35).3) Thefocal lengths (measureafrom the principal points) ar :

1- First focal length fl = -17.05~m. [-15.70 + (-1.35)].2- Second focal length f2 = 22.75 mm [24.10 + (-1.35)].

4)The 'dioptric power of the reduced eye (, ~:Is 58.6 D.5) The nodal point N lies:

1) 5.65 mm (7.00 - 1.35) from the ideal spherical surface.2) 17.1 mm (24.1'-7.0) from f2.

(2)Donders reduced eye Donders further simplified matters by using values forthe reduced eye which are useful in calculation of size a/images:1)Distance of the ideal spherical (curved) sUrface from the anterior surface

of the cornea = 2 mm.2) Radius of curvature f the ideal spherical surface = 5 mm.3) First (anterior) focallengthf= 15 mm.4) Second (posterior) focal lengthf= 20 111111.

5) Index 0 refraction of the idearsurface = 1.33.Uses of the reduced eye:The form of the reduced eye is used in all subsequent

discussions of ophthalmoptics to reconstIuct retinal image formed underVaDOUS conditions and to calculate its size. '

THE FORMATION OF RETINAL IMAGESCONSTRUCTION OF THE RETINAL IMAGES: Using the reduced eye it is

simple to construct the retinal image formed in ametropia (Fig. 7.7):-(1)The reduced eye itself is represented by two parallel lines which indicate

the principal point P and the retina R:1)These two parallel lines intersect principal axis (optical axis) at right angles.2)The nodal point (N) is indicated by a point behind the line P.3) The anterior principal focus F1 is indicated by a point in front of the line P.4) The posterior principal focus F2 falls on line R (retina in emmetropic eye).

(2)Two rays construct the image formed by parallel light incident upon eye:1)A ray passing through F 1 which after refraction at P, continues parallel to the

principal axis.2) A ray passing through N, undeviated.

Fig.7.7:Retinal image formation. Fig.7.8:Calculation of size of retinal image:lst method.CALCULATION OF THE SIZE OF THE RETINAL IMAGE: The size of the

retinal image can be calculated from construction of the retinal image by two raysoflight incident upon the eye, by one of the following methods:(1)First method:With one ray passing through F 1 and the other ray passing

through N (Fig. 7.8): '"1)The parallel light subtends angle 00 at the nodal point N as well as at the

anterior principal focus F1.2) Therefore the image size is directly related to the angle subtended by an

object at the nodal point which is called the visual angle:-I

Tan 00 =- So, 1=Tan co ~ nflWhere: I = The retinal image size.

00 = The visual angle.f1 = The first focal point.

3) As an object of a given size approaches the eye, it subtends a greater visualangle and thus appears larger (Fig. 7.9).

Ao1J U.

Fig.7.9:Visual angle of a near object. Fig.7.10:Calculation of size of retinal image:2nd method.(2)Second method:With both rays passing though N (Fig. 7.10):

1) ABN and CDN are similar:I V

As, 0 =U _So, I = 0 ~ V I U

Where: I = The retinal image size.0= The known size of the object.U =The measurable distance of the object from the eye.V = The distance between Nand F2 from the reduced eye.

2) The calculation of the retinal image size is important to measure the size ojthe diseased area of the retina I: Which corresponds to the scotoma 0 in thefield of vision.

REAL AND APPARENT POSITION OF IRIS AND SIZE OF PUPILAPPARENT POSITION AND SIZE: When the iris and the pupil are seen from a

less dense medium (air) while they are situated in a more dense medium (corneaand aqueous humour) we see:(1) The apparent position of the iris which appears nearer to the cornea with a

shallower anterior chamber.(2) The apparent size of the pupil which appears larger.

IRIS RELATION TO CORNEA:(1)The iris is placed (between P and F2): As an object placed behind a lens

between its principal point P and its second focal point F2 ..,(2)Therefore by turning a thin lens into a simple magnifying glass, it produces

a pupil image which is:I) Virtual. 2) Erect. 3) Magnified.4) On the same side between the second focal point F2 and the cornea.

CONSTRUCTION OF PUPIL IMAGE (Fig.7.11):(1)A ray from F2 joining Oland emerges fro~ cornea parallel to optical axis.(2)A ray from 01 parallel to the optical axis and comes to a focus at Fl.(3)The two rays are extended backwards to meet at II which is the image of 0 1.(4)A similar construction for 02 to get 12(5)Therefore 1Il2 is the image of the pupil 0102 which is between the pupil

and the cornea and is larger in size.

'ze ofJ1 the

Fig. 7.11: Real and apparent position of the iris and size of the pupil.CALCULATION OF THE POSITION OF THE IRIS AND THE SIZE OF THE

PUPIL (FIG. 7.11):(1)The position of the iris (the pupil):

nlr n?rl)fl= 2 l)andf2= 2- 1 (Chapter 4).n -n n -n

Where: nl (air) = 1, n2 (aqueous) = 1.336and:' r (of anterior surface of cornea) = 7.7So, fl = 1X. 7.7 / 1.336 - 1 = 22.91 mm and

f2 = 1.336 X. 7.7/1.336-1 = 30.61 mm .. So, fl of the cornea = 22.91 mm and

f2 of the cornea = 30.61 mm.fl f2 22.91 30.61

2)Pl + P2 = 1 So, PI + P2 = 13) P2 = Real distance of tt.\e iris from the anterior corneal surface = 3.5 mm and

22.91 30.61So, PI + 3.5 = 1 and So, PI =3 mm and P2 - PI =3.5 - 3= 0.5 mm.

Therefore the apparent position of the iris is 0.5 mm nearer to the cornea.(2)The size of the pupil:

I f2 II1)1112=flf2andM=- =- =-. 0 12 fl2)0 = real size of the pupil and equals 4 mm, f2= 30.61 mm and 12= f2 - P2 =

30.61 - 3.50 = 27.11 mm.I 30.61

So, 4 - 27.11 So, I = 4.5 mm and 1-0 = 4.5 - 4 = 0.5 mm.

Therefore the apparent size of the pupil is larger by 0.5 mm.

THE EMMETROPIC EYEDEFINITION: It is the condition of the eye in which parallel rays are focussed on

the retina without accommodation effort to form a circle of least diffusion.OPTICAL CONDITION:

(I)The far point of the emmetropic eye is at infinity and so rays from a point on theretina of an emmetropic eye come out of the eye parallel (in Fig.7.12, I is apoint 01' the retina R of emmetropic eye).

(2)Tl.erefore the retina of an emmetropic eye and infinity are conjugate foci.(3) Retina is comparable to the sensitive film of a box-camera and in Fig.7 .13, the

retinal image I II of object 001 formed on the retina R is a real, inverted anddiminished image as in the camera (Fig. 7.13), but the anterior and posteriorprincipalfoei are not the same (as the dioptric system of the eye has an airmedium in front and aqueous and vitreous medium behind).

fig.7.12:Emmetropic eye. Fig.7.13:Image on the retina orE eye(as box-camera).

THE DIOPTRIC SYSTEM OF THE EYE:(1)The dioptric power of the cornea:

1) he aioptric ower of the cornea= 40-45 D as the major part of the ocularrefraction takes place at the anterior surface of the cornea for two reasons:I-Its greater curvature.2-Big difference in the refractive index of air and cornea( 1.376 - 1 = 0.376).

2) Calculation:1-Power of the anterior surface of the cornea (D l):

n2 - nlDl= f1Where n1 (air) = 1, n2 (co1nea) = 1.376 and f1 (of anterior surface of

1.376 - 1cornea) = 7.7 mm. So, Dl = 7.7 = 48.83 D.

2- Power of the posterior surface of the cornea (D2):

\D2 = n2r~nl I

Where, n1 (cornea) = 1.376, n2 (aqueous) = 1.336 and r2 (of posterior1.336 - 1.376

surface of cornea) = 6.8 mm. So, D2 = 6.8 = -5.88 D.

3) Total power of the comea (D):(a)First method: ID = DI + D21 = 4iu"5J T (-5.<S<S) = 4L.Y5 u.

I n2 - n1 I(b)Second method: D = r

Where n1 (air) = 1, n2 (aqueous) . = 1.336and r (of anterior surface of cornea) = 7.7

1.336 - 1SO, D = 7.7 = 43.63 D.NB:Wben a swimmer opens his ~es under water:Hefinds Iris vision to be

blurred as the difference ill refractive index between water and cornea isonly 0.040 (1.376-1.336) and tllis problem is eliminated by wearing goggles.

(2)The dioptric power of the crystalline lens:1)Tlie dioptric power of the clystalline lens 16-20 D and the greater power of

the cornea than the lens is due to the greater difference in the refractive indexbetween air and cornea as compared to aqueous and vitreous and the lens:I-Difference in refractive index between air and Cornea = 1.376 - 1 = 0.376.2- Difference in refractive index between aqueous and vitreous (1.336) and

the lens (1.406) = (1.406 - 1.336) 0 2 = 0.14 as both anterior and posteriorlens surfaces contribute to the power of the crystalline lens.

2)A thin lens immersed in a refractive medium such as water (a an intraocularlens inside the eye) is treated as a combination of 2 spherical refractivesurfaces with the total power of the lens (D) equal to the sum of the powers ofthe 2 surfaces (D1 and D2): So, D = Dl + D2 = n2 - nl/ rl + n2 - nl/ r2

3)Calculation of the power of I OL of 20 D when measured in air:The power ofIOL of20 D is 63.35 D in air due to greater difference betweenthe refractive index of air and cornea (1.376-1) than between aqueous andvitreous(1.336-1.336): Dair / Daqueous = n IOL- n aiar / n IOL - n aqueous.

So, Dair=1.491-lI1.491-1.336D. So, IOL of20D = 63.35D.NB:Vision is blurred under water:Due to the little (Urrerevce between water and cornea

(1.336 and 1.376) titan between air and comea(l and 1.376).

ular~:

C PT 8ABERRATIONS (OPTICAL DEFECTS)

1 ABERRATIONS OF THE EYEOCULAR PHYSIOLOGICAL OPTICAL D.£FECTS)

DEFINITION: These are the physiological optical defects of the refracting system ofthe eye which theoretically lead to imperfect image formation, but there arecorrecting nl~chanisms built in the eye it self.

TYPES:(1)Chromatic aberration:

v C aracter: The total dispersion from the red to the blue image is about 1.5 D inthe emmetropic eye which is (Fig. 8.1):l)Focussed for the yellow portion of the spectrum.2) Hypermetropic for the red portion (+0.5 D).3) Myopic for the blue portion (-1.00 D).eutra ization:1)Small pupil size.2) The eye makes a sharp image from the yellow portion and neglects images

from the red and the blue portions of the spectrum.-- 3) Chromatic difference of magnification.Clinica application~. 1) Duochrome test (Chapter 16).

2) Cobalt blue glass (Chapter 16).orrection: Achromatic lens (combination of a convex lens of high refractivepower andlow dispersive power with a concave lens of low refractive powerbut higher dispersive power)NB:The earliest achromatic lenses were made bv combinin!! a convex iens 01 qQY.

glass and a concave lens of flint glass.

W__.hlt=8 ==:~.t t t•• C "0

::J ••••m~a:r0;

~Foci

ParaxIal {zone

Fig. 8.1:Chromatic aberration. Fig.8.2:Spherical aberration.(2)Chromatc difference of magnification:

1)When an object is little to the side of the optic axis, the image produced at thefovea(which is to one side of the optic axis) by shorter wavelengths(blue) issmaller than that produced by longer wavelengths(red) .

2)Chromatic difference of magnification neutralizes chromatic abelTation to a

large extent and so correction is NOT needed.(3)Spherical aberration:

Character: The rays passing tlu'ough the periphery of the crystalline lens aredeviated more and come to a focus earlier than those passing through theparaxial zone of the lens periphery of the lens because the periphery of thelens has a higher refractive power (higher prismatic effect) than the paraxialzone of the lens (Fig. 8.2).etection: pin is brought closer to the eye till doubled7then a card with a holeis held in-between the pin and the eye->the pin becomes clear and notdoubled.

eutrallzation:l)the flatter corneal periphery acts as an aspherical (aplanatic) surface and is

of value when the pupil is dilated.2) Small pupil size.3)The nucleus of the lens has a higher refractive power (due to greater

density with higher refractive index and layers of greater curvature) thanthe lens periphery.

4) The retinal cones are more sensitive to light that enters the eye paraxiallythan to light that enters the eye obliquely through the peripheral cornea(this is called Stiles-Crawford effect; Chapter 13).

Nll:Night myopia is due to:(1)Spherical aberration witlt dilated PI~OiJ(2)fncreased sensitivity to blue ill dark adapted relilla.

CorreJ !ioll:l)Occlusion of the periphery of the lens: By the use of "Stops".2)The lens form adjustment: For example a Plano-convex lS better than

biconvex lens.3)Aplanatic (aspherical) lenses: In which the surfaces are aplanatic i.e. the

peripheral curvature is less than the central CUfvatur (Fig. 8.3).

Fig.8.3:Aplanfitic(aspbcrical)lcllS. Fig.8.4:Asphcric doblet.4)A doublet: I-It consists of a principal lens and a somewhat w~aker lens of

different refractive index cemented together (Fig. 8.4).2-The weaker lens must be of opposite power and its spherical

aberration will reduce the power of the periphery of theprincipal lens more than the central zone.

5)Aplanatic achromatic lenses:A doublet is usually designed to be bothaplanatic and achromatic to correct both spherical and chromatic aberrations.

aplanatic and achromatic to correct both spherical and chromatic aberrations.(4) Peripheral aberrations: These are aberrations caused by incidence or

emergence of light away from the optic axis of the eye(i.e. all are off-axis (1)aberrations and not axial or paraxial) as the image formed at the peripheral (2)retina is less defined than that fonned at the central retina:- (3)1)0 Iique astigmatism: (4)

Character: It occurs when rays of light traverse the refracting surfaces of theeye obliquely.

eutralizatio :I-The aspherical (aplanatic) curvature of the cornea.2-The retina is a spherical and not a plane surface.3-The image due to oblique astigmatism falls on the periphery of the

the retina which has a poor resolving power compared to the macula.Correcfion

I-Orientation of the lens: So iricident light is parallel to the principal axis.2-Restriction bfthe lens aperture:Limiting rays to the axial area of the lens.3-The lens fOlm adjustment: For example meniscus lenses are better than

biconvex and biconcave lenses (Chapter 17).2)Coma Aberration.

Character1-It is really a spherical aberration applied to light coming from points notlying on the optic axis of the eye.2- It results in unequal magnification of image formed by different zones ofthe lens-7elongated image like a comma.3-Its neutralization is as in oblique astigmatism.

Correction1-Limiting rays to the axial area of the lens: As in oblique astigmatism.2-Using the principal axis of the lens: Rather than a subsidiary axis

3) mage aistonion- The edges of an object seen through the periphery of thelens are distorted due to increased prismatic effect and its neutralization is asin spherical aberration and oblique astigmatism.

(5)Curvature of the field: A plane object gives rise to a curved image and thecurvature of the field is neutralized by the curvature of the retina.

(6) Diffraction: The diffraction of light passing through the converging crystallinelens is more serious with smaller pupil than 2.8 mm(Chapter 2).

(7) Decentring:1)The centring of the refracting surfaces of the eye is never exact but the

deviations from normal are functionally negligible.2) Therefore, there is decentring of the ocular media as follows:

1-The centre of curvature of the cornea is 0.25 mm below the axis of the lens.2- The fovea is 1.25 mm down and lateral to the optic axis.

(2) ABERRATIONS OF OPTICAL SYSTEMS(1) Chromatic aberration:See above.(2) Chromatic difference in magnification: See above.(3) Spherical aberration: See above.(4) Peripheral aberrations:

1) Oblique astigmatism: See above.2) Coma aberration: See above.3) Image distortion:

Pln-cushiori" ]. distortion ." !

!r•............ ,• I· .· ., :· - .· .: !•.••• • •• 1

(a)Plalle glass. (b)Concave lens. (c)Convex lens.••• Fig.8.5:Image distortion by lenses.

1-Image distortion ue to peripheral magnification: traight lines appearcurved except when viewed through a very small axial zone of the lens andthe image distortion will be either:(a) egative barrel distortion: By a high concave lens (Fig. 8.5b).(b)Positive pin-cushion distortion: By a high convex lens as in aphakic

spectacle lenses 8.5c).NB: The linear environment thus appears to be curved with a change in the

shape as the patient m'oves his eyes and looliS through different zones of hislenses7this" effect is unavoidable but the patient can decrease imagedistortion by:(l)Restrictitlg his gaze to the axial zone of the lens.

(2)Moving his head rather than his eyes to look around.(3)The use of lenticular lenses.

2-Distartion effe.l:ts of cylinders: Rectangles appear as rhomboids due to arotatory deviation in which linear objects appear to slant and flat surfacesto slope especially' when the visual axes are not parallel( so it is NOT acylindrical aberration).

3-Prisma ic effect of the periphery of glasses (Fig. 6.12): It is common withhigh errors as aphakia and high myopia and produces:,(a) ing scotoma all around the edge of the lens Which causes patients to

trip over unseen obstructions in their path.(b)Jack-in-the-box phenomenon: bjects appear out of the ring scotoma or

disappear into it when the patient moves his eyes with a change in thedirection of the scotoma.

(5) Curvature of the field: The effect depends upon RI of the lens surfaces.(6) Diffraction: Due to spread of light when its wave front encounters the edge of

obstruction( Chapte2).

(7) Decentring: (Chapter 17).(8) Spectacle lens aberration due to ocular rotation to see hlteral objects: Will

lead to a blurred image as the eyes are not fixed to the direction of the opticalcentre of lenses and is corrected by:-1)Head movement: To keep eyes fixed to direction of optical centre of lenses.2) Lens bending: By varying the radii of the two surfaces to get a sharp image.

(9)Other optical defects of bifocals:Chapter 17.DEFI:r-ITTIONS OF TERMS OF REFRACTING SYSTEM OF EYE:

The optic axis: It is the line upon which the various refracting surfaces of the eyeHe centred (ANB in Fig. 8.6).

The visual line:(1)Line joining the fovea, nodal point and object of regard (ONM in Fig. 8.6).(2)It is 5° nasal to and 4° above the optic axis at the cornea.

The nodal point(N in Fig.8.6):(I)It lies upon the optic axis and is placed 7 mm behind the apex of the cornea.(2) It is the point of intersection of the optic axis and the visual line.(3)So all light rays entering the eye pass through it.(4) It corresponds to the centre of a thin lens and lies near the posterior pole of

the crystalline lens.(5)A small paraxial pencil oflight enters the pupil and is refracted through the

, nodal point(so a small posterior cortical cataract produces grossimpairment of vision when the pupil is small).

The fixation axis: It the line joining the centre of rotation of the eye and theobject ofregard(OC in Fig.8.7).

The centre of rotation of the eye:(I)It is the point along which the eye rotates.(2)It lies on the optic axis and is 12-13 mm behind the apex of the cornea in an

emmetropic eye (C in Fig. 8.7).(3)It coincides with geometrical centre of curvature of posterior part of globe.

The central pupillary line:(l)A norn1al to cornea passing through centre of pupil (APNB in Fig.8.8).(2)It cuts the optic axis at the centre of curvature of the cornea i.e. 8 mm behind

the apex of the cornea.(3)It cuts the cornea slightly to the nasal side of the optic axis.

Angle alpha(Fig. 8.6):(l)Dejfnition:lt is t e angle Between optic axis and visual line at the nodal point.

NBI :The visual angle is the angle of the two extremities of the object at the nodalpoint:So visual angle differsfrom angle alpha (Chapter 13 & Fig. 13.1).

MB2:Distancc betwecn the fovca and the optic axis = 1.25 mm in an emmctropic eye.(2)Measurement of angle alpha ~ ig. 8.9):

MN = Distance between the retina and nodal point = 15 mm from reduced eye.

0__

. l' I"'h MB" 1.25 S· O(fr bl')SO, Sm ang e a p a = MN = li = In 5'. om ta es.

Therefore, angle alpha = 5 ° in an emmetropic eye.(3)Importance 0[ an Ie alph!:L

(1)AbnOlmal angle alpha occurs in ametropia ~nd leads to pseudo-strabismus:.1)Angle alpha of morethan 5°: In hypermetropia and leads to pseudo-

exotropia.2)Angle alpha of less than 5 ° or even-ve angle:In myopia and leads to

pseudo-esotropia.(2)lii positive angle a pha·Nisual axis cuts cornea on nasal side of optic axis.(3)In negative ang e alpha: The visual axis cuts the cornea on the temporal

side of the optic axis because the posterior pole of the eye becomes on thetemporal side' of the macula due to changes in the posterior portion of theocular coats in high myopia.

Fig. 8.6: Angle alpha. Fig. 8.7: Angle gamma~ Fig. 8.8: Angle Kappa .. Angle gamma:Angle between the optic axis and the fixation axis (In Fig. 8.7 with

the optic axis ANB, fixation axis OC, centre of rotation C and nodal point N).

g. Fig.8.9:Measurement of angle Alpha. Fig.8.10:Measurement of angle Kappa by synoptophore.Angle kappa:

d (l)Definztion:;Angle between the central pupillary line and visual line(In Fig. 8.8with the central pupillary line APNB, centre of the pupil P,visual line APNBand nodal point N).

(2) easurement of angle kallpq:(I)By the synoptophore (Fig. 8.10):

1)A slide with a row of numbers and letters at one degree intervals (e.g. E DI. C B A 0 1 23 45) is placed in front of the patient's eye who looks to 0:

1-If the corneal reflex is seen nasal t<;>the pupil, the angle kappa is+ve.2- If the corneal reflex is seen temporal to the pupil, the angle kappa is-ve.

2)So, the angle is ACPwhich is a little greater than angle kappa, ANP.

(2)By the perimeter:Obsolete nowadays.SIGNIFICANCES OF ABERRATIONS OF THE EYE:

(1)Circlesof diffusion: D1) The combine effect of the different aberrations of the eye:

1-No clear image is formed as a point focus on the retina, but as a circle oflight which causes slight blurring of the image called a circle of diffusion. T

2- The image of the line appears as a broad band and each point of the line isconsidered as a circle (Fig. 8.11).

2) The circles oJ/east aiffusion: he smaller the circles of diffusion, the greaterthe visual acuity and so in emmetropia and in corrected ametropia we obtainthe circles of least diffusion.

(a)Line. (b)Broad band. (c)Circles of diffusion.Fig. 8.11: Circle of diffusion: Fig. 8.12:Effect of size of pupil.

(2)Small pupil size (Stenopaeic aperture, Fig.8.~2):Demonstration' )1)InFig.8.12, the object 0 forms an image at F with dirninished diffusion

circles on the retina R and a smaller pupil.2)Beam of light within eye is cone shaped,with its base at pupillary aperture.3)Smaller pupillary aperture~smaller section of the cone with a clear image.

Aavqntages:I)A small pupil size reduces the aberrations of the eye caused by the

periphery of the lens: I-Peripheral and spherical aberrations.2-Diffraction(except with small pupil than 2.8 mm).

2)A small pupil size leads to a more clear image due to stimulation of lessnumber of cones (which is of advantage in refractive errors, where the apexof the light cone does not fall on the retina).

Clinic applications: )Stenopaeic slit test for visual acuity.2)Ametropes prefer bright light, which leads to miosis.3)Frowning in myopes(half-close their eyes to see better).

AMETROPIA (Ji::RRORS Ol? REFRACTION)(PATHOLOGICAL OCULAR DEFECTS)

DEFINITION: A condition in which the eye fails to bring parallel light to a focus onthe retina without accommodation (i.e. the second principal focus F2 of the eyedoes not fall on the retina).

TYPES:1) Main types: 1) Spherical errors: 1- Hyperme,tropia.

2- Myopia.2) Noizspherical error: Astigmatism.

(2) Acces ory types: 1) Aphakia.2) Anisometropia.3) Aniseikonia.

(l)HYPEHM I:TROPIA (1I)DEFINITION: A condition of the eye in which parallel 'light comes to a focus

behind the retina without accommodation (i.e. the second principal [OCllS F2 ofthe eye lies behind the retina, Fig. 9.1).

Fit;.9.1:Hypermct opin. Itig.9.2:Far point in hypermetropia.AETIOLOGY:

(1)Axial hypermetropia: Due to d(",creased.'ntero-posterior diameter of the eye.N131:Hypermctropia is the commonest I"cfrnctivc cr 'or in infants,NB2:Axial hypermetropia is the commonest rCfl':lctive cr OJ' in 'ufmlts nLo.

(2) Hefractive hypermetropia: Due to decreased refracti Ie power of the eye:-1) Cw-vature hypermetropia: Due to decreased curvature of the ant rial' surface

of the cornea or of the lens, as in cornea plana.2) lnaex h)!permetropia: Due to decreased refl'active index of the whole lens (in

old age and in diabetics un er treatment) or increased refractive index of thelens cortex ( in senile incipient and immature cortical cataract).

(3) Posterior dislocation of the lens: Usually after it"\iuries.(4)Aphakia: Chapter 9.

OPI'leAL PRINCIPLES:Optical condition:

(l) Parallel light comes to a focus behind the retina to form a circle of leastdiffusion, a d so a larger circle of diffusion occurs on the retina (Fig. 9.1).

(2)Rays emerging from a point on retina will emerge divergent, so that they willappear to meet behind retina at the far point, i.e the image is.virual (Fig. 9.2).

Varieties of hypermetropia:(1) Total hypermetropia (Ht, hypermetropia with atropine): It is measured by

the strongest convex lens which gives the maximal visual acuity underatropine.

(2) Manifest hypermetropia (JIm, hypermetropia without atropine): 11 ismeasured by the strongest convex lens which gives the maximal visual acuitywithout atropine, and is increased with age.NB: Ht becomes Hm in old age due to loss of accommodation.

(3)Latent hypermetropia (HI, the tone of the ciliary muscle): It is measured asthe difference between total and manifest hypermetropia (HI = Ht - Hm) ,and it equals 0.5-1.0 D.

(4)Facultative hypermetropia (HI, the part corrected by accommodation): It ismeasured as the difference between the manifest and the absolutehypermetropia (Hf= Hm - Ha).

(5)Absolute hypermetropia (Ha, the part not corrected by accommodation): It ismeasured by the weakest convex lens which gives the maximal visual acuitywithout atropine.

NB1: Ht = Hm + HI (Ht - Hm ).NB2: Hm = Ha + Hf (Hm -Ha).

Change of hypermetropia with age:(1) ~n chi! ren: J!\ll hypermetropia is facultative.(2).As tFzein ividual becomes olden:Accommodation becomes weaker and so

part of hypermetropia becomes absolute.(3)1n old age: ypermetropia becomes absolute with defective vision for

distance ..Angle alpha in hypermetropia: > 5°-7pseudo-exotropia(Chapter 11).Accommodation in hypermetropia: A hypennetrope cannot see clearly for far

or for near except with accommodation (Chapter 10) :(1) Fa far: e accommodates equal to his error.(2) Ern : he part he accommodates for far is added.

Relation between accommodation and convergence in hypermetropia:Accommodation is in excess of convergence in hypermetropia which leads totrue esotropia (Chapter 11).

PRINCIPLES OF OPTICAL CORRECTION OF HYPERMETROPIA:(1)A hypermetropic eye is focussed for convergent rays:

I)A convex lens is used with its focal point coinciding with far point of H eye.2)This will deviate parallel incident light,to be converging towards the far point.3)So, the light will then be brought to afocus by the eye on the retina (Fig. 9.3).

(2)The effective power of the lens is calculated as follows:1)If the correcting lens is,at the principal plane:

1 1 (. ..)D = - =- r ISposItlve. f r .

"'!here: D = The power of the lens in dioptres. _f = The [ocal1ength of the lens in metres.r = The distance of the far point from the principal plane in metres.

The reciprocal of r in metres, is symbolized by R, expressed in dioptres (R isknown as the ametropic error or the static refraction).

The value of r: is negative ill frollt of the principal plane(ill M) amI positive behiudtlte principal plalle(ill ll).

~

.~~ 1/\......_--- ..•............._ ff'-..- ~------,

t (

(a) Uncorrected. (b) Corrected.Fig. 9.3:Correction of hypermetropia.

2) he co/'reclin ) lens is wle! ill spectacles at 1 I1l1n in frollt of 1eprmcipalfJ (me oflle ey :

Ds = 6(d is negative) IWhere: f = The focallengtb of the lens in metres at the principal plane.

d = The distance of spectacle lens from the principal plane- (13 111111).

Ds = The power of the lens required at the spectacle plane.NH:Thc value of d: (l)ls positive (so D is increaseu) if the lell!f is moved towUI"ds

the cyc(as in contact lenses).(2)ls negative (so n is decreased) if moved away from the eye

. (as in spcctncles).Example: A hypermetrope of +4D sphere:

1000f=:: -4- = 250 111111 and d:::: -13 n 111.

1000-263 = +3.8 D spher

Therefore the pOl,vel' of the correcting COJ1V -'x lens ill h)'lJermetropia held atthe spectacle p silion (J 3 nlln ./;·om the eye) is less than the actualreji'({ctive error o.lthe eye.

3) if tlie original corre ting lens power is to be c/tanged to a contact lens pOl\1er:

Ds (i' ..)Dc = 1 _ dDs ( IS posItIve

Where: bc = The power of the contacllcns in dioptres.Ds = The. :~ of the original lens at the spectacle plane in dioptres.d = The distance moved in metres.

E ample: A hype metrope of +8 D specta les at 13 mm distance in front ofthe principal plane of the eye, needs a contact lens:

SO. Dc = +8 / 1- 0.0 13 X, -8) 8.93 D Sphere.

1 1000Ds:::: f - cI = 250 - (- I 3) ::::

(3) Refractive surgery: For high hypermetropia especially hypermetropia due toaphakia(Chapter 32).

(2) MYOPIADEFINITION:A condition in which parallel light comes to a focus in front of the

retina without accommodation (i.e. the second principal focus F2 of the eye lies infront of the retina, Fig. 9.4).

AETIOLOGY:(1)Axial myopia: Oue to an increase in antero-posterior diameter of the eye and its

types are: I-Simple. P, 2-Progressive.3-Congenital.

.'1B:Progressive myopia is characterised by :(1) Genitic in origin.. (2)Common ill Japanesse.

(3)Star from childhood.(4)Progresses to more than 20 D.(5)Retinal degenerative changes in old age.

(2)Refru~tive myopia:Due to decreased refractive power of the eye-7 types:J) Curvature myopi : Due to increased curvature of the anterior surface of the

cornea or of the lens, as in keratoconus and in lenticonus.2) Index myopia Due to increased refractive index of the lens nucleus (in senile

nuclear cataract) or decreased refractive index of the lens cortex ( in diabeticmyopia due to uncontrolled diabetes).

(3)Anterior dislocation of the lens: Usually after injuries.

"~r~•

P A

=====p+.Fi~.9.4:Myopia. Fig.9.5:Far point in myopia.

OPTICAL PRINCIPLES:Optical condition:

(1 )Parallel light comes to a focus in front of the retina to form a circle of leastdiffusion, and so a large circle of diffusion occurs on the retina (Fig. 9.4).

(2)Rays emerging from a point on the retina will emerge convergent, so thatthey will meet in front of the eye at the far point (Fig. 9.5).

Varieties of myopia:(1) True myopia Axial and refractive myopia.(2) False myopia: Due to ciliary muscle contraction with exceSSive

accommodation as in diamox therapy.Angle alpha in myopia: The angle alpha is less than 5° and may be negative which

leads to pseudo-esotropia (apparent convergent squint, Chapter 11).

Accommoda ion in myopia:(I)A myope cannot see clearly for far as his far point is.in front of his eye(Le. at

a finite distance).(2)Therefore, accommodation cannot help a myope but increases the el~ror

(Chapter 1.0).Relation between accommodation a ld convergence In myopia: Convergence is

in excess of accommodation in myopia which leads to true exotropia (truedivergent squint, Chapter 11.).

PRINCIPLES OF OPTICAL CORRECTlON OF MYOPIA:(1)A myopic eye is focussed for divergent rays:

1.)So we use a concave lens with its focal point coinciding will the far point oft.hemyopic eye.

2) This lens will deviate parallel incident light, so that it appears to be divergentfrom the fa' point in myopia.

3)So, the light will then be brought to a focus by the eye on the retina (Fig.9.6).

. p' A \

~-~--r •.

-L~~P rA

Ff' I ••- ••

-"'·c::::------. --- _.;,

(a) Uncorrected. (b) Corrected.l?ig.9.6: Corn~ ·tion or myop· ••

(2)Effective power of the lens is calculated as foil ws:1)1 tite co,.rectin. hms is at the principal pl,m :r'~~'-~~ . « -'-'~l~ = +- = '~(l'is I e~.ative)J

2) the correcting lens is h "ld in pe 'facles t I 111m distance in,li'ont of th 'principal plane a/the eye:

~s - f _~Cd is_negativ~

1000Example: A myope of - 4 D spher ,: f= --4 - = ,·250 mm and d::-: -13 mm.

I 1SO, Ds = -250 _ (-13) = -237 0:: -4.25 D sphere.

Therefore, the power olthe correcting concave lens in myopiaheld at the spectacle position (13 mmjh)m the eye) is more thanthe actual refractive error of the eye.

3) If the original correcting lens power is to be changed to a contact lens power:

DDc = 1 _ ;D

S(d is positive)

Example: A myope of - 10 D spectacles at 13 mm distance in front of theprincipal plane of the eye, needs a contact lens of:

Dc = -10/ 1-(+0.013 x: -10)= -8.85 D sphere.(3) Refractive surgery: Chapter (32).

(3) ASTIGMATISMDEFINITION: The refractive power varies in different meridians and so no point

focus is formed.TYPES: (1) Regular astigmatism.

(2) Irregular astigmatism.1) REGULAR ASTIGMATISM

DEFINITION: A condition in which the major and the minor meridians are at rightangles and the change from one meridian to the other is gradual and regular (theimage is formed as a Sturm's conoid,Fig. 9.7).

AETIOLOGY:(1)Corneal curvature astigmatism: 1) Congenital.

2) Postoperative as after cataract extraction.(2) Lenticular astigmatism: Rare.(3)Obliquity of the eye elements(rare ):

1) Obliquity of the lens: Y\s in subluxated lens.2) Obliquity of the retina: As in high myopic posterior staphyloma.

OPTICAL PRINCIPLES:Rule of regular astigmatism:

(1)Astigma tism with the rule (90%). Vertical meridian is more curved than thehorizontal meridian. '

(2)Astigmatism against the rule (10%): Vertical meridian is less curved thanhorizontal meridian.

Types of regular astigmatism:(1)St~aight astigmatism: The major and the mi'nor meridians are one horizontal

and one vertical and both are at right angles.(2)Oblique astigmatism: The major and the minor meridians are oblique but are

at right angles.NB: Bi-oblique astigmatism: In wlllc}, the major and the minor meridians are obU,rte.

and are not at right angles but with regular gradual changes in-between.Symmetry of regular astigmatism:

(1)S)7mmetrical astigmatism: n which there is a symmetrical position ofdeviation of the principal meridians from the median line with their axes ofthe same sign and together equal 1800

•

(2)Asymmetrical astigmatism: There is no symmetry in the relationship of theprincipal meridians to.the rl1edian line.NBl:Asymmetrical astigmatism is less common than symmetrical astigmatism.

NB1:Torticollis: occurs in asymmetrical more thall in symmetrical astigmatism ifullcorrected.

Classification of regular astigmatism:(1) imple astigmatis : One meridian is emmetropic while the other at right

angles is ametropic:1) Simple m 0 ic astigmatism: 1fhe rays in one principal meridian focus on

the retina while those in the other focus in front of the retina.2) Simple hypermetropic asti matism The rays in one principal meridian

focus on the retina while those in the other focus behind the retina.(2) Compound astigmatism The two principal meridians are either myopic or

hypennetropic:1Compound ,myo ic astigmatism~' Rays in all meridians come to a focus in

front of the retina. ' '2)Compound hypermetropic astigmatism:Raors in all meridians come to a

focus behind the retina.(3) }'1ixed astigmatism: One principal meridian is myopic and the other is

hypermetropic i.e. the rays in one meridian come to a focus in front of theretina while those in the other come to a focus behind the retina(llieads to adecreased visual acuity less than in compound astigmatism).

Accommodation in regular astigrrlatism:(1) 11 compoun hypermetropic astigmatism Accommodation corrects the less

hypermetropic meridian.(2)In other types q astigmatism: ccommodation decreases the hypermetropic

meridian, changes the emmetropic to a myopic meridian and increases themyopic meridian.

Optical c ndiUon of regular astigmatism:oca lines The eYf~ has two foci (two focal lines) in regular astigmatism andso the retil a hu'" two c njugate foci and no clear image will be fonned on it( hus astig1l1a ism cannot be corrected by a spherical lens alone).

Re J'activn b I an ([sli 7matic corneal sur ace (Fi . 9.7):(l)C rucfon:"

V = Less curved vertical meridian orthe cornea,H = More curved horizontal meridian of cornea.FV = Line focus formed by V.FH = Litle foc,us formed by H.FVFH = Interval of Stunn (Focal interval) = Difference in the refractive

power of the two meridians (Measure of astigmatism).(2)The eye las two oci F1Vand FH and so a line focus or a i usion:

ircle wil be formed with 7 possible positions on retina as fof ows~1)Compound hypermetropic astigmatism.2)Simple hypermetropic astigmatism (V hypermetropic).3)Mixed astigmatism (V hypermetropic and H myopic).

4)Mixed astigmatism with the circle of the least diffusion, where thedivergent rays leaving Fy meet the convergent rays going to FH (theleast amount of distortion occurs because the two opposing tendenciesare equal and opposite).

5)Mixed astigmatism.6)Simple myopic astigmatism.7)Compound myopic astigmatism.

(3) The circle of least diffusion: Occurs in the plane where the two pencils oflight from the vertical meridian and from the horizontal meridian intersect.

(4)Blur circle imagesc At all other planes lying between Fy and FH.(5)Sturm's conoid It is the figure fornled by the whole bundle of light rays

between the two line foci Fy and FH.

I IF FI(._

OPT(1)(2)

Fig.9.7:Refraction by astigmatic surface:7 image positiolls relative to the retilla. (3)The clinical applications of Sturm's conoid: (4)

(1)The oblique astigmatism of optical systems such as: (5)1) Spectacle lenses. (6)2) Ophthalmoptic instruments.3) Refracting system of the eye.

(2)The image seen by an astigmatic corneal surface.(3) The fogging method of ~efraction (Chapter 16).(4)Thecross cylinder (Chapter 16).(5)The spherical equivalent of spectacle prescriptions (Chapter 17)

PRINCIPLES OF OPTICAL CORRECTION OF REGULAR. ,

ASTIGMATISM: It is corrected ifit leads to eye strain:(1) Eye glasses:

1) Cylindncal lenses: In simple astigmatism (the axis of the lens is placed atright angle to the meridian to be corrected).

2) Sphero-cylindricallenses: In compound and mixed astigmatism as follows:1-Astigmatism is changed to simple astigmatism by giving the suitable

spherical lens.2- A cylindrical lens is added to correct lhe simple astigmatism.

(2) Contact lenses: 1) Hard contact lenses.2) Rigid gas permeable contact lenses.

(3) Refractive surgery: Chapter (32).

2) IRREGULAR ASTIGMATISMDEFINITION: It is the type of astigmatism in which all meridians are not alike and

the m~jor and I~1ino"r11le'ridians are not at right angles.AETIOLOGY: (l) Corneal opacities.

(2) Keratoconus.DIAGNOSIS: Placido disc or keratoscope (Chapter 16).CORRECTION:

(1) Corneal astigmatism: 1) Contact lenses: For early cases.2) Keratoplasty: For late cases.

(2) Lenticular astigmatism: Len~ extraction.4) APHAKIA

DEFINITION: It is the absen.ce or removal of the crystalline lens from the eye.AETIOLOGY:

(1) Congenital; Very rare.(2) Acquired: 1 )Af1er cataract operations.

2)After ocular i"njury (with absorption" or injured lens in children).3 )After lens extraction in high myopia.

"OPTICAL PROBLEMS'OF APHAKiA:(1) Hypermetropia: Of marked degree.(2)Astigmatism against the rule: In postoperative aphakia (up to +2 D cylinder

axis 1"80°).(3)Anisometropia: In unilateral cases (Chapter 9).(4)Aniseikonia: In unilateral cases (Chapter 9).(5)Accornmodation Is abolished. "(6) rob ems of spectac e correction in aphakia:

1) Speclaclemagn{!lcal[on q Image IJ1aphakia.:1- Ie re su y w r mm n ro

aphakic relativ spectacle magnification ISapproximaLly L33. "

2- ec an os a e ant rl r 0 a pomt 0 ~produces a n~Jati"e spc~f:acle magnification becakuhlted as foHm 'S (Fig. 9.8a):Emmetropic anterior focal length fe == 17.05 mm.Aphakic anterior focal length fa == 23.23 i1ll11.

AS = DE := emmetropic image size.AC := DG == Corrected aphakic image size.

" AC' DC' I' ""'3 "'"'. In _ ._-,Thei'ef()/'e, RSM = AS =: DE =: te = 17.05 = 1.36

3- lierefore, he image pro u e in the cur ec ell Ul I' 'ye is OIIl'

third larger than the image formed in a~! emmetropic eye which willlead to altered depth perception:

(a) Magnification causes the patient to misjudg~ distances (so objectsappear to be closer to the eye than they really are because of increasedvisual anglesubtended at the eye).

(b)Difficulty with hand-eye coordination.NB: The image magnification results in enhanced performance of standan~

tests of yisualactiity: For' example, a level 6/9 for all aphakic spectacleIvearer is equJvalellt to 6/12 for an einmetropic eye.

4- he relaf e stacie ma nification is ed,.rv(a)A contact leDs which reduces the RSM to 1.1 (Fig. 9.8b).v{b )An intraocular lens which reduces the RSM to 1.0.

," E, ------0c

________Mo"

(a) With spectacles at the anterior focal poillt. (b) With cOlltact lells.Fig.9.8:Relative spectacle magnification (RSM) in corrected aphakia.

2) Spectacle. lens aberrations: Especially pin-cushion distortion and nng.scotoma with Jack-in-the-box phenomenon (Chapter 8).

3) The roblems of the lens material and designs:1- Keavy high powered glass h~nses:l[hese lenses cause the spectacles to slip

down the patient's nose, thus altering the effective power of the lenses,and are uncomfortable to wear.

2- Plastic lenses: Are lighter but tend to scratch.3- Lenticular lenses: Lead to a more reduced field of vision (Chapter 17).4- Minor misadjustment in the vertex distance, pantoscopic tilt or eye height:

Is not tolerated.NB:lncorrect priscription may be due to incorrect vertex distance.

4)Limited Vlsua jiefd With glasses. .5)No Binocular vision: ith glasses due to anisometropia and aniseikonia.6) Cosmetic roblems: -The eyes of the patient appear magnified.

2-Eyes may be grossly displaced when viewed obliquely.3-High powered lens with unattractive appearance.

OPTICAL PRINCIPLES AND CORRECTION OF APHAKfA:(1)The optical system of tile aphakic eye consists of:

1)The cornea.2)Air in front of the cornea.3)Aqueous and vitreous behind the cornea.

(2) The anterior and posterior principal foci of the cornea (f1 and f2) in theaphakic eye are calcu.lated as follows (Fig. 9.9):

C nil" n2rL~ n2 - 111 llnd 12 = 112_ nl and 1'= 7.7 mm, nl = I and n2 = 1.336

50,[1 = 22.91 mm in front of the cornea and f2 = 30.61 mm behind the cornea.NB:Expcrimcntally:JI oJllte aplwkic eye = 23.23/11111 alldJ2 =31.23111",.

(3)Ttle dioptric power of tile aphakic eye:1) The cornea is the only refracting system reli,aining in the aphakic eye and the

anterior principal foclls of the aphclkic e·ye is 22.91 mm.1000

2) Thus the dioptric power of the aphakic eye equals 22.91 = about 43.64 D.(4)The refractive state of the aphakic eye:

1) The aphakic eye which was previoLlsly emmetropic becomes highlyhypermetropic with the far point behind the aphakic eye (which is focussedfor convergent rays).

2) The aphakic eye which was previously highly myopic with spectaclecorrection of -18 0 to -20 0 is rendered emmetropic after extraction or the·crystalline lens.

(5)The power of the correcting convex lens is calculated as follows (Fig. 9.9):I) U ·in,.' Newlon's lall' (1112 =fIf2):

1]12 = 22.9] X, 30.61 and II = /2 -length oUhe eye = 30.61 - 23 = 7.61lllff\ .

So~ 12= 92.15 mm.Theref re, the fa point is 92.15 mm from the Clnlcrioi: principal locus F I or.the aphakic eye.

]) There.fiJl'l! the correcting lens with a jhcal length 92. /5 111111 jJlaced at theanterior principal focus FI of the aphakic eye: 'Will cause parallel rays tocOll/erge and come to a focus on the 'etina, ani the p ~ vel' or the correctinglens placed at F 1 =.: 1000/92.15 :;-.:-I- 10.85 0 sphere.

3) As Ihe plasses are worn /3 mm in .Ii-ont of the cornea i.e. nearer to the eyeIha/7 Fj by 22.91-13 = 9.91 111m: The focal length of the lens becomes 92.15- 9.91 = 82.24 mm and thus, the power of the correcting lens worn at 13 I11Ill

distance = 1000/82.24 = + 12.16 D sphere.Tl1eretore a lens of a .higher power with a shorter focal Lngth is needed ifthe correcl ing COllvex lens is worn at IJ 111m in rmnt or the cornea.

(6)The astigmatism against the rule: Is due to the vertically flattened corneafrom the fibrosis of the corneo-scleral section in cataract extraction cases and iscorrected by up to +20 cylinder axis 180°.

Fig.9.10:Spectacle correction in aphakia(withthe spectacles at the antedor focal point).(7) Correction of unilateral aphakia:

J) Spectacle correction:Disadvantages:!

1-This can achieve a clear retinal image . but, with a RSM of 1.33( theimage in the aphakic eye is olie third larger than the image in the normalfellow eye,Fig. 9.10) and so the patient is unable to fuse images of suchunequal size (aniseikonia) and complains of seeing double.

2- Heavy thick "lens. .Calculation of the angular magnification of an aphakic spectacle lens:

1- Aphakic eye corrected with +12 D spectacle lens placed 17 mm anteriorto the crystalline lens former position: Is an emmetropic eye with anerror lens of -15 0 added at the crystalline lens position(Fig.9.11).

2- The spectacle lens and the error lens constitute a Galilean telescope:When the focal points of both lenses coincide at the far points of the eye.

3- When both spectacle and error lenses are combined with the emmetropiceye, a focussed but magnified retinal image is produced.

4- Magnification of the Galilean telescope as it turns out:Power of eyepiece 15

M = .. So M = - = 1 25)(,power of objective ' 12 .

So, Angular magnification of an aphakic spectacle fens is 25%.

Forpointplane

IIIIIIIIII

f:66mrn 'If:83mm----_, .

~0~V·1-17mml.

Fig.9.11 :Calculation of angular magnification of an aphakic spectacle lens.l) Iseikonic tenses But these lenses have fallen into disuse (Chapter 9).3) Contact lense8' Reduce the RSM to 1.1 and restorebinocular vision (Chapter

DEFIdinWea

AETI(1)(2)(3 )(4)(5)(6)

OPTTy~

(l(2

Vis'

18) and the original correcting lens is to he changed to a contact lens power

by the formula: Dc = 1 _D;DS

(d is positive)

Where, Dc = Refraction at the corneal plane.Ds = Spectacle refraction.

d = Vertex distance in metres.Example: An aphakic patient with a spectacle correction of + I0 D 'at 13 mm

distance in front of principal plane of eye needs a contact lens of:Dc = +10/ 1- 0.013 X. -10 = 11.61 D sphere = 11.61 D sphere.

4) Intraocular lenses(IOLs)I-PC-IOLs:educe the RSM to 1 (i.e. image size in aphakic eye becomes

the same as in phakic eye) and restore binocular vision ..2-AC-IOLs Reduce the RSM to 1,03 (i.e.3 % , Chapter 19).

5) Relractive surgery.: Chapter (32) .. (5) ANISOMETROPIA

. DEFINITION: It is marked 'inequality in the refraction of the two eyes ,~ilh adifference of morethari 2-4 D between the eyes usually(20'-30% of spectaclewearers have some degree of anisometropia).

AETIOLOGY:(1) Congenital (common).(2) Progress of myopia in one eye ..(3) Recession of hypermetropia in one eye:(4) Operative trauma.(5) Ocular diseases.(6) Unilateral.aphakia.

OPTICAL PRINCIPLES:Types of refraction in anisometropia:

(1) One eye is emmetropic and the other eye is ametropic.(2) Both eyes are ametropic.

Vision in anisometropia: . . .(l)Binocular- vision: In small differences in refraction of both eyes (up to 6D

rarely), but usually there is only a superimposition of images with occurrenceof accommodative asthenopia due to a continious accommodativeeffort(more than in hypermetropia):l)A difference of 0.250 in refraction leads to 0.5% difference in size of

the two retinal images. .2) A difference of 2.5'0 ( 5% difference in size) is usually the limit tolerated

for binocular vision.(2)Alternating vision.' If one eye is myopic and used for near vision and other

eye is emn1etropic or moderately hyptmnetropic and is used for far vision.(3) Uniocular vision When one eye is amblyopic.

Differentiation between the three types of vision in anisometropia:(1) Colour fest: .

1) he wor E D will be presented to the patient as:,1- F.I.N. in green letters.2- R.E.D. in red letters.

2) Then ut a red glass in front of the right eye and a green glass in frontuf the left eye:i-It he reads FRlEND-7 binocular vision.2-Ifhe reads alternately F.I.N. then R.E.D.-7 alternating vision.3-If he reads only F.I.N. or R.E.D.-7 uniocular vision.

(2) Wort's four dots test:Principle;'

1) Same principle as the colour test and is suitablc I()J' chi ldrcli.2)The apparatus consists of a box containing four sheets of gl:Iss; one red,

two green and one white which are illuminated internally (Fig. 9.12) .. ethod: '

1)The patient wears red-green goggles with the red glass in front of theright eye and the green glass in front of the left eye.

2) The patient is seated six metres away froin the illuI11inated apparatusand he noticed one of the fonowing observations:I-Four dots, al;e seen if there is binocular vision:

(a) One red, one pink and two grcen as thc right eye is usuallydominant.

(b)One red, one palegreen and two green if the left eye is dominant.2- Two red dots: Indicate left suppression (uniocular vision).3- Three green dots: Indicate right suppression (uniocular vision).4- Alternately two red dots, then three green dots: Indicate alternating

suppression (alternating vision).5- Five dots,(2 red and 3 green):Indicate diplopia (paralytic strabismus).6- Four dots, may be seen in spite of presence of manifest strabismus:

Which indicate abnormal retinal correspondence.The optical condition in anisometropia: There is a difference in refraction in

anisometropia and so if an object is held in the midline between the two eyes,no sharp image will be produced in each eye, when equal degrees ofaccommodation are exerted in each eye.

CORRECTION OF ANISOMETROPIA:(1)Small grades: With any degree of binocular vision are corrected as follows:

l) /1 childrel :Full corre.ction as children can tolerate up to 6 D difference.2)7/1 aau ts Less correction to be tolerated (up to 4 D difference).

(2) Large grades: .1)With al el}lafing vision:

I-The myopic eye is used for near and the hyp,ermetropic eye is used for far

by one of the following: .(a) Full correction, separate glasses for far and for near (if full correction

fails) or under-correction of the more ametropic eye (if the patientcannot tolerate full correction specially in old age).

(b) Anisometropic spectacles to minimize peripheral prismatic effect bymaking the margin of the stronger lens weaker (Fig. 9.13).

2- Contact enses (Chapter 18), intraocular lens (Chapter 19),01' refractivesurgery (Chapter 32).

2) With uniocular vision (one eye is amblyojJic):1- Correct the good eye.2- Pleoptics for the amblyopic eye and occlusion in young age.

'·0""~,T '..., •.) '. \.'" i,:,X:¥:' ..

. .~: "

(I ., C :::::J' t:======:::::JR L

Fig.9.12:Worth's four dots test. Fig.9.13:Anisometropic spectacles..' (6) ANISEIKONIADEFINITION: It is marked inequality in the size and the shape of the retinal images

in the two eyes.AETIOLOGY:

(1)Optical aniseikonia: Due to difference in refraction in anisometropia as In

corrected uniocular aphakia with 33% magnification of spectacle lenses.(2)Anatomical aniseikonia: It is due to a difference in the distribution of retinal

elements (i.e. wide separation of visuai elements leads to small retinal imageand vice versa).

RETINAL IMAGES IN ANISEIKONIA:The amount of difference in ~hesize of retinal images:

(l)Right eye. (2)Left eyeFig.9.] 4: Imagc differences in aniseikonia: (ll)Symmetrical ill al/meridiillls; (b)Symmetrical

i1lo1le meridia1l: (c)Asymmetrical witlt aile side larger; (d)Asymmetrical witlt distortioll.The amount of difference in the size of retinal images:

(I) A slight difference in size and shape of retinal images: For normal stereopsis.

(2) (jPIO"5%'differ~nc¢:.i u~ual~nd of nosignificance. . " ' ,,;(3) ,JO'J(, di.ffe,.~nce~·l"CorreCted unilatcml aphakia with contact lenses, and in

" ", :::;'···:,.'~ome cases of st•.abis~~s~ , , ", ,.,"" '. '. . ,"". ," . " .'. " ''(4)3J'H,diflerence~'; ncorrected unilateral aphakia with eye spectacles.

. .'....The type~f difference In 'the size and shape of r~tlnallmag~:

., (l)S)'",,,ieirical.: One image being larger than the other: .'1)ln'all meridians (Fig: 9.14a).·. . .

. '2)ln one meridian only (fig. 9.14b) ..(2)ifsymmelrica/: The image is some~hat distorted: .' , ,....: I)Larger in onedirectionand smaller in another, as a prisin (Fig. 9.14c).

, '2)The shaPe may be progressively distorted (Fig.9.J4d).SYMP.TOMS: " ' '. ,'.,','(1)Blurrlng 'of vision with: ~) Photoph~bia .

. , .2) Headache.', , (2)Dlplopla which,may lead tQ: 1) SuppresSion.

. . . 2) Amblyopia.. '.. . 3) Strabismus.

. '(3)Abn.onna·l·loCalization in space. '""DIAGNOSIS:. '. ' .' , .,' , , . ' " , . ""

" . (1)Standard ,elkoriometer:Two dissimil~r objects of the same si~ arep'resented to Rneasurethe disparity in size of the retinal hnages(as a'synoptophore): l)The'noi'malperson~fuses both object~. '

. ,2)ln ~niseikonia~disParity. in size 'ofth~ two retinal images., (2)Space'eiko'n'ometer:Many lines in different planes are viewed against a '',. . bright backgro~nd: 1)The "annal person sees the lines in riOrma~relation ..

. '. 2)ln anisei~onia 'there is a[jnomlal relation due to ..-disturbed s'pati.allocalizatio,n.· . . .. .

(2)Conl(3)lnW(4)Retr

(1) PHY!1) The

pro2) Hyp

1- Pa

2-J.r

3) As1veragenie

(2) PAl1) CH1- H)

2) TICP

..CORREcTION: ..(1)Aniselkonic lenses:

"1 )h~oilsists of plates of gl~sses 'with parallel sides arid so it causes angular. magni.fication only and has no ref~ctivepower ( Le. afcicallens with.

, . magn·ification,Fig.9.15).. . ..... , . .2)A refractive correction may be imposed on an i'Sci~onic.lens._3) It· has' fanen into' disuse as it achieves. only 5% magnification practically .

(whiCh,isinsufti.cie'lt in cQrrecti()nof~Jllilateral ap~akia) and .it is expensive.

(2)Contact lenses: Chapter 18.(3)lntraocular lens; Chapter 19.(4)Refractive surgery: Chapter 32 ..

CHANGES IN REFRACTION(1) PHYSIOLOGICAL CHANGES: ,'i) There is a change from hypermetropia at birth to emmetropia as growth

proceeds: A change which may progress to myopia.2) Hypermetropia occurs in advancing years which may be:

1- Apparent increase in' the hypermetropia due to decreased power ofaccommodation. .

2- Absolute increase in the hyperlnetropia due to, changes in the size andrefractivity of the lens.

3) Astigmatism: Astigmatism with the rule (due to greater corneal curVature in the, vertical meridian) in the early years which tends to change to astigmatism

against the rule (in which there is a greater corneal curvature in the horizontalnieridian) later in life.

(2) PATHOLOGICAL CHANGES:1) Changes in dynamic refraction due to paralysis or spas·m of ciliary muscle:1- Hypermetropia due to paralysis of the ciliary muscle(with iess accommodation):

(a) Atropine instillation.(b) Nervous diseases.(c) Trauma to the eyeball.(d) Lens subluxatjon and tilting. ,

2- Myopia due to spasm of the ciliary muscle(with more .accommodation) as in:(a) Iridocyclitis:(b) Musculqr imbalance.

2) Transient myopia due to toxic states and drugs:Causes: 1- Toxic states as jaundice. 2- Drugs as diarl10x. 'Pathogenesis:These causes lead to transient myopia by one of following effects:

1- Hypersensitivity effect.2- Irritative toxic ciliary spasm.3- Central toxic irritation of the parasympathetic centres.4- Lenticular changes (if the myopia is, unchanged with atropine).

Degree of myopia: From 1 to 4 D.Duration of myopia: From few days to few weeks.

3) Changes in glaucoma:I-In buphthalmos: High degree myopia due to antero-posterior stretching but is

neutralized by the flattening of the cornea and lens and the relativedisplacement of the lens backwards.

2- In other glaucomas: .(a) Small degree myopia: Due to antero-posterior stretching which is less

marked than in buphtha:Jmos as the ocular tissues are fully consolidated.

(b)Loss of accommodation: Due to pressure on the ciliary muscle with earlypresbyopia.

3- Ajier anfi- rlaucuma uperatiun:(a) Marked myopia: Due to collapse of anterior chamber after the operation.(b)Hypermetropia: Due to reformation of the anterior chamber and recession

0f the lens will follow in the weeks following theop"eratibn.4) Diseased ocular coats:

1 ,Irrl ular astigmatism: Due to corneal diseases as in kerato.conus.2- Myopia: Due to sCleral stretching as in severe scleritis.

5) Changes in the refractivity of the lens: "1-In early stages of cataract:

(a) Index hy ermetropia: In senile incipient and immature cortical cataractdue to an increase in the refractive index of the lens cortex.

(b)lndex myopia:I senile nuclear cataract due to increased refractive indexof the lens nucleus.

2-1n diabetes:(a)High sugar concentration of the blood leads to myopia: Due deci'eased

refractive index of the lens cortex from hydration of the cortical layers ofthe lens relative to the nucleus in which the fluid tends to flow into thethe lens from a decreased osmotic pressure of aqueous humour.

(b)Low sugar concentration of the blood leads to hypermetropia: Due todecreased refractive index of the whole lens from a reverse osmotic flowwith hvdration of the nucleus of the tensNil: Refractive state in diabetics:

(l)A sudden myopia suggests diabetes and if in a diabetic patient it suggests"ullcolltrolled diabetes.

(2) A sudden hypermetropic change in a diabetic patient suggests over-treatment especially with insulin.

(3) Spectacles should be prescribed after control of diabetes except as allemergellcy and temporarily.

6) Pressure on the globe from outside: This leads to slight changes in refractionmechanically, depending on the point of pressore as follows:I-Orbital tumour leads to hypermetropia or hypermetropic astigmatism due to

axial "pressure. "2-Pressure by the finger, a tumour, a buckling procedure for retinal detachment

or a swelling in the lids leads to a transient astigmatism.

a(3)1n

w

TYPE(1)F

tC(

(2) PPirE

j\1ECI(1) II

at(2) Ii

ar(3) II

tll(4) M

mOEFIr

Far(1

(2)Nea

ob

tACCOMMODATION AND ITS DISTURBANCES

(1) COMMODATIODEFINITION: It is the ability of the eye to change its dioptric power:(1)ln Fig.l 0.1 a, emmetropic eye rays from infinity are focussed upon retina R.(2)Whena 'leal' object A is looked at, a focus isformed behind the retina (beyond

the second Drincipal focus F2) and no clear image will be formed on the retinaa at A'(the conjugate focus).

(3)lnorder to bring this focus forwards to R, the lens increases its convexitywith an increase in its dioptric power as illustrated by aCCOffilTIodation.

~

. - ..._-. -')~----- .A ~~ __ •• ,_ ~ •• »_--.--.--A

(a)principle. (b)Lens form change.Fig.l0.l:Accommod~tion. Fig.l0.2:Purkinje-Sanson images.

TYPES OF ACCOMMODATION:(1) Physical accommodation: It is accommodation related to a physical change of

the curvature of the lens and is measured in dioptres-'7an increase in theconverging power of the eye by 1 D leads to Gxertion of 1 D accommodation.

(2) Physiological accommodation: It is accommodation related to the contractilepowerof the ciliary muscle in myodioptres which are needed to raise therefractive power of the lens 1 D.

MECHANISM OF ACCOMMODATION:Young-HelmhoJtsz theory:II) Thelens is held suspended under tension by the suspensory ligament Which

attachesit to the ring of the ciliary muscle.\2)Theciliary muscle contraction reduces the tension on the suspensory ligament

andthe lens, allowing the lens to assume a more globular shape (Fig. 10.1 b).(3)Thecurvatures of the lens surfaces and the lens thickness are increased and thus

thedioptric power is increased.(4)Mostof the changes in the curvature occur at the anterior lens surface, which

t movesforwards slightly towards the cornea.DEFINITIONS OF CERTAIN TERMS:

Far point of distinct vision (punctum remotum, r):(J )It is the position of an object such that its image falls on the retina in the

relaxed eye i.e. in the absence of accommodation.(2)Jtis at infinity in emmetropic eye theoretically (but at 6 metres practically).

Near point of distinct vision (Punctum proximum, p): Is nearest point at which anobjectcan be seen CLEARLY when maximum accommodation is used i.e. withmaximumrefraction(it is nearer in emmetropic than in hypermetropic eyes).

Near point of accommodation:tl )The near point of 'att mmou-at\o1\is th~\\~'t\I~,:;t~\\\t(\t v.•hich the ob~ect is

seen BLURRED when the maximum accommodation is used.(2)lt is variable with age as follows: 1) Till 10 years age = 7 cm.

2) At 35 years = 14 cm.3) At 45 years = 25 cm.4) At 60 years = 100 cm.

Range of a.ccommodation: It is the distance betwee-n the far point and the nearpoint of distinct vision, over which accommodation is effective.

Amplitude of accommodation, A: Is the difference between the dioptric powerof the fully' accommodated eye(at near point)and eye at rest (at far point):(I )Dioptric power of resting eye (with atropine) is called stdtic refraction.(2)Dioptric power-of accommodated eye(without atropine)is dynamic refraction.

MEASUREMENT OF THE NEAR POINT OF ACCOMMODATION:The eye isemmetropic or rendered E (with correcting lenses) at first:(1)Dynamic retinoscopy: Chapter 14. -(2)The card test: The patient is asked to approximate the card close to his eyes just

till the letters appear blurred (but not doubled).(3) The strongest concave lens test: The focal length(in cm) of the strongest

concave lens that the eye still sees blurred the smallest test type 6/6, is thenear point of accommodation(the strongest convex lens test till blurring for fargives the far point of accommodation).

(4)The Scheiner's experiment: Look at a pin's head at different distances from theeye through two small holes in a card with a distance between these two holesless than the pupil diameter, till a single blurred image of the pin's head is seenwhich is the near point of accommodation.

THE FORMULA OF THE AMPLITUDE OF ACCOMMODATION: .(1 )The dioptric value of the near point distance P is the reciprocal of the near point

- - ~ -

distance-p- in-~etres :P= 1/p(2)The dioptric value of the far point distance R is the reciprocal of the far

point distance-r-in metres: R=l/r(3) The amplitude of accommodation A in dioptres is given by the formula:A=P-R

Al\1PLITUDE OF ACCOMMODATION IN DIFFERENT REFRACTIONS:(1)The amplitude of accommodation in emmetropia:

J) In emmetropia, distant vision is normal without using accommodation as thejar point r is at infinity:

1R = . t-' = 0 and A = P - 0 So, A = P for emmetropia.

111!luty

2) If accommodation is paralysed: he eye still sees distinct vision 6/6 b~lt thepatient needs a convex lens of P power to see at the near point.

(2) The amplitude of accommodation in hypermetropia: The hypermetropic eyesees nothing at all clearly without accommodation:

l)To see far objects: e exerts an amount of accommodation(R) equal to hishypermetropic error.

2) 0 see near objects: He must add to that amount needed, the amount exertedin seeiilg far objects in order to put him on the same level as emmetropia:

Ip is measurcd( to get P) and I' (to get R) = t f J:: t·. . amoun 0 relrac lve error.

(3) The amplit'tde of accommodation in myopia: The myope has his far point infr0nt of his eye.-7 p is measured( to get P) and r is measured( to get R which isequal to the amount of the myopic error).

(4) The amplitude of accommodation in astigmatism:1)Accommodation in the two eyes. are equal and simu'ltaneous and so never

becomes dissociated (canno~ act unequally) in astigmatism7so astigmatismcould not be corrected by accommodation.

2) Accommodation in astigmatism will:I-Correct the least hypermetropic meridian.2-Change the emmetropic to a myopic meridian.3-Chang,e the 111yopicto a more myopic meridian.

3) Thcrd'ore no clear image could be obtained by accommodation inastigmatism7so, eye strain develops'.

(5) The amplitude of accommodation in anisometropia:I) Accommodation in the two eyes are equal and simultaneous and so never

becomes dissoci-ated as in astigmatism7so, anisometropia could not becorrected with accommodation.

2) Accommodation in anisometropia will:I-Try to correct anisometropia with a small di (Terence (less than 4 D) in the

refraction of the two eyes, but accommodative eye strain occurs.2- Cannot correct anisometropia with a high difference (more than 4 D) in the

refraction of the two eyes because the image of the' eye with a higherrefractive error is blurred with neglected 'vision and so vision is uniocular.

i\lEASUREMENT OF AMPLITUDE OF NACCOMMODATION(A):(1)Measurement of the near point(p): To get P and so P=A as R =O( as r is at------

infinity) .(2)Accommodation (Prince's) rule:This combines a reading card with a ruler

calibrated in cm and D.(3)Method of spheres:Summation of the number of strongest concave and convex

lens tests for p and r.MEASUREMENT OF FATIGUE OF ACCOMMODATION BY ERGOGRAPHOF BERENS:

(I )Repeatedly approximating to eye a target carrying an object as a dot until itbecomes bi urred.

(2)The excursion of the target being recorded automatically on a drum.

CATOPTRIC (PURKINJE-SANSON) IMAGES:. Definition: Catoptric images are images fonned by reflections from the surfaces of

the refracting media of the eye (these images were described ·by Purkinj'e andwere used for diagnostic purposes by Sanson).

Principles: Each refracting interface in the· eye acts also as a spherical mirror(convex or concave) reflecting a small porti<m of light incident upOD it:(l)Practically four catoptric images are formed by reflection at the following

four surface :1) Image L: Is formed by reflection at the anterior corneal surface.2) mage II: Is formed by reflection at the posterior corneal surface.3) mage IIu Is formed by reflection at the anterior lens surface.4) mage IV. Is formed by reflection at the posterior lens surface.

(2)Other refracting surfaces in the eye form more images such as:I) The anterior surface of the lens nucleus.2) he posterior surface of the lens nucleus.3) he ankrior surface of the vitreous.

Chara~ters of.catoptric images: Thc· charactcrs of catoptric imagcs are prcscntedill Table 111and Fig. 10.2:(l)Jmages I, Il and Ill: Are erect and virtual because they are formed by convex

reflecting surfaces.(2)The image IV: ls inverted and real because it is formed by a concave

reflecting surface.Table III: Characters of catoptric images.

Ima!!e 1 /ma!!e II Ima!!e IIIBy reflection at By reflection at By reflection atant. Corn surface. oost.Corn.surlace. ant. lens surface ..Erect. Erect. Erect.Virtual. Virtual. Virtual.Bright. Vcry .faint. Faint.Lanre Smallest. Lamest.- -Just behind ant.· Close behind In the vitreous.

. lens caosule. imal!e I.- -

(7) During No change in size No change in sizeaccon1l11odation: and site. and site.

Ima!!e IVBy rcllcction atoost.lens surface.Inverted.Real.Faint.Small.In the anterior lenssurface.

Smaller in size Smaller in size butand approache~no change in site.image I.

NBl:When using images II, III and IV, th~ refraction of the reflected light as itemerges from the eye must be considered (i.e. the apparent position of theimages as that of the iris when viewed by·the observer must be ~onsidered).

NB2:Changcs in lens during accommodation is recorded by slit lamp photogrnphy.Uses of. catoptric images:

(1) Use of image 1:1) Study the anterior corneal curvature by:

1-The placido disc which examines the reglJlarity of the corneal curvature.2- The keratometer which measures the radius of curvature of the cornea.

2) Diagnosis and. measurement of the angle of squint. .

racrers(J) Formation:

(2) Position:(3) Nature:(4) Brightness:(5) Size:(6) Actual site:

VARIdecli

DEFIbey

AETIO)T.

(2) T.1

(3)(4)

ON,-( 1

FA((

(2) Use o.f images /1/ and I V.·Study the changes in the lens position and Cllrvatllreduring accollllllodation (by indirect measurements like the optical conslantsor the eye).

VARIATION OF THE AMPLITUDE OF ACCOMlVIODATION WITH AGE: Itdeclines steadily with age:( 1)Till 10 years of age = 14 0 (i.e. near point = 7 cm).

(2) At 35 years = 7 0 (i.e. near point = ]4 cm).(3)At 45 years = 4 D (i.e. near point = 25 cm).(4)At 60 years = I 0 (i.e. near'point = 100 cm).

NB:The decline in the amplitude of accommodation with advancing age at about 40 to 45years: Gives rise to tile inability to focus nearobjects w"ie/lis called presbyopill.

(2) PRESBYOPIADEFINITION: It is a physiological recession of the near point of distinct vIsion

beyond the normal reading or working distance with accommodation li.llly exerted.AETIOLOGY:Hecent theories are:(J)The lens: I) Becomes more rigid due to sclerosis of the lens fibres in old age

with decreased physical accommodation.2) Loses elasticity.3) Reduction in optical properties.

(2)The ciliwy muscle:I) \Veakness of ciliary muscle with decreased physiological accommodation

(decreased tension due to atrophic changes).2) Decreased muscle innervation.3) Changes in the neuromuscular junction ..

(3)The zonule. Loses elasticity.(4) The choroid: Becomes stitTer.

ONSET:(I) To focus 011 an objcet at a rcading clj~;tallc~ or 25 cm, the elllll1etropic eye I1lllSt

accommodate by 4 b~ ~ .(2) For a comfonable near vision, onc third of available accolllmodation must be

kept in reserve.(3) Therefore the paticnt will begin to experience dilliculty or discomlort lor near

vision at 25 cm, \",hen his accommodation has decayed to 6 D.FACTORS AFFECTING THE ONSET 0 i' PRESBYOPIA:

(1)Age:Presbyopia us I3lly occurs between 40 alld 45 year~ of agc in emmetropia.(2) Refractive error:

I) It occurs earlier in a hypermctrope (acc<1l11ll1odates more for nellr vision) 1111d

later in myopia.2) In myopia or 4D presbyopia never occurs and the patient can always read

without glasses.(3) The patient's preferred wor~~ing dist nee: A person doing line needle work

will experience diniculty earlier than a typist.

SYMPTOMS:(I) At first, eye strain, headache and indistinct small prints on reading in arti ficial

Iight at night.(2)Then, the reading becomes impossible in day light also.(3) Lastly, the reading becomes impossible even in strong light.

CORRECTION(1)Optical correction:

i)Reading glasses: A convex lens for near work is prescribed as follows:j - The error of refractiort: Is corrected for distance if pl'esenL2-The near point of distinct vision~p: Is determined.3-The amplitude of accommodation, A: Is calculated from the reciprocal of the

ne~r point of distinct vision lip which equals the dioptric value ofaccommodation for near point distance, p (as there iS,no accommodation forfar with the error of refraction corrected (or distance i.e. R =0).

-l-Re:ern: UCCUIIlIllUdut;ul1: l/3 accommodatl~n is kept in reSCI'VCto give thepresbyope a range for reading(by theci\iary muscle action) and so correctonly the renlaining 2/3 of accommodation ..

5-The allowed accommodation: Is the remaining two thirds of accommodationto be corrected.

6-The lVorking distance is measured:"1) 33 cm is the accepted distance for reading a textbook.2)Mid-distance glasses of more than 33 cm mClYbe needed(with a di (Terelll

correction of the near point) e.g. lor reading music.7-lens needed for correction ofpresbyopia is determined: tt is the difference

between the accommodation at the working distance and the allowedaccommodation.

8-Lens neededfor near vision is determined. It is the sum of the lens needed[or near to the amount"Q£the refractive el'ror.

9-Decelllralion of lenses in old presbyope!:(a)This is indicated because convergence is necessary in spite of weak

accommodation.(b )Decentration of lenses occurs nasally on both sides.(c )Prismatic power gained by decentring a spherical lens is given by the

formula: P = 0 x,h . Where, P = Prismatic power in prism dioptres.D = The lens power in dioptres.h = The decentration in centimetres:

j()- The interpupillary distance for reading: Is measured (Chapter 17).CorrectIOn Examp : A presbyope aged 50 years with +2D for distance:

1- +20.2- Say 40 cm.

1003 - -, )roo- 40 - -. .

1 -4- 2.5 X. 3 = 0.750 approximately.

5- 2.50 - 0.75= 1.75D.6- Say 33 cm which needs 100/33= 3D accommodation.7- 3 - 1.75 = 1.25 D.8- 2 + 1.25 = 3.25 D.9- Average 2.4 mm.

10- Average 64 mm.])Bifocal contact lenses: Chapter(18).

(2)Surgical correction:To achieve monovision:IJBifocal or accommodative intraocular/ens ':Chapter (19).2) Conductive keratoplasty: 'Chapter (32).3)PhotoreJractive surge,y. /- PRK: Chapter (32).

2- LASIK: Chapter (32).·I)Sclera/ surge,y: (£'/wpter (32/

(3) AN MALIES OF 1 CCOMMODATION _(I )INCREASED ACCOMMODATION:

Aetiology:

I) Excessive accommodation: ue to excessive near work with:1- Local cause. (a) Errors of refraction(H more than M).

(b) Excessive convergence.2- General causes' (a) l3ad illumination.

(b) GClieral debility and ill health.2) Spasm of accommodation:

l..l...ocaJ ausc: (a) rvluscular imbalance.(bj--I-rTdocycliUs.(c) Miotics (e.g-. p.ilocarpine).

2- u f.~uCJJem) )01 _ "lea/ioll (c:g. (1I;1tf~ 'KJ.CJmical f.atures: l- Arti tkiat myopia.

2- Accommodative asthenopia.Diagnosis: The difference between dynamic rcfraction (witholll <llropinc) and static

rdi'action (with atropine) is more than normal i.e. more than! .00 D.Treatment:

J- I.ocal {reaimen :(<:1) Rcfi'uctiol1 L1ndcl'atropinc and the ~orn:ctioll is onlcrl:d.(b) Cycloplegic treatment with atropine.

2- General treatment:(a) AvoiJ near work lor a period.(b) The genera! health should be improved and thc habits ~;hould be changed.

(2) n,J\tJlNISHED ACCOMMODATION:1) Insufficiency of accommodation:

Acrio!o':!,.l':l-LenticuJar sclerosis with early presbyopia.

2- Weakness of ciliary muscle (muscular fatigue) with excessive near work:(a) General debi-lity and toxaemia.(b) Early chronic simple glaucoma.

Clinical features: 1- ~sthenopia. _2- Near work is blurred and difficult.3- Disturbance of convergence.

Diagnosis Tests of the amplitude of accomnlodation.Treatment:

1- Local treatment:·(a) Correction ofrefraciive error, : a) For far.

b) For near as in presbyopia.(b)Prism base-in: fthere is nvergence insufficiency.(c) Practice by exercises as the llCCOl11l1w{/atioll test card. A black vertical

-line drawn on a white card is held at a considerable distance away andthen is brought closer to eye until the line appears blurred and indistinct.

2- General treatmcnt:(a) Treatment of the cause.(b) General health should be improved.(c) Avoid excessive near work.

2) Paralysis of accommodation:Aetiolog): Paralysis of the ciliary muscle or the third nerve by diseases such as:

(a) Local causes: a)Cycloplegics (e.g. atropine).b) Chronic simple glaucoma.

(b)General causes: a) Cerebral syphilis.b) Diabetes.c) Chronic alcoholism.

Clinical features:(a)F"e~tures of yaralysis of acco~mo~atio~l: Near ~ision is blurred and far

VISIon maybe blurred (due to dilatatIOn 01 the pupil). -(b) AssoCiated features:a) Dilatation of the pupil is usually present.

b) Extraocular muscle palsies may be present.Treatment:(a) Treat the cause, rest to the eye and general tonics.

(b) Pl"esbyopic glasses if recovery is delayed 01: prognosis is bad.N B:Anisocycloplcgia: Tlte tlVO ~I'es sltow II considerable variabili(viil their response to

cycloplegic.s (up to 10 D difference in tlte deptlt fij cycloplegia between tlte two qe.\).

CHAPTER'II,-B--I."..",N"='O...",.C,.......,U,.....L-A~R~MUSC U COO DIN ATI ON

(1) ORTHOPHORIA AND BINOCULAR VISIONORTHOPHORIA: Orthophoria is the condition in which the balanced action of the

extraocular muscles allows fusion without effort.BINOCULAR VISION:

Definition: It is the ability to see a single image of an object using both eyes .. Detection of the grades of binocular vision: Three grades CCl!1 be detected on the

synoptophore (Chapter 30 and Fig. 11.1):

(a) Simultaneous perception. (b) Fusion. (c) Si{!reoscopic vision.Fig. 11.1: G nldcs 0 f hinocu la r vision.

1) Grade 1 (Simultaneous pcrc.eption): If an object slide is presented to eachc./' eye (like the cow and the moon) and the two eyes see both· objects

simultaneously.[/2) Grade II (F lsion): If a composite picture is presented to each eye so that

each half of it is incomplete, fusion of the two images occurs and both eyessee one complete image.

3) Gntde HI (Stc.·coscopic vision): If a slightl·y dissimilar objects (as two ballsv with a sJight difference) are presented, both eyes see one complete image

with three dimensions (leng~h, width and breadth).Measurement of the fusional reserve (verging power): By' the prism vergence

test (Chapter 12).Development of binocular vision:

1) At birth.·No binocular vision but the baby can only fix a light for a moment.J) AI J-.1 11'eeks f?(age: aintained fixation lor few seconds by one eye.3) At 5-6 l11onths:B nocular fixation and maintained fixation.4) At 6 years: Fully developed fusion(training of fusion is successful alter 6

years of age).(2) CONVERGENCE

DEFINITION: Is the process by which the visual axes are turned inwards (direct 'dupon the object of attention).

TYPES OF CONVERGENCE:(1)Volutltary convergence: It is initiated in the frontal lobe (can be acquired by

training). .2) Involuntary (reflex) convergence: It is a psycho-optical ·reflex centred in the

peri striate area of the occipital cortex.

DEFINITIONS OF CERTAIN TERMS:The far point of convergence: It is the relative position or the eyes when they are

completely at rest with a slight divergence (therefore, it is beyond infinity).The near point of convergence: It is the nearest point for which the eyes can be

converged without producing diplopia (it is 7-8 em normally)..'m:Near point of vision is when the object is seen clearly, of accommodation is when

the object is blurred and of convergence is when the object is doubled.The range of convergence: It is the distance between the far and the near point of

convergence.The amplitude of convergence: It is the difference in the converging power

required to maintain the eyes in convergence at the near point and in divergenceat the rar point or convergence:1) Positive convergenc , It is the part of convergence between the eyes and

infinity.2) Negative convergence: Part of convergence beyond infinity i.e. behind eye.

MEASUREMENT OF THE NEAR POINT OF CONVERGENCE: The nearpoint is theoretically measured from the base line joining the centre or rotation orthe two eyes( 1'2-13mm behind the cornea), but practically it is measured frorn theanterior focal point F 1 of the eye() 5mm in front of the cornea) with addition of anaverage correction of 25 mm (2.5 em):(I) Bring a small object as a wire stretched vertically on a frame or a luminous slit,

near to the eye till it becomes double.(2) The near point of convergence is measured when the test object is doubled

while the near point of accommodation (near point of distinct vision) ismeasured when the testobject is blurred only.

MEASUREMENT OF THE AMPLITUDE OF CONVERGENCE:(1)By angular displacement of the visual angle in metre angles (m.a):

1) The patient looks at infinity: With the visual axes parallel.2) Then he looks at an object at one metre distance from the base line (between

centre ofrotation of the two eyes): '1'his will converge the eyes to one metreangle (which is the unit of convergence) on the median line between the twoeyes (0 and 01 are the centres of rotation of the two eyes. Fig. 11.2).

3) The angular. displacement varies with the distance between the two eyes:With an interpupillary distance of 64 mm:1-The convergence at 2 metres is 0.5 m.a.

.2- The convergence at 1 metre is 1 m.a.3- The convergence at 0.5 metre is 2 m.a.-+- The convergence at 0.33 metre is 3 m.a.

2) By prism in prism dioptres (Ll):)) Positive convergence:

I-It is measured by the strongest prism base-out placed in front of one eyeand is tolerated by the patient without diplopia (in prism dioptres which

eqw2- This

l) Negari1- it is

is tal

2- This I

NB:N,+

MEASUREI\'OPHTHALl\'

In the sarrapproxin(3)REL

ASSOCIATIAccol11mo(. .IS numenc;

DISSOCIAl(1)Accom

I) In hy2) Use

cony(2) Conver

1) In m2) In 013) Atro

retai4) Use

ac;:cc

equals 30 li , and may reach 6011 by training).2- This makes the eye to deviate inwards to avoid diplopia (Fig. I 1-3a).

]) Nl.!aolivl.! convergence:

1- it is measured by the strongest prism base-in placed in front of one eye andis tolerated by the patient without diplopia (in prism dioptres of 4-811 ).

2-Tl is make the eye to deviate outwards to avoid diplopia (Fig. 11.3b).NB:N()rnutl amplitude of converl!cnce is 10.5 m.a or 34-38 L\ (9.5 ni.a. or 30 h.

+ve convel'gence and 1 m.a. or 4-8 L\ -ve convergence)

a) Positive COIlVer!(ellc. b) Ne!(ative COllver!(ellce.(a)ln metre angles(m.a.). b)ln prism dioptres (6).

Fig. 11.2: Measurement of convergence.

MEASUREMENT OF FATIGUE OF CONVERGENCE BY THEOPHTHALMIC ERGOGRAPH OF BERENS:

In the same way as for accommodation fatigue, a target is repeatedlyapproximated towards the eyes until a fine line thereon appears double.(3)RELATJON BETWEEN ACCOMMODATION AND CONVERGENCE

ASSOCIATION OF ACCOMMODATION AND CONVERGENCE:Accommodation and convergence are associated movements (accommodation in Dis numerically equal to convergence in m.a and so 30= 3m.a at 33cm).

DISSOCIATION OF ACCOMMODATION AND CONVERGENCE:(1)Accommodation in excess of convergence:

1) In hypermetropia leading to convergent squint.2) Use of concave lenses in front of the eye to exert ~ccommodation without

convergence.(2)Convergence in excess of accommodation:

1) In myopia leading to divergent squint.2) In old age as accommodation is diminished but convergence is retained.3) Atropine instillation as accomJilodation is paral)'zed but convergence is

retained.4) Use of prisms 111 front of each eye to exert convergence without

accommodation.

RELATIVE ACCOMMODATION:Definition: Relative accommodation is the amount of accommodation which IS

possible to exert or to relax for a given constant amount of convergence.Types of relative accommodation:

(l)Positive part of relative accommodation: 1t is the amount of accommodationexerted with a given constant amount of convergence.

(2)Negative part of relative accommodation: It is the amount of accommodat"ionrelaxed with a given constant amount of convergence.

Relative range of ac ommodation: It is the distance between the near and the farpoint of relative accommodation.

Relative accommodation in the ernmetropic eye:(1) When looking at infinity: All the relative accommodation is positive.(2)As the eye looks nearer Positive part decreases and negative part increases.(3)As the near point is reached: All relative accommodation is negative with no

posi tive part.NI3:A far point, a ncar point and a range of rcl~tivc accommodation: For eve(J'

degree of convergence.Measurement of relative accommodation:

(l)1n emmetropic eye By the reading test types in front of the eyes at thereading distance of 33 cm (with a constant amount of ~onvergence of 3 m.a.):1) The strongest concave lens:Placed in front of both eyes which can be

tolerated is the measure of the positive part of relative accommodation.2) The strongest convex lens:ls placed in front of ~oth eyes which can be

tolerated is the measu"re of the negative part of relative accommodation.(2)1n ametropia: The patient i"srendered emmetro.pic first by glasses.

RESERVE ACCOMMODATION: .Definition:Part of available accommodation kept as a reserve to avoid straining of

the ciliary muscle which is usually one third of the available accommodation.Importance:The positive part of relative accommodation should be as large as

possible and at least greater than the negative part for comfort of the ciliarymuscle and so we must keep one third of the correcting glasses of presbyopia.

BINOCULAR ACCOMMODATION: When both eyes are used, theaccommodation increases due to the stimulus· of act" of convergence (thisaccommodation excess is usually 0.5 D).

RELATIVE CONVERGENCE:Definition: Relative convergence is the amount of convergence which can be

exerted or relaxed with a given constant amount of accommodation.Types of relative convergence:

(1) Positive part: It is the amount of convergence exerted with given constantamount of accommodation.

(2) Negative part: It is the amount of convergence relaxed with a given" constantamount of accommodation.

gIveACCOM(AC/A):

(1) Norelle

(2)Forl)N

il

Measurement of relative convergence: The sali1e as for relative accommodationby accommodating for a fixed object at 33 cm and varying convergence by:(l) Prisms base-ollt: ~oget the positive paIi.(2) Pnsms base-in: 0 get the negative part of relative convergence as usual.

RESERVE CO VERGENCE:(1) There 1 lUst be excess positive convergence in reserve for comfOit to the ocular

muscles.(2) Pa+ienL are able to' exercise only the middle third of convergence without

fatigue and so if their work has to go outside this area (either in the positive orthe negative part of relative convergence), prisms or exercise courses should begiven.

ACCOMl\10DA TIVE-CONVERG ENCE/ ACCOMMODA TION-RA TIO(AC/A):

(1) Normally: The effort of accommodation is accompanied by a correspondingeJTnrl or convergcncc (I D = I Illa).

(2) For practical purposes:J)Normal ACIA ratio: = ~ / D of accommodation (=3.5 ~/ D normally) and

it is usually constant 101' each person bLitdifferent for different individuals:I-Uncorrected 10 hypermetrope accommodates ID for distance but with no

convergence effOli.2-Uncorrected myope converges without accommodative effort to see clearly

at the eye far point2) AelA ratio can be measured by relaxing accommodation:

1- Heterophoria is measured with the target distance fixed at 33 cm beforeand after putting of +3 OS in front of both eyes.

2- AC/ A is the di rference betweerl the 2 measurements divided by 3.Nll:Abnormal AC/A may cause asthenopia 01' strabismus and so it should he

taken into consideration when presuibing cOlTcctive Icnses.

s.no

the.a.):

be

e as\iarya.the

(this

CHAPTER 12A MUSCUL\.NO LIES

(1) HETEROPHORJA (LATENT STRABISMUS)DEFINITIOI : Ocular deviation with abnormal direction of the visual axes of both

eyes reimive to each mher vvhen the binocuiar VISion is dissociated as by Jaugue VI

coverIng one eye.AETIOLOGY: J 1) Congenital weak muscles.

J2) Uncorrected refractive error.\.,./(3) Q"erstraining of the eye.

VARIETIES OF HETEROPHORIA:(1) Exophori : Both eyes tend to deviate outwards.(2) Esophona: Both eyes tend to deviate inwards.(3) Hyperphoria: One eye tends to d~viatc upwards or downwards.(4) Cyclophoria:

I) E.n:yL'fopllOric : Latci t extorsion or onc or both eyes.2) /nc Iclophoria Latent intorsion of one or both eyes.

SYMPTOl\lS OF HETEROPHORIA:(1)Compensa ed cases: No symptoms.(2) Non compensated cases

1) ivluscular asthenopia (eye strain).2) Blurred vision and intermittent diplopia.3) Reflex symptoms as headache, vertigo and restlessness ..

INVESTIGATIOi SO IETEROPHORIA:(1)Examination of. visu acuity and state of refraction:Chapter 13 and 16.(2)Examination of the ocular mov ments: In principal positions or gaze.(3)Measuremen of e deviation:' he tests depend on the principle of

dissociating the images in the two eyes so that the stimulus for binocular vision isremoved and the eyes take up the position of rest

1)The cover test:1- Binocular uncovering test!

(a) While the patient is fixing an object, one eye is covered with a card forabout 30 seconds, then the card is removed.

(b) The covered eye is seen to move under the card and then to move in theopposite direction when the card is removed.

2- Uniocular uncovering (alternate cover) testlEach eye is covered in turn andthe behaviour of each on uncovering is noted.

2)The Maddox rod test:ndicafio17 : 1- To test the muscle balance for distant vision.

2- Orthoptic exercises.The Maddox rod and the Maddox groove:'

1-The original M· '0 : Consists of a series of red powerful convexcylindrical lenses of glass placed side by side in a trial lens (Fig. 12.1 a).

2-The MadClox roOy : Consists of glass beveled to the same generalconfiguration so that it acts as parallel rows of double prisms (mostMaddox rods are of this type now).

'PrincIple of the Maddox rod.·f The Maddox rod acts by dissociating theimages in the two eyes by changing the shape of one iniage as follows:1- The patient views a distant white point source of light through the

Mwlrlox rod which is placed close to the eye, in the trial frame.2- The spnt light must be far enough away Jor its ray to be para!lel on

reaching, the patient (at least 6 metres) as follows:(a)Light in the meridian parallel to the axis qf each c linder:

a) It passes through undeviated and is brought to a focus by the eye(Fig.12:Ib).

b) The Maddox rod consists of a row of such cylinders, and thus arow of foci is formed on the retina (Fig. 12.1 c).

c) These loci join up and arc sccn ils a local line of light wl-,ich lics at90 to the axis or the Maddox rod.

(b)Li ht inci ent on Maddox rod -in the meridian at 90° to its axis:a) It is converged by each cylinder to a real line focus(real image)

between the rod and the eye (F.ig. 12.1d).b) This focus is too close to the eye for a distinct composite image

to be formed on the retina by the focussing mechanism of the eye(as this light is scattered over a wide area of the retina, Fig.12.1 d, and does not confuse the perception of the composite imagedescribed above, Fig. I2.Ic).NB: Focal line image:

(l) Remember tltat tlte lille (focallille image or composite image) seellby tlte patient lies at 90° to the axis of the Ma(ldox rod and isformed by the focussing mecltanism of the eye (it is not tlte reallille.image of the Maddox rod). .

(2) The glass of the Maddox rot! is tinted red ami so composite Iilleimage seen by tlte patiellt is also red.

Reye In merldlan01 axie nays In meridian

al 90' \0 a"l.

Rays In m~rldlen01 a"ie

(a) (b) (c) (d)Fig.12.1 :Optics of Maddox rod:(a)M. rod;(b)Ugltt illmeridicill parallel to axis of M. rod; (c)Ullevffoci by adjacellt elemellts of M. rod;(d)Ligltt illcidellt ill meridian at 90° to axis of M. rod.

Testing the muscle halance for distant vision hy the Maddox rod:1-The Maddox rod is placed close in front of the right eye in the trial

frame, and the distant white spot light (at 6 m) is viewed with both eyes,with the patient wearing his corrccting Icnses.

2- The right eye, therefore, sees a red line at 90° to the axis of the Maddoxrod, while the left eye sees the white spot light.

3- Thus the two eyes see dissimilar images and are dissociated, allowingany muscle imbal.ance to become manifest.

4- To test the horizontal imbalance, the rod must be horizontal to give avertical line and vice versa.

5- Remember that the eye behind the Maddox rod (conventionally the righteye) is deviating in the opposite direction to that indicated by red line.

Fi!!.ll.l:1\ I:HhJox n)tl hdol'c I'it;hl CYC:(lV Horlzo/ltal mtlUJfJIIOrlll: (£) f.ml'llar'a..;(d) Vertical ortilOpllOria;(e)Right hyperpllOda;(f)Right hypophoria (= left Ityperp 110 ria).

o 0 0'

(aJRight exophoria with crossed diplovia. (bJ Right esophoria with ullcrossed diplovia.Fig. 12.3: Diplopia in exophoria and in esophoria

The results with the Maddox rod infront of the right eye:.11- Horizontal orthophoria: The red line will be vertical and runs through

the white spot light(Fig. 12-2a).2- Exophoria: he red line will be vertica,l and to the left of the white spot

light" (Fig. 12-2b), as the patient has crossed (heteronymous) diplopia(Fig. 12-3a in which 0, object; F, ,fovea; P, peripheral retinal point; and0' projected image of object 0).

3- Esophoria: The red line will be vertical and to the right of the whitespot light (Fig. 12-2c), as the patient has uncrossed (homonymous)diplopia in esophoria (Fig. 12-3b).

4- Vertical orthophoria: The red line will be horizontal and runs throughth~ white spot light (Fig. l2-2d).

5- Right hyperphoria: he red line will be horizontal and below the whitespot light (Fig. 12- .

6- Righ hypophoria (= left hyperphoria):' he red line will be horizontaland above the white spot light (Fig. 12-2f).

7- C clophoria' The red line wifl run obliquely when the Maddox rod ishorizontal or vertical.

JHeasuremellt of the angle of deviation:1- Maddox rod with a raduated tan ent scale (if add.o (Fig. 12.4):

The tangent scale with a white spot in its middle is placed at 6 m fromthe patient in a dark room and the angle of deviation is measured on the"calL directly.

2- Mt r l with prisms before tlie Ie t eye:(a ( measure the deviations in exophoria or in esophoria: By

prisms with their bases in or out respectively until the horizontal0l1hophoria is achieved.

(b) 'I { re le (evia ion i I)perp lOna or typop loria: TheMaddox rod is rotated at right angles and prisms with their basesdown or up resrectively until the vertical orthophoria i3 ~chieved.

(crl) ncasurc{lc cvia ioa) fPil lonE /' addox rod:

a-The Maddox rod is tilted till the oblique red line becomeshorizontal if the Maddox rod was vertical or becomes verticalwhen the Maddox rod was horizontal.b-The number of degrees through which the Maddox rod has tobe tilted gives some' indication of the amount of torsion.

b) Wit 1 two lvfad ox rods:ith different tints and one is tilted.

Fig. 12A:Thc Maddox tangent scale. Fig. 12.5: the Maddox wing.3- The Maddox fUlIldframe:

(a)It consists of a Maddox rod placed in front of right eye, and arotating prism 10 IS in front of left eye.

(b) The prism is rotated until the red line traverses the white s'pot light.3)The Maddox wing test:

Jndicatiod' To test the muscle balance for near vision.Maddox wing consists (~r--

1- Small blackboard which contains 2 arrows pointing to the zero mark of 2, scales with 2 rows of numbers.2- Two eyerieces, diarhragm (to separate thc 2 eyes) and a handk.

Principle of the Maddox wing: The Maddox wing. acts by separating thevisual fields presented to each eye by a diaphragm and thus dissociatingthem (Fig. 12.5).

lvfethod of testing muscle balance for near vision by Maddox wing test:1-The p~tient wears hI correcting lenses with the addition of near Vision.2- The' Maddox wing invesfigate every type of heterophoria at a distance

of33 cm.3- When the patient looks through the two slit.,.holes in the eyepieces of the

instrument, the fields which, are exposed to each eye are separated by adiaphragm in such a way that they glide tangentially into each other:(a) Ie rI )' I eye se Ie arrows only:

a) A w Hte arrow pointing vertically upwards.b) A'red arrow poi nting horizontally to the left.

(b) Ire left ~re ,..,-eestie sca es "2 rows () i,um ers) OIl()I:

a) i\ white horizontal scale (row or numbers.b) A red .vertical scale (row of numbers).

4- The horizontal and veitical rows of numbers seen by the left eye arecalibrated to read in degrees of deviation:(a)The white arrow is pointing to the horizontal row of numbers and the, red arrow is pointing to the vertical row of numbers.

(b )Normally the two arrows should be at zero and any deviationindicates exophoria, esophoria or hyperphoria, the amount of whichcan be read off on the scale (odd numbers indicate esophoria andeven numbers indicate exophoria).

(c) It is possible to estimate the deviation with each eye fixing in turn byasking the patient first to concentrate upon the arrow and then uponthe figure to which the arrow points.

4)The orthoptoscope (Calibrated synoptophore): This is one of the morecomplicated amblyoscopes which measures the deviations of muscularimbalance accurately and rapidly

(4)Measurement of the strength of EOMs by the prism vergence test:Indications:

1)To measure ,the fusional reserve (the verging power) i.e. the capacity tomaintain fusion.

2) To assess the presence of binocular vision in children under two years old.Phnciple of the prism vergence test:

1) The test should be carried out with:1- The patient wearing his full correction.2- He should fix a point of light at 6m distance.

2) Prisms of increasing power are placed before one eye: Until fusionbreaks down with diplopia.

3) This gives the prism power whi<;h maintains fusion as follmvs:

I-Prisms base-ou~ Measure the amount of positive fusional reserve (Fig.12.6) i.e. of convergence (may reach 30-60 1:1 ).

2-Prisms ase-il: Measure the amount of negative fusional reserve i.e. ofdivergence (may reach 4 1:1 to 8 1:1). And so that of convergence is morethan that of divergence.

3- Prisms base-up or down Measure the amount of vertical fusionalreserve i.e. of veltical deviations (may reach 2--4 ~) and so horizontal ismore than vertical fusional reserve.

Fi~.12.6: MC~ISU rcment of capacity to Imlintain fusion. Fig.12. 7: Rotatory (dou hie) prism .. Methods of testing thefusiona! reserve by ti1eprism verg.ence test:

l)0,rdinary prisms(of increasing strength) or prism bar:ll Fig.12.6,objecto ~ppears to the right eye R to be displaced to 0' by the prism base-out P,and double vision should be produced and the tendency to diplopia isovercome within limits by the power of fusion.

2)A rotatory nrislll (double prisms):1- A rotafor }prism is two prisms wnic 1 can be rotated asfo//ows:

(a) \Vhen they are lying apex to base the combination becomes a glassplate and the prismatic effect is nil.

(b) When they are lying with their two bases in corresponding positionsthe total etTect is equal to double the effect of one.

tc) Each intermediate position has an intermediate prismatic value.2- TJz~ deviation can be calibrated 011 a scale: Both for horizontal and

vertical displacements (Fig. 12.7).3) The phorometer: \Vhich consists of two rotating cells each carrying a

prism of 101:1 which rotates in unison (under control or a gearwheel) and ascale measures the strength of the refracting angle of the prism used.

4)' hc ' 'noptophol"c. Chapter 31.TREATMENT OF HETEROPHORIA:

(1) Compensated cases: No treatment.(2) Non-compensated cases:

J) General treatment:Ceneral health improvement, rest and suitable exercises.

2) Correction of refractive errors.3) Orthoptic exercises:

Indication :lfthe above treatment fails.Disadvantages:I-Useless in hyperphoria and less su.ccessful in esopl~oria and in cyclophoria.2-The symptoms may be relieved but the heterophoroia is unchanged usually.

oI"I,

I \I \

I \1 \

I 'I \

\I \I \I

1I

(ajOr exophoria by prism base inwards. fbJOfesopllOr;a ~ prism hase tJufwa,.J s'.Fig.. 12.8:Correction of heterophoria by relieving prisms.

Method: Orthoptic exercises both for distant vision and for near vision overlong periods:.1- xercising (adverse) prisms: ith their bases towards the direction of

deviation.2- Orthoptic treatment on the synoptophor : Is needed in difficult cases.3- Two Maddox rOGspaced perpendiculnrly one in front of either eye tp

exe"rcisetorsion in cyc op11Oria:(a) In cyclophoria the two red lines are inclined at an angle, the size or

which varies with the degree of the defect.(b) One of the rods is then rotated until the two lines are fused.(c) Then this rod is moved alternately forwards and backwards in the

direction which will exercise the muscle, while the patient is trying tokeep the line of light from doubling.

4- Two lines drawn on stereoscopic cards in the amblyoscope: Each linecan be rotated against the other to exercise torsion in cyclophoria (sameprinciple of the two Maddox ,rods).

4) Relieving prisms:Indications: Rei i.eving prisms with their bases against the direction of the

deviation (Fig. 12.8) are used (in hyperphoria mail~ly) to correct the effectoptically if the above measures fail and their importance are:1- They compensate for the imbalance.2- They relieve eye strain and so may stimulate fusion.

Prescription of relieving prisms:1- Full prismatic correction in hyperphoria and half prismatic correction in

exophoria and esophoria (not successful in cyclophoria).2- The prismatic correction is divided equally between the two eyes.

5) Operative treatment: If the above measures fail.(2) HETEROTROPIA (MANIFEST STRABISMUS)

DEFINITION:Muscle imbalance with one eye deviated out of its proper direction.1) CONCOMITANT STRANISMUS

DEFINITION: Ocular deviation with abnormal direction of the visual axes or botheyes relative to each other in which angle of deviation is constantin different directions of gaze.

IMPORTANCE OF REFRACTIVE ERRORS: Heterotropia occurs when thestimulus for fusion is lacking ill:(I) Accommodative esotropia nssociatecl with hypermetropia.(2) Exotropia associated" with myopia.(3) Manifest esotropia clue to congenital or infantile myopia.(4) Habitual divergence lor ne8r and then for distance in congenital astigmatism.

VISION IN CONCOMITANT STRABISMUS:(1) Diplopia:

I) Diplopia occurs in concomitant strabismus starting in early childhood (but thepatient usually adapts himself to it later on).

2) Late in life, diplopia may reappear and is overcome by eit.her suppression orthe image of the deviating eye or mental re-orientation of the displaced image(false projection):

(2)Strabismic nmbly pia'Definition: Act've inhibition of vision in one eye.Principle: To avoid diplopia, the hrain neglects the blurred image seen by the

squinting eye in unilat.eral squint.FiXQ r ion:

1) Centric (foveal) fixation: By the macula.2) Eccentric (cxtrafoveal) fixation: Occurs in unilateral squint in which an

eccentric part of the retina takes the function of the macula which ismarkedly suppressed i.e. a false ma "ula.

V ARIETl.E._ OF CONCOMITANT SQUINT:(1)Esotropia: Which is convergent str~bismus.(2) Exotropia: Which is divergent strabismus.(3) Hypertropia and hypotropia: Which is vertical strabismus (the eye moves

upwards or downwards).INVEST CATIONS OF CONCOMITANT STRABISMllS:

(1) Examination of the visual acuity and of the state of refraction.(2) Exurninatlon of the ocular movements.(3) Measurement of the ar gle of deviation:

1) Corneal refle : Inspection of corneal reflections of white spot light ( angle is15 degree if the light spot is at the edge of the pupil, 30 degree if mi.dwaybetween the papillary margin and the limbus, and 45 degi-ee if at the limbusi.e 1mm=7 degrees) and should be FOLLOWED by the cover test.

2) Cover test: Binocular uncovering test.3) Prism (or prism bw) test: Prisms of increasing power are placed in front of

the fixing eye with the prism base against the ocular deviation until thesquinting eye becomes straight and the corneal reHex on the centre of pupil.

4) Prism cover test:· Prisms of increasing power are placed in front of squintingeye with the pi-ism base towards the deviation while covering the other(fixing) eye until the movement of squinting eye is abolished (Fig. 12.9).

5) FOllr dioptre prism test: This is a delicate test for small degrees of esotropia(microtropia):1- A four dioptre prism placed bnse- out before the deviDting eye causes no

movcment as thc imagc rcmains within the suppression scotoma.2- When placed before the normal (fixing) eye, movement occurs.

6) Synoptophore.(4) Investigations of amblyopia:

1) Measuren'lent of the corrected visual acuity.2) Worth's four dots test.3) Synoptophore.4) Detection of the type of fixation (centric or eccentric fixation) by:

1- Cover test.2- Visuscope.

5) Response to occlusion.(5) Investigations of binocular vision:

1)Detection of binocular vision: ~y the Worth's [our dots test.2) Detection of the grades of binocular vision: By the synoptophore.3) Detection of the retinal correspondence: 1- Synoptophore.

2- After image test.TREATMENT OF CONCOMITANT STRABISMUS:

Aim of treatment:(l) To correct the deformity (cosmetic).(2) To restore binocular vision.(3) 1'0 improve visuai acuity.

Methods:(J)Atropine eye ointment 1%: For accommodative strabismus (esotropia due to

hypermetropia) in children below 2 years of age till he can wear eye glasses(to relieve accommodation and associated convergence).

(2)Correction of any refractive error.(3) Correction of amblyopia:

1) Occlusion: Is more effective below the age of 6 years:I-Amblyopia with centric fixation. Occlusion of the eye with better vision

for a month or more as long as any improvement is obtained.2-Amblyopia with eccentric I alion:

(a cc USIOIl () tl a eeted e e flrst: For one month or more toweaken the eccentric point.

(b) Ie" oce USIOIl 0 t Ie bet er eye: With foveal exercises to theamblyopic eye (pleoptics).

2) Pleoptics: Is de~igned to encourage foveal stimulation and to develop fullvision in an amblyopic eye with eccentric fixation after occlusion of thesquinting (amblyopic) eye for 3-4 weeks by:-1-AJter image met lOd: The euthyscope is used to suppress the parafoveal

region with a strong light and then the patient looks at a white screen toget a negative after image.

2- Direct !()veal stimulatiOl : The projectoscope is used to stilllulate thelovea with a strong stimulus aller dazzling the eccentric area.Nil: lIaidinger's brushes test: 011 tile coordilla(()r or Oil the ~YII()ptof1lrore call he

used fnr trailling of the fovea (~fter tirefoveal fixatioll is established.(4JResforafion of binocular vision (orthoptics):To encourage development of 3

grades of binocular vision by training exercises on the synoptophore [or:1)Orthoptic training alone cannot cure a case of squint with a large angle of

squint unless combined with other methods of treatment of squint as:-1-After correction of refractive error.2- After occlusion and pleoptics in amblyopia.3- Before and after strabismus operation.

2) Done after the age of 4 years (cooperative child) till the age of 10 years.(5) Qperative [reatment:

1) A curati e operation: Is required in cases of squint in which thedeviation persists in spite of improved visual acuity and uniocular visionby spectacles, pleoptics or orthoptics.

2) A cosmetic operation: Is required to improve the appearance of thepatient when the binocular vision is absent.

2) PARALYTIC STRABISMUSDEFINITION: Ocular deviation with abnormal direction- of the visual axes of both

eyes relative to each other due to paralysis of one or more of the extraocularmuscles in which the angle of squint is variable in different directions of gaze.

DIAGNOSIS OF PARALYTIC STRABISMUS:(1)Signs and symptoms:Diplopia,ocular deviation,limitation of movements, ...etc.(2) Examination of ocular movements: To detect the paralysed muscles.(3) Diplopia chart:

Aim: To determine the paralyzed muscle (especially muscle paresis).Method: 1)The patient wears red-green goggles with the red glass in front of the

right eye and the green glass in front of the left eye of the patient (todifferentiate between the 2 images), in a dark room.

2)A torch with a stenopaeic slit is used to project a linear light in ninedirections of gaze.

3)The patient moves his eyes and not his head and can see 2 images(red and green):

I-Clear image seen by the normal eye.2-Blurred image seen by the paralyzed eye.

4)The patient is asked about the following data to investigate diplopia:-l-Ai'eas and type (uncrossed or crossed) of diplopia.2-Relative position of the 2 images.3-Distance between the 2 images.

Example' Right LR paralysls(Fig.12.1 0).(4) Hess screen:

Aim: l)To determine the degree of the paralyzed muscle which is comparedwith subsequent examinations.

2)To determine the secondary changes affecting the other muscles (i.e.overaction, underaction, ... ete).

Principle. ~ screen (Fig.12.11) records the degree of false projection in differentdirections of gaze by dissociating images of 2 eyes with red-green goggles.

TREATMENT:(1)Treat the cause: As diabetes, hypertension, ... etc.(2)Occlusion of one eye: To avoid diplopia till the muscle regains its function

with recovery within 6 months:I) Occlusion of the paralyzed eye.2) Alternate occlusion with orthoptic exercises is better.

(3)Surgical treatment: If no recovery occurs after 6 months.(3) ANOMALIES OF CONVERGENCE

(1) INSUFFICIENCY OF CONVERGENCE:Types: l)A bsolute insufficiency:

1-When the near point (in absence of presbyopia) is greater than II emfrom the base 1 ine.

2- When there is di fficulty in attaining 30° convergencc.2)Relative insufficiency: When there is convergence insuf1iciency for a

preferred working distance.Aetiology:

1)Local causes: 1- 'Wide interpupillary distance: With exophoria.2- Accommodative difficulties:

. (a) Uncorrected myopia.(b) Recently corrected hypermetropes.

3- Disuse of the eye as in:(a) Marked hypermetropes.(b) Anisometropes.

2) General causes:- General debility.2-Myasthenia gravis with MR paresis.

Clinical features: I )Voluntary convergence is usually impo~sible.2)Characteristic features of absolute and relative insufficiency.3)The causal disease.

Diagnosis: 1) Orthophoria for distance.2) Remoteness of the near point beyond 9.5 cm.3) Prism convergence is less than 15 ~.4) Prism divemence is norma:.. -

Treatment:I) Elimination of the causalfactors: 1- Local causes.

2- General causes.2) Orthoptic exercises:

1-Exerdses for involuntary convergence:, (a)Increasing power ofstereopsis. By stereograms of increasing difficulty.(b)Ductional exercises oy prisms base-out To get convergence in distance.(c) Vergence exercises on the amblyoscope: In difficult cases.(d)Pencil-nose exercise aL home (Lasupplement vergence exercises): The

patient attempts to approximate the near point by fixing a pencil as itapproaches his eyes until it appears double.

2- Training of voluntary convergence:. (a) The patient must recognize and control position ofhis eyes:At first.(b)Then he demonstrates Lahimself physiological diplopia asfollows:

a) Holding a pencil in front of his eyes he sees two pencils while fixinga distant object.

b) Looking at the pencil he then sees two objects which separate andapproximate as the pencil is moved forwards and backwards.

c) The pencil is then removed and the patient tries to retain the twoobjects apart as long as possible, without moving his eyes.

d)The exercise is completed by doubling the objects without the aid ofthe pencil.

(c) Then exercises with the stcreoscope are carried 0 It.

(d) Lastly fusion of stereoscopic pictures by convergence without the aid ofthe stereoscope will cure the case with a good fusional amplitude.

3) Correctio/7 of reji'active errors: Undercorrection for hypermetropia and fullcorrection for myopia.

4) Relievllig prisms: fthe above lines of treatment fail.2) CONVERGENCE EXCESS:

Aetiology. i\ habitual excess or spasm is common(due to innervational influences).Types:

1) Convergence excess with increased accommodation as in:1- Uncorrected hypermetropes.2- Recently corrected myopes.

2) When the desire for clear vision calls for unusually accentuatedaccommodution.1- In children starting near work for the first time.2-ln industrial workers starting a sedentary life or concentration.

3) irritative conditions of the central nervous ~ystem: As meningeal irritation.Clinical features: I) Difficulties in near work and re.ading with blurring.

2) F:ll iguej headache and diplopia.Treatment:

I) Elimination of the causal factors.. .2) Correction ofre,fi'active errors and of heterophoria.3) Near work should be reduced.4) Orthoptic exercises:

1-Exercises producing voluntary relaxation: The patient looks at a distantobject through two transparent slides and is taught to obtain physiologicaldiplopia of markings on the transparencies (this will necessitate relaxationof both accommodation and convergence).

2- Divergen~e exercises with the amblyoscope; Which may follow therelaxation exercises.

DEFII"disti:

THE'extn

THE~Defi

(6

Prin

FilLTHE~

(1)1/1

(2)1ft

96 ...:.97.:..-- _

of ER 13VISUAL FUNCTIONS

1111 (1) VISUAL ACUTIYDEFINITION: It is the ability to appreciate the form of the smallest retinal image

and is measured by the smallest object which can be seen at a certain distance.1) FAR VISION

s). DEFINITION: The power by which the details of visible far objects can bedistin5uished from one another.

THE VISUAL ANGLE: It is the angle formed by two lines drawn from theextremities of the object through the nodal point of the eye (Fig. 13.1).

THE MINIMUM VISUAL ANGLE(FIG.13.1):ed Definitlon:lt is the smallest angle, I minute of a degree in test charts for 6/6 = I ,

(] 0 minutes of adegree for 6/12 =0.50 and 100 minutes or a degree lor6/60=0.0 I )subtended at nodal point of eye by 2 small points to be seen separate.

Principle:Two separate cones should be stimulated, while one intermediate cone isunstimulated, if two separate points are to be distinguished by the retina:-(1) The cone diameter at the centre of the fovea: Is 1.5 !J.m(0.00] 5 mm).0) The linear separaiion of2 cones separated hy a third cone: Is 0.003 mm.(3) The angular resolving power(Chapter2): ,0.78 minutes of arc (47 seconds).

Fig.13.] :Landolt's broken ring. F.ig.13.2:Separation between segments of each targetthe THE SIZE OF THE RETINAL IMAGE:

(1) The size of the retinal image depends on:/1) The size of the object:

1-The smaller object size, the smaller is the image siz~ and vice versa.2-1n Fig. 13.3a, CB is half AB and so its image C 1B I is half Al B 1.

./ 2) The distance of the objectfrom the eye:1-The shorter the distance of the object, the larger is the image size and vice

versa.2- In Fig. 13.3b, AB is at a distance twice CO and so its image J\ 1131 is hal r

('IOj.

(2)These principles (of size of object and its distance from eye) ·have beenfollowed in Snellen's test types and Landolt's broken rings for measuringthe visual acuity by finding the minimum visual angle V as follows:

DV = M Where, D = Distance of object from eye and M = size of the object.

(3)The minimum distance at which the test type is used can be calculatedfrom the reduced eye as follows:1)1 the object is at infinity: Its image will be at 20 mm which is the length of

the globe (approximately).2) if the object is at 5 metres: Its image will be at 20.06 mm. .

N~:The difference 0.06 mm (20.()6-20.00) is the thickness of the sensitive layer of theretina: So five metres is the minimum distance at which the test type is used.

Q-6,11,C,

(a)Smaller object site.-7 smaller image size. (b)sllOrter distance of object .-7larger image size.Fig. 13.3: The size of the retinal image.

VERNIER ACUITY (ALIGNING POWER OF THE EYE):Definition: It is the ability of the eye to detect a break in a line or a displacement in

a contour.Explanation: One to one relationsqip of cone, bipolar cell, ganglion cell, and nerve

fibre is the most accepted theory.Character: It is as fine as 3-5 seconds of arc (as it. is only a fraction of the angular

diameter of a foveal cone which is 36 seconds of arc).Clinical applications:

(1) Vernier alignment is used as an endpoint in several ophthalmopticinstruments: As the keratometer, lensmeter and applanation tonometer.

(2) Vernier test on a Vernier scale: ls a delicate test for detection of maculardamage when vision is 6/6 as measured by Snellen chart.

(3)Amsler grid chart:1)Vernier acuity is the basis of Amsler grid chart of lOX. 10 cm which is

divided into small squares of 5 X. 5 mm and containing a central whitefixation spot. .

2) Patient is asked to look at the central fixation spot with the uncovered eye.3) Any distortion, wavy lines, blurred areas or black spots indicate a macular

lesion as in Vernier test.TYPES OF VISUAL ACUITY:

(1) Unaided visual acuity: It is the efficiency of the unaided eye without correction.(2) Optical visual acuity: It is the visual acuity of a single letter.(3) Maximum visual acuity:The visual acuity of the smallest line of test objects in

which the patient can correctly recognize more than one half of targets.

(h)(5)lnt

(6)Fi)pe

(7)Te(8)5t

gaSNELL

(1)Th

(2) rI).2}

(3) D~

CLU(:

(4)Absolute visual acuity: It is the visual acuity with the accommodation relaxedand the refractive error corrected by spectacle lenses situated at the anteriorfocal point of the eye;

(5) Relative (best corrected) visual acuity: It is the visual acuity of the eye with itsrefraction fully corrected by spectacle lenses worn in the·ordinary position.

FACTORS AFFECTING VISUAL ACUITY:(1) Refractive error: leads to decreased visual acuity.v

v(2) Contrast: Decreased contrast leads to decreased visual acuity.(3) Pupil size: Decreased visual acuity with:-

J) Pupil size helow 2.5 mm:Due to diffraction.2) Dilated pupi/:Due to aberrations and uncorrected errors of reti·action.

(4) Stiles-Crawford effect:(a) First kind: Rays striking the .photoreceptors at oblique angles are 110t

efficient as the parallel rays.(h) Second kind: Oblique rays cause a different colour sensation than axial rays.

(5) Intensity of illumination: Decreased intensity of illumination leads to decreasedvisual acuitv.

(6)Fixation: Fine ocular movements which occur during fixation lead to imageperfection.

(7)Te r film: Its abnormalities lead to defective visual acuity.(8)Stimu·lated retina: Visual acuity decreases outside the fovea due to decreased

ganglion cells.SNELLEN' ,RACTION:

d(l)The visual acuity = D :\Vhere, d = Distance between the test object and the eye.

D =Distance from eye at which target subtends an. angle or one minute nt the.nodal point.

(2}Snellen's n tation:l)/n metres' 6/4, 6/5, 6/6, 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60.2)lnfeet( metres=20feet):20/20, 20/30, 20/40, 20/60 20/80, 20/120 and 20/200.

(3) Decimal notation: 1 (6/6),0.7 (6/9), 0.5 (6/12),0.3 (6/18),0.25 (6/24), 0.17(6/36) and 0.1 (6/60).

CLINICAL TESTS FOR FAR VISION(VISUAL ACUITY TESTS):(1) Verba! vision tests:

I) Test charts:1-Ttle targets () t e . c tarts:

(a) Each target consists o["'~' etters(in Snellen's test types),C broken rings{in Landolt's charts), or numbers: E letters are less confusing than Cbroken rings.

(b )Each tar Jet is constructe 0 subtend a visual angle or 5 minutes or adegree (5 ') when viewed from the specified distance: Therefore torecognize a target, the eye must have a limit of resolution or one

!1 IS

l1ite

ye.liar

minute of a degree (1 ') which is the angle subtended at the edges ofeach letter (Fig. 13.1).

(c) The separation between tl e segments of each target: Is one fifth theoverall size of the target (Fig. 13..2).

(d)The targets are of diminishing size: The largest having a viewingdistance of 60 metres (m), with smaller letters for distances of 36 m,24 m, 18 m, 12 m, 9 m, 6 m, 5 m, and 4 m.

2- The distance between the patient and the targets of the test charts:(a) Usually at six metres from t e chart:

a) A normal eye reads the six m size from a distance of six m and issaid to have 6/6 vision.

b)A weaker eye may only be able to resolve the larger letters, e.g. the24 m size and is said to have 6/24 vision.

(b )If for any reason the patient reads the chart fr in a different distance:The numerator of the acuity is changed accordingly, e.g. if the test isdone at four metres the above examples become: a) The normal eye4/6 vision. b) The weaker eye 4/24 vision.

(c) A normal person w 10 reads the six m etters from a distance of six 111

(i.e. 6/6): Is be able to read the four m size letters liOln the samedistance (i.e. 6/4).

2) Chart projectors:1-These are optically identical to slide projectors (Fig. 13.4)!

(a )Slide projector:a) Slide 0102: Lies between F1 and F2 of the projection lens L,

through which the light source S is imaged after passing throughthe condensers C and the slide,

b) Image 1112: Is inverted, magnified and projected on the screen.(b) Chart projector: The charts are incorporated in rotatory discs

(between the illumination source and the projection lenses).2-Components:

(a)Light source. T~e filament of the bulb is imaged at or close to theprojection lens system (after passing through the condensers andthe chart) to get the maximum amount of light which passes throughprojection lenses and keeps illumination of screen even (Fig. 13.4).

(b)The projection lens system: Is usually composed of two lenses, bothofwhich can be independently moved back and forth to allow theexaminer to alter the magnification of the chart to the correct valuetor the projection distance used. .

(c)The charts: Incorporated in otator1' discs situated in-between the lightsource (bulb filament) and the projection lens system.

(d)The screen: The majority of screens used specularly reflect theincident illumination in order to retain any polarization.

ROTATING OISC'

WHICH CONTAIN THE

DIFFERENT CHARTS

(a)Slide projector. (b)Cltart projector.Fig.13.4:0ptics of slide and chart projectors.

3) Video acuity tester:1-This is a type of test chart which is based upon a television monitor.2- The display on the monitor is controlled by a keyboard which allows the

examiner to select lett~rs at different sizes and store, in a small memory,the visual acuity of the patient.

4) The auto-acuitometer:1-This is an instrument used for automatic measurement of visual acuity

and near vision.2- It contains a small computer, a random access slide projector and a joy

sti ck Iever:(a) The computer would present to the patient a' slide which contains a

single, Landolt C and then wait for the patient to respond by pushingthe lever in the direction of the gap in the C (Fig. 13.5).

(b) Depending upon the· patient's response, the computer would thenpresent either a smaller or larger symbol.

(c)At the end of a series of measurements, the computer would then\:'aku\ate the patient s acuity and print out the result.

(2) Nonverbal vision tests:These methods determine the potential visual acuity in infants and youngChildren mainly, in illiterates,in malingerers and in mentally handicapped:l)Assessment offixation:

I-Fixation preference: The corneal reflex is observed while the child isfixing a pen light:(a) Central steady maintained fixation (C S M): Means good vision.(b) ncentral, unsteady and unmaintained fixation: eans poor vision.

2-Basc-up prism: To dissociate the eyes with better assessment offixation of each eye.

3-Fixation and following of light or toys: Can be done even with lessthan 6/60 vision.

4- ouc ing or picking up of small beads or obje .-Confirms that thechild is seeing.

5-Brueckner method:' The observer looks through an ophthalmoscope toobserve both the red reflex coming back from the eye and the pupilbehavior-7lf the patient fixes the light, the pupil gets smaller and the red~'etlexbecomes darker than when he is not fixing.

2)PreI'r0J1.zallooking:Movirig striped and grey cards are presented to theinfant-7lJased on the observation that infants are more interested inlooking at patterned (striped).than other equivah:mt stimuli (Fig 13.6).

3)Catford drum (evoked optokinetic nystagmus): drum with striped pattern(of white and dark bands) is moved across the subject's visual field in onedirection to elicit optokinetic nystagmus and its presence noted byobservation or by electro-oculogram to record eye movements (Fig. 13.7).

(a) Normal: (b) Abnormal.Fig.13.6:Pr·cfcl·cntiallooking. Fig.13.7:Catford drum. Fig.13.8:Visual evoked response.

4) Visual evoked re::,pOJ1se(VER):efinition: It is a method by which the action potential produced byelectrical activity of the visual cortex is recorded in response to light orpattern stimulation of the eye (Fig. 13.8):(a) Flash VER' Represents the function of the central 20° of the retina.(b)Pattenz VER: Represents the function of the fovea.

orma response: I

(a) Double-peaked response, the smaller wave being related to visualacuity (the smp.ller wave is altered by affections of the eentralvisionas in macular or optic nerve disease affecting the papillo-macularbundle).

(b)Opacities of the ocular media do not affect VER.Value:

(a) Assessment of refractive error.(b)Objective testing of the visual acuity in infants and young chi ldren.(c) Electro-physiological study of the optic nerve disease, colour

blindness and amblyopia.

(2)(3)(a)~

(b)NVv\.

(5)(6)(7)

TES~(I)0)

CONby

GLASPA"

De

2) NEAR VISION .DEFINITION: The power by which the smallest types can be read comfortably.FACTORS AFI·ECTJNG THE NEAR VISION:

(1) Retrdctlve errors:1) Hypermetropes and corrected aphakics for far hold the card beyond 33 em.2) Myopes hold the card nearer than 33 em.

(2)Age: In old age with presbyopia the near vision distance is more than 33 em.(3),l'r;cOl"Ylmodation: Diminished accommodation leads to an increase in the near

vision distance.(4) Convergence: Convergence anomalies affect the near vision (affect the near

point of convergence).(5) Pupil size: Increased pupil size leads to peripheral and spherical aberrations.

CLINICAL TESTS FOR NEAR VISION:(1) The Snellen's letters (Snellen's equivalent) for near vision: A photographic

reduction oCthe Snellen's letters to 1/17 times is used.(2)Jaeger's test types: These are graded s.izes of le~ters of pleasing types.(3) Modified Jaeger's test types: Graded sizes of modern types on test cards

(J I ,J2, ... etc.).(4)The Faculty notation:

I) Used Times Roman typeface with standard spacing and specimens of printingare given in sizes 5 pt., 6 pt., 8 pt., 10 pt., 12 pt., 14 pt., 18 pt., 24 pt., 36 pt.,and 48 pt.) upon a white paper.

2) The near vision is recorded as N followed by a number indicating the typeface size (i.e. N.5 is printed in 5 point type, N.8 in 8 point type, ... etc.).

(5) Numbers and broken rings with Snellen's letters.(6)Arabic letters or numbers.(7) The auto-acuitometer.

TESTING FOR NEAR VISION:(I) The patient sits on a chair: With a good light thrown over his left shoulder.(2) The smallest test type:Which he can read comfortably with a note of the

appropriate distance at which the test card is held is the near vision (forexample: N.V. = J 1 at 33 cm in Jaeger notation).

(2) CONTRAST SENSITIVITY AND GLARECONTRAST SENSITIVITY:ls the resolution of picture elements of visual image

by the retina-brain processing system which is better with finer picture elements.GLARE: It is a strong unpleasant light which leads to dazzling.SPATIAL FREQUENCY:

Definition: It is the number of black and white bars or strips (cycles) on a target asthe letter E on Snellen's chart within degrees of angular subtense (degrees of theangle subtended by the spacing between Je black bars) and is described interms of cycles/degrees (cpd).

Explanation:(l )One black and one white ban. = 1 cycle.(l){(this cycle subtends 1 degree (60 minute~) if arc: It is 1c/d (1 cpd).(3) Therefore spatial frequency of 30 cycles/degree (30 cpd), means that:-

1)Each black bar of letter E on Snellen's chart subtends 1 minute of arc at 6/6.2)Each white bar of letter E on Snellen's chart subtends 1 minute of arc at 6/6.

180(4JSpafial frequency: ::;:Snellen's denominator in metres

600Snellen's denominator in feet

180 ( 600 )So, 6/6 vision in metres (20/20 in feet) has a spatial frequency ofT = 20

= 30 cpd. So, 30 cpd=6/6=20/20.Sine waves and sine squares pattern:

(l)Sine waves If the light intensity is plotted across the retinal image of a blackbar against a white background (white bar), a sine wave pattern is obtained.

(2)Sine squares: The sum of sine waves of different spatial frequencies,amplitude and phases gives rise to a sine square pattern.

FACTORS AFFECTING THE CONTRAST SENSITIVITY:(l )Spatial ji'equency:Contrast sensitivity is decreased with more spatial

frequencies.(2) Age: Contrast sensitivity is decreased with age due to:

1)Increased scattering by the lens.2)Decreased ability of the retina-brain processing system to enhance contrast.

NB:ln young patients with eye disease a~ cataract or corneal oedema: Contrastenhancement mechanism will improve the fuzzy picture ami its details.

(3) Hypoxia: Contrast sensitivity is decreased in high altitude hypoxia due to retinalor cerebral disturbance.

(4)Light scattering by opaque tissues.;Contrast sensitivity is,decreased with opaquecornea or cataract 'in the face of glare source (in spite' of good visual acuity)which causes light to spread diffusely over large areas of the retina withfonnation of a poor retinal image and.

(5) Wavelength: Contrast sensitivity is decreased ( with poor discrimination) withlarger R-G WLs.

(6JIllumination: Contrast sensitivity is decreased with decreased illumination 'in thescotopic range.

(7)Bar width: Contrast sensitivity is decreased with decreased bar width.(SJCrating llIo/ion: Contrast sensitivity is decreased with increased motion.

NB:The cornea is transparent due to: The arrangement 0/ the collagen fibrils (withrefractive index of 1.47 i.e. close to that of glass) ami the mucopo~vsaccllaride matrix(with refractive index of 1.33 i.e. sim'l(ar to water) with spacing of less than half awavelength light. )

3GLAF

DefVall

l

CLINICAL CONDITIONS AFFECTING CONTRAST SENSITIVITY ANDGLARE:(1) Optical conditions:

1)Corneal conditions Decrease contrast sensit.ivity due to increased lightscattering in:1- Corneal oedema. 2- Contact lens problems.3- Keratoconus. 4- Keratoplasty and refractive surgery.

2) Lens coridi/ions.1- ataract and opacified posterior capsule: Decrease contrast sensitivity

by increasing light scattering as in corneal conditions.NB: 4 mm YAG laser capsuh)tomy is good for dayiiehi imi 6 mm Yl\(; laser

capsulotomy may be needed at night (with pupil dilatation): To avoid a"annoying glare from an a"noying IIeadligllts.

2- Intraocular lenses (IOLs):(a)Dis ocaled IOL: .Light rays coming through the aphakic portion of the

pupil are focussed only by the. cornea with a res·~lllant poor retinal imagewith decreased contrast sensitivity.

(b)Jvu7t[(oca/lOL: Concentric bifocal IOL with distance and near opticalpowers results in one blurred image and another sharp image on theretina with decreased contrast sensitivity.

(2) N<Hloptical conditions:1)R~tinal diseases 1- Diabetic retinopathy (due to macular thickening).

2- Macular degeneration.2) Optic nerve affections I-Papilloedema.

2-0ptic neuritis.3-Glaucomatous cupping and atrophy.

~B:Contrast sensitivity c~m be used for ~lsscssmcnt of diminished visual function dueto (:arly glaucoma:Because about IIalf of tile. optic nerve fibers may be lost beforedetection of allY scotomata by routine perimetry.

3) Strabismic ambZvopia. Can be assessed by contrast sensitivity testing.GLARE AND CONTRAST SENSITIVITY TESTS:

Definition: Are clinical tests for visual function which are described by a graph.Va'lue:

(l)Contrast sensitivity lestin : It describes a number of levels of vision and sois more accurate than chart visual acuity for assessment of the progress of theabove clinical conditions.

(2)Glare testing: It describes the inc·rease in light scattering in different clinicalocular conditions.

Principles:(1)Contrast sensitivity targets. Different size lines of targets with different

grades of grey.(2)Glaring light: rr'o decrease the contrast of the target if the patient has a light

scattering lesion.

Elements of testing:(1) Contrast sensitivity targets:

l)lllumination:.oftarget is needed for tests done under dim illumination.NB: Standard luminancc:(l) For visual acuity projectors:; J 20camlela/11I2.

(2) For contrast sensitivity targets: 85 candela/m2.2)Target details:

1- Type of targets' (a) Letter targets.(b) Grid targets.

2-Size of targets: Targets should subtend an angle of 20-60 becausecontrast sensitivity is not only a foveal function:-(a)The size of the E letter on the Snellen's visual acuity chart depends

on the angular subtense of the letter:a) 6/6 letter subtends an angle of 5 minutes or arc.b)White spaces between each black bar of E subtend Iminute of arc.

(b)The alternating grid pattern: Uses cycles per degree (cpd).3- Gradations within targets' Different levels of contrast sensitivity and of

spatial frequencies.3}Test media: 1- Printed tests (plates).

2- Slides in a projection device.3- Video-display channel.

MACUI(1) Me

1)2)3)4)5)6)

(Y 7,

(2)Glare testing:1) Intensity of glare:'

1- From 100-400 candela/m2: For cataracts and corneal conditions.2- More than 400 candela/m2: For special tasks as night driving.

2) Position of the glare source: 1- Adjacent to the target.2- Surrounding the target (is better).

3) Standards for glare disability. To simulate the work of the patient.(3)Contrast sensitivity curve:

1) Shape: ~s the same in all groups of patients.2) Position: 1!... Normal position.

2- Moved towards lower contrast.3.1\10ved towards lower spatial frequencies.

The clinical tests:(1) Contrast sensitivity testers:

1) Printed tests:!1- Pelli-Robson letter chart. Consists of 8 rows of 6 letters all of the same

size but decreasing in contrast every group of 3.2- Arden gratings: pattern of stripes is exposed from low to high contrast

in 6 plates studied at 57 cm with spatial frequency increasing from 0.2cpd to 6.4 cpd, each being double the frequency of the previous one.

2) Slides in a projection device: 'With slides of letter or grid targets similarto those of printed tests.

3) Video-display C lannel- By which the letter or grid targets are presentedon the screen.

(2) Glare testers: 1 Brightness acuity tester (BAT).2) Miller Nadler glare tester.

(3) MACULAR FUNCTIONSDEFINITION:The visual sensations on stimulation of macula (cones) with light:

1- Form sense (details).2- Colour sense (colours).

MACULAR'FUNCTION TESTS:(1) Macular function tests with clear media:

I) Visual acuity.2) Pupil/my reactions3) Colour vision tests.4) Ophthalmoscopy.5) Slit-lamp biomicrosco y(with jfruby lens and Volk lens).6) Amsler grid test.

(Y 7) PhotoslreS~'i lest: . .,. _ .1-The patIent, wIth corrected distance vIsual acuity, fIxates a 1Ight from a

pen~torch held about 3 cm away for 10 seconds.2- The recovery time after which the patient can read any 3 letters of the pre-

test line is:(a) Mo.re than 50 seconds in macular disease.(b) Less than 50 seconds normally.(c) Within normal limits 1::1 optic nerve lesions.

(2) Macular functio ests !\lith opaque media:These are tests for pot.ential visual acuity in patients with opaque media as incataract to confirm that the patient is seeing but most of these tests give littleinformation about the actual level of vision (except. the interferometers):1) Perception of-light (PL : PL means abn0l1Tlal retina or optic nerve.2) Pupillary_ reactions. Arc abnormal in optic nerve damage.3) Colour Zl£ferenfiation ,'Ith rea. green and blue coloured glass discs: Is good

if the macula i" healthy.4) Nonverbal vision test: Chapter 13.5) A1addox rod test.

1-Maddox rod is put before the catara_ctous eye, while covering the other eye2- The spot of distant light appears as a red line (which is unbroken) if the

macula is healthy.6) A blat.:k disc with multiple pelJoratio/1s or with 2 pinholes):

~-Is placed in a trial frame in front of the eye and light is thrown over it.2- The patient can count the number oflights (holes) if the macula is healthy.

7) Fovea! ERG: s abnormal with diseased macula.

8) Entoptic retinal view test (Purkinje vascular tree): A 'patient with a normalfundus can see the entire retinal vasculature if his eye is closed and the globeis massaged with a torch through the lower lid (scotomata may be noticed bythe patient).

9) Haidinger's brushes. Is an entoptic phenomenon seen when viewing a plane-polarized field through a blue filter:1-The normal macula can appreciate a flare as the Henle's layer in the macula

is oriented to polarize incident light.2- The coordinator utilizes this phenomenon in evaluation of macular function

and testing of eccentric fixation in an amblyopic eye.10) Laser interferomete . Chapter 26.

C E 4RETINOSCOPY (SKIASCOPY)

DEFINITION :Accurate method of objective measurement of refractive state of eye.METHODS OF RETINOSCOPY:

Fig. 14.1: Simple method of i'IIumination in ret.inoscopy.(1) The simple method of illumination: A plane or concave mirror ret1ects an

external source of light. (was used before the use of the self luminousretinoscope, with the same principles:I)A light source is located beside the patient's head.2) The light is reflected from a plane or concave mirror, held by the observer

into the observed eye.3)The observer views the observed eye through a small hole in the mirror:

(a)Light is reflected from a plane mirror (the image is erect, virtual, same sizeand at same distance behind the mirror, Fig.14.1 a) or,

(b)Light is reflected from a concave mirror (the image is inverted, real and isbetween the observer and the observed eye! Fig.14.1 b).

ICiJ'. r:==:

~, \ I I

Fig~ 14.2: Electric retinoscope...(2) The self luminous retinoscope:

Components:l)A light source.2) A strong convex lens (the ·condensing lens).3) A mirror at 45°: 1- To give both systems of illumination (plane and

concave mirror effects) .2- To project the light onto the patient's eye.

Plane and concave mirror effect Both can be provided by altering the distance.between the light and the condensing lens by moving this lens within theshaft of the instrument to vary the angularity of the light leaving the mirror: .1) At one extreme with the shaft of fhe instrunient at its lowest position:.

A plane mirror effect (the rays are parallel) is obtained.2) At the other extreme with the shaft of the instrument at its highest.

position: A concave mirror effect IS obtained (the rays converge at a pointclose to the instrument).

3) At an intermediate position: A focussed image of the retinoscope bulbfilament falls on the patient's eye and the effect is that of a concave mirrorof focal length equaling the distance between the observer and the observedeye and is of NO value in retinoscopy (as the image of the light source iscoincident with the observed eye).

4) Just below the intermediate position7fhe retinoscope acts as a concavemirror of focal length slightly exceeding the distance between the observerand the observed eye, but with a plane mirror effect (as a virtual image S 1 ofthe light source is formed just behind the observed eye, Fig. 14.2) which isuseful in clinical practice for two reasons:

(a) A brighter illumination (than that produced by the plane mirror effectwith the shaft at its lowest position) which makes retinoscopy easierwhen the pupil is small and when there are opacities in the media ofthe eye. ()~

(b) The plane mirror effect is retained (which most observers prefer).The types of self luminous retinoscopes:

1) The focussing spot retinoscope:' The source of light used produces acircular image.

2) The streak retinoscope: The source of light used produces a linear image(Chapter 14).

3) The dynamic retinoscope:/ Which is used for dynamic retinoscopy(Chapter 14).

STAGES OF RETINOSCOPY:(1)The illumination stage:

Definition. It is the stage in which light is directed into the observed eye toilluminate the retina.

The source of illumination.1) When the plane mirr.or of the retinoscope with the plane mirror effect

is used as seen in 'Fig.14.3a:(n) Movement of light across the observed fundus is with the movement of

the plane mirror.(b) Vi11ual images S land S2 of light source S throw light via the nodal

point and so the movement of illumination at the retina R1 and R2 isWIth the movement of the l"nirror M I to M2. ,

Z) When the concave mirror or the retinoscope with the concave mirrore eel is used.The concave mirror of25 cm focal length (i.e. less than thedistance between the observer and the observed eye) or the retinoscopewith such effect is occasionally used for retinoscopy as seen in Fig. 14.3b:

I-Real images Sl and S2 of the light are formed between the observedeye and the observer and act as a light source via the nodal point forillumination of the observed retina.

2- The movement of illumination of observed retina is in the oppositedirection or against the movement of the mirror t-.11 to M2.

Conclusion: The illuminated fundus area moves with the movement of the planemirror and against the movement of the concave mirror, irrespective therefractive state of the observed eye.

fl)MoFemellt of light is with (b)Movemf!llf 0/ illumillatioll at retina R I Rl.as that ofpilllie mirror. is with that of mirroriW [to M2.

Fig.14.3: Illumination stage of rctinoscopy:Phlnc mirror or plane mirror effect.

Fig.14.4: I lIuminalion stllgc of rctinos'~opy:C()"C(lve mirmr or COllcavemirror effect.(2)The luminous reflex (the red reflex) stage:

Definition. ~he formation of the image of the illuminated retina of the observedeye at its far point(E at infinity, H behind the eye and M in fi:ont of the eye).

COliS ruetion oIffze image of the illuminated retina of the observed eye at its jarPOitl :An image Al B] of the illuminated retina of he bserved eye is formedat its far point and C<:!n be constructed by two rays:

i-A ray from a ret.inal point B passes parallel to the optic axis and comes toa tocus at Fa (Fig] 4.5).

2-A ray from B pas<::csthrough the nodal point N undeviated ..NB:Thc following description ~lIld the diagnnuswith the plane mirro.· cffc ~l:

Are usually preferred in Retinoscopy.

.81-; ...• ~:::.::::.::-I U-

AI . A

n p

~.~

.::::;;:~1A· N .

81

a)Emmetropia. (b)Hypermelropla.Fig.14.5:Reflex stage of retinoscopy.

(3) The projection stage:Definition: It is the stage in which the image at the far point of the observed eye

(the reflex stage) is located by moving the illumination acro"ss the fundus andnoting the behaviour or the luminous reflex seen (projected) by the observerin the pupil of the observed eye.

"':--<o:'~____ 'H'·A, 0* ~~ -u.

(b)

-B - n -~ ....__ -~'''~'_- -:::--- •.._---:~-- .... _- ------

---_ No

1 ~.8 .. ~ No Ao 1\_1---=--;;:: --- --IAN· -- [l I_ 0 I

. -='=" :.:.:::~J,o1

Fig. 14.6: Projection stage of retinoscopy: .(a) Emmetropia: (b) Hypermetropia; (c) Myopia less titan -I D for working distance of I metre,'(d) Myopia of -1 Dfor working distance of J metre (neutflll point); (e) Myopia of more tltall -I Dfor working distance of I metre; (f) Myopia of a ftigher degree tfum ill (e).

. Constructi0l1 of the pi'ojected image by observer in the pupil of observed eye:1) Image A IB I of illuminated retina of observed eye at its far point is

constructed first (Fig. 14.5).

2) Then the projected image by the observer in the p~piJ of the observed eyeis constructed' by drawing an additional hypothetical ray from B 1 passingthrough the nodal point No of observer to observer's retina RO (Fig. 14.6).

3) This ray locates the point Bo, the image of B 1 on the observer's retina.Viewing of the projected image by the observer.' .

1) The observer views the image Al B 1 of the illuminated retina AB from aconvenient distance usually 2/3 or I m.

2)The observer does not see actual image AIBI but rays from AIBI are seenas illuminated area or reflex which is 'projeeted in pupil of observed eye.

Behm'ior of the luminous red reflex rprojected in the pupil of the observed eye)in relation to the movement of the illuminated fundus area of the observed ~Veusing a plane mirror:

1) he direction of t· IC n~d I'CtlCX IllOVCl11cn (:

1- In an e111117efl'Opic(E) ohserved eye: The lumiltous r.ed reflex moves inthe same direction as the illull)inutcd fundus area or the observed eyeand so in t.he same direction .of the movcment of the plane mirror (withmovement) (Fig. J4.Ga).

2- In a lypermetropic (H) observed eye: The same as in an emmetropicobserved eye (with movement) (Fig. 14.Gb).

3- /n a myopic (1\1) observed eye: The movement of the luminous redreflex varies with the position of the observer in relation to the 1~11'point(conjugate focus) of the myopic observed eye:(a) f ar 011 01" tIe myopic 0 serve eye IS Be HIIO ( lC ° server':

a) The myopia of the observed eye is less than the dioptric value orthe observer's working' distance (the observer?s working distanceis the distance between the observer and the observed eye).

b) Therefore in M observed eye of less than ID at 1m observer'sworking distance, the red reflex 1 loves in the same direction as inEar H~bserved eye (with movement, Fig. 14.6c).

(b) c ar p-omf of. olJscrvcl cyc coincil cs '\'11I tiC 0 cr 'cr'sa pomt:

0)1 0 una rc '. can bc1ixmed on observer's relina (Fi J. ". ~c; .

a-At this point 110 movement of the luminous red reflcx can beseen in the pupil of the observed eye.

b-The observer sees a diffuse bright red reHex with nomovement because movement of red refl~'x is infinitely rapid.

b) ell/rul pain. Which is the principle ofretinoscopy test in which all conditions of refraction are mademyopic cqual to the dioptric valuc of the observer's workingdistance (myopic value of working distance) i.e. a11conditions ofref/'action are myopic of 10 at 1m observer's working distance.

(c) t e ur oint of M obscn:cd cye is in front 0 he 0 )SCI"vcr':

a) If the far point of M observed eye is in front of the observer(between the observer and the observed eye), the myopia of theobserved eye is more than the dioptric value of the observer's

. working distance.b) Therefore in M observed eye of more than 1D at 1m observer's

working distance, the luminous red reflex moves in the oppositedirection to the illuminated fundus area of the observed eye andso in the opposite direction of the movement of the plane mirror(against movement) (Fig. 14.6e and 14.61).

2) The rapiility of the red reflex movemcllt:rrhe angle AoNoBo in Fig. 14.6decreases with less rapidity of the Ted reflex as a refractive error becomesmore than the dioptric value of the observer's working distance(I 0 at 1m)and so the red renex appears to move more rapidly as this point isapproached ( at this point no movement is seen because it is very rapid).

THEDe

1

Cal(

Fig.14.7:Field of illumination in retinoscopy. Fi!!.14.8:Field of view in retinoscof)',THE FIELD OF ILLUMINATION:

Definition:The part of the observed fundus covered by the image of the immediatesource of light (which is the image of the original source of light of 25 mmdialheter and at 25 cm behind the observed eye).

Calculation:

(I) Using the plane mirror, the field of illumination can be calculated from thefollowingforl71ula (Fig. 14.7):I v

- Where,o LJ

0= The immediate sourceoflight (which is the image of theoriginal source oflight of25 mm diameter situated 25 cmbehind the observed eye). .

I = The illuminated area of the observed eye ..v = The distance from the nodal point to the retina and is 15 111111 from

the reduced eye.u = The distance of immediate source of light from the nodal point of

the observed eye.(2) The distance u is calculated as follows:

1) The distance between the observer and the observed eye is 100 cm.

2) The distance of the original source of light from the plane mirror = 100 +25 = 125 cm.

3) The immediate source of light is 125 cm behind the plane min·or.4) Therefore the immediate source of light is 125 + 125 = 250 cm (2500

mm) from the nodal point of the observed eye.(3) Therefore thefield of illumination J will be: I = 2 5 X. 15/ 2500 = 0.15 mm.

THE FIELD OF VIEW (FIELD OF VISION):Definition: The part of the observed fundus covered by the image of the observer's

pupil assuming that the observed eye is myopic 1 0 with 1 metre observer'sworking distance.

Calculation:(1) TheJield of view can be calcualtedfrom thefollowingformula (Fig. 14.8):

I b ~~ I Where,

0= The diameter of the observer's pupil (4 mm).I = The image of observer's pupil which is the lilnit offield of view.

v = The distance from nodal point to retina (15 mm from reduced eye).u = The observer's working distance (the distance between the observer

and the observed eye) = I metre (1000 mm).(2)Therefore the field of view: 1=4 X 15/ 1000= 0.06 mm.

Characters:(l)Thefield of view (0.06 mm) which is the portion of the observed fundus seen

by the observer is much smaller than the field of illumination (0.15 mm).(2)This small field of view is surrounded by darkness and so we must notice

that:I) If the plane mirror (i.e. the immediate source of light) is moved, the

illuminated area of the fundus will be followed by a dark area called theshadow.

2) In retinoscopy, it is more accurate to observe the movement of the centralpart of the red reflex (the red reflex movement) than to observe thejutiction between the illuminated area i.e. the red reflex and thenonilluminated area i.e. the shadow (the shadow movement).

THE OPTICAL PROPERTIES OF TI-IE RED REFLEX:(l)The brightness of the red reflex: The brightn,ess of the red reflex (the intensity

of the illuminated fundus area of the observed eye) is affected by the followingfactors:1) The intensity of the source of light Increased intensity of the source of light,

increases the brightness of the red reflex and vice versa.2) 'he type ofmirror or mirror effect used:

1-A concave mirror or a concave mirror effect of the retinoscope gives abrighter red reflex.

atem

2- Retinoscope with the condensing lens acting as a concave mirror but with aplane mirror effect also gives a brighter red reflex.

3- The plane mitror or a -plane mirror effect of the retinoscope gives a lessbright red reflex.

3) The distance between the source of light and the observed eye: Increaseddistance will diminish the brightness of the red reflex'and vice versa.

4) The refractive state of the observ.ed eye: he lower the error of refraction thebrighter the red reflex and vice versa (Fig. 14.9):1-The red reflex is brighter in M observed eye of 1 D because the illuminated

fundus area.,R 1 is small with a great amount of light per unit area.2- The red reflex is dim in M observed eye of 10 D because the illuminated

fundus area R2 is large with a very small amount of light per unit area.

Fig.I4.9:A smaller illuminated fundu~ al'ca on RI of -I 0 than on R;Zof -10 D myopia.(2) The movement of the red reflex:

1) The rapidity o.fmovement of red reflex: his depends on the following:I-Tile raplility of movement of the mirror or retinoscope The more rapid

is the movement of the mirror or retinoscope, the more rapid is themovement of the red reflex and vice versa.

8,_10

Fig.14:10:Effect of refl"active state of observed eye on rapidity of red reflex movement.2- Tlle refractive state of the eye: ,

(a) In emmetropic observed eye (R I in Fi"g. 14.10), with the dioptric valueequal to the observer's working distance(i.e. 1 D for 1 m) there is nomovement of the red reflex because it is infinitely rapid(as the distanceAl B 1 is very long)~

(b)In a low refractive error of the observed eye the movement of the redrcncx is rapid.

- -(c)In a higher refractive error of the observed eye the movement of the red

reflex is slow:ln a myope R3 of -10 D, the movement of the red reflexAlB 1 is slower than in a myope R2 of -5 D (the distance AlB 1 iscovered by the same movement in both cases and so the shorter distancebetween AIBI the slower the movement of the red reflex).

3- The distance between the observer and the observed eye.4- The distance of the original source of light and the mirror: When the

mirror method is used.2) The direction of171Ovement of the red reflex:

1- With movement:t If the refracted meridian of the observed eye is E, H orM of less than 1 D with 1m observer's working distance (or less than 1.5 Dwith 2/3 m working distance).

2- A:gainst movement: If the refracted meridian of the observed eye isi11yopic of more than 1 D with 1 metre observer's working distance (ormore than 1.5D with 2/3 m working distance).

3- No movement: lf the refi-acted l1}eridian of the observed eye is myopic of _ID with observer's working distance 1m (or 1.50 with 2/3 m observer'sworking distance).

Fil?14.U:The shapes oHhc I·cd refleX.(a)ROlillded RR;(b) Oval RR with curved sides; (c) Oval RR with Jlraiglzt side.\- (banded red

rellex):((/) Scissors RR: (e) Spil1niflf! RR: (f) Bright RR with paracentral sluulOlv.(3)Theshapes of the red reflex:

I) Rounded red reflex If the observed eye is E, H or M (Fig. 14.1Ia).2) Oval red reflex: If there is regular astigmatism of the observed eye because

l11agniJication ditTers in the two principal meridians:1- Oval red reflex with curved sides: If both meridians are ametropic (Fig.

14.1 Ib).2- Oval red reflex, with straight sides (banded red reflex): If one meridian

is E which appears as a straight line along which is the axis of the cylinder(Fig 14.11c).

3)Scissors red refleJ;. In irregular astigmatism( corneal opacities or posteriorstaphyloma (Fig 14.11 d).

4) Spinning red reflex: in keratoconus of the observed eye (Fig. 14.11 e).5)Bright red reflex with paracentral shado'w: In spherical aberration produced

by the lens of the observed eye whose pupil is dilated (Fig. 14.11 I).

(4)Aberrations in the red reflex:I)· egafive aberration:

Caus : Keratoconus of the observed eye with more curved central part of thecornea.

Princi Ie (Fi . 14.12}:1- 'he central rays: Come from the observed eye E and pass through the

more curved (highly myopic) central part of its cornea and so the centralpart of the red reflex moves slowly against the movement of the planemllTOr.

2- The aracentral ra s: Do not enter the pupil of the observer eye Eo andso there is a paracentral shadow in the red reflex.

3- ~ e perl eral rays: Pass through the less curved (less myopic)peripheral part of the cornea but are refracted more than the paracentralrays and enter the observer's pupil and so the peripheral part of the redreflex moves rapidly against the movement of the plane mirror.

Effcct: The red reflex movement is rapid peripherally which appears to spinsround the central part of the red retlex which moves slowly i.e. a spinningred reflex which can be avoided if we look to the central part of red reflex.

Fig.14.12: - vc abcrration(kcratoconus). Fig.14.13: + vc abcrration(sphcrical aberration).2) Positive aberration:

ausc' Spherical aberration produced by the lens of the· observed eye whenthe pupil is dilated.

Principle (Fig. 4. 3)~1 The centra rays: Come from the dilated pupil of the observed eye E and

enter the pupil of the observer's eye Eo and so the red reflex is brightcentrally.

2- The paracentral rays: Do not enter the observer's pupil and so there is aparacentral shadow in the red reflex.

3 'Ie peripheral rays: Pass through the periphery of the lens and are morerefracted than the paracentral rays due to spherical aberration and enterthe observer's pupil and so the red reflex is bright peripherally.

Effect: )1 paracentral shadow is seen in the red reflex whieh can be avoidedif we look to the central part of the red reflex.

THE NEUTRAL POINT (POINT OF REVERSAL):Definition: It is the principle of retinoscopy test in which all conditions of

refraction are Imide myopic of a value equal to the dioptric value of theobserver's working distance(i.e.l D at 1m or 1.50 at 314m).

The optical condition at the neutral point:

(1)1\0 movement of the red reflex: This is because the observed eye is renderedmyopic of a value equal to the observer's workillg distance i.e. with theobserver at the far point (conjugate focus) of the observed eye (i.e. 1mdistance to lender the obsel;ved eye myopic ID or 213m to render theobserved eye myopic 1.50, Fig.14.6d).

(2)Brightness of the red reflex:

(a)Brightest. (b)DulI(dim). (c)Dark.Fig.14.14:Brightest RR at neutral point. Fig.14.1 S:3:Changcs in brightness of RR at NI>.

I) Brightest rcd .·eflex: At the point of reversal, with no movement of thered reflex (Fig.14.14):

1- AB is the image of the observer's pupil on the observed retina R at thepoint of reversal (this is the field of view).

2-X Y is the illuminated area of the observed fundus (this is the field orillumination which is larger than the field of view).

3-AjBlis the image of the pupil of the observed eye on the observer'sretina Ro.

4- Therefore AB and A IB I are conjugate foci at the point of reversal.5- Therefore at one position of the mirror, AIB I will be at maximum

illumination i.e. the red rcnex is brightest when AB is inside XY (Fig.14.14 and 14.15a).

of 2) ull (dim) red reflex:iL\ dull red reflex Occurs if the mirror is changedslightly(Fig. 14.15b shows that XY has moved with slight change in theposition of the mirror so one edge of the image AS of the observer's pupilis in darkness while the other edge is in light).

3)Dark red reflex: A black red reflex occurs with further movement of themiITor(Fig. 14.15c shows that the mirror has moved more and XY hascompletely left AS and now the image AS of the observer's pupil iscompletely dark).

)

CLINICAL PRACTICE F RETINOSCOPY:(1)The visual acuity test: 1) Without glasses.

2) With corrective glasses for far if present.(2)A myd. iatic- ycloplegic drug prior to retinos.copytest:

1)Alropine sulphate ~ye ointment 1%: Is given t.d.s. for 3 days to dilate thepupil and to paralyze completely the accommodation in children under 12years.

2) C cIo entolate 1% drops or tropicamide 1% eye drops: very 15 minutes forone hour to dilate the pupil and to paralyze partially the accommodation inpatients aged 12.,..40years.

3) Without any mydriatic: In patients over 40 years but if necessary the tensionshould be taken first and a miotic given after the retinoscopy test to constrictthe pupil again for fear of glaucoma.

(3)The retinoscopy test in the dark room:1) Vie distance between the observer and the observed eye:

] - convenient distan~c of 1 or 2/3 Ill: distance of 1m (at which theobserved eye is rendered myopic 1D) or a distance of 2/3m (at which theobserved eye is rendered myopic 1.5D) is convenient for:

(a) Putting lenses in the trial frame easily.(b) Seeing a good red reflex.

2- A shorter distance of 1/2 III : Brovides a brighter red reflex but with ahigher error.

3-A longer distance of .s III : Provides a less error but with a less bright redreflex and a difficulty in putting lens.es in the trial frame.

2) The ource of light:1,;.With the plane or concave mirror method: lectric bulb on a stand or a

bracket.2- The retinoscope: It is self-luminous.

3) The mirror or mirror e.fJectused:1- Plane mirror with: (a) A diameter: Of 4 em.

(6) A sight hole: or 2 or 4 mm diameter.2-Concave mirror with: A focal length of25 cm.3- The retinoscope with: (a) A plane mirror effect.

(b) A concave mirror effect.4) The position of the observer and the observed eye:

1- The observer should wear his corrective glasses if present.2- The observer's eyes should be in level with the observed eye.3- The observed eye should be straight ahead to avoid accommodation and to

refract the macula.5) The movement of the mirror or the retinoscope:

1- T e type of the mirror or the mirror effect:A pl,ane mirror is the reverseof concave mirror as regards the direction of movement of the red reltex .

2-The direction of the movement of the mirror;. The movement of themirror should be vertical first and then horizontal.

3- The rapidity of movement of the mirror.. Should be neither quick nor tooslow.

6) The optical properties of the red reflex:1- The brightness, rapidity & direction of movement, shape & aberrations of

red reflex: Are assessed.2- We work on the central part of the red reflex: As it corresponds to the

macula to avoid:(a) Negative and positive aberrations.(b) The shadow.

7) Detection and checking of the neutral point:Metlioils of etection of t Ie neutral point:

J - With the plane mirror method or with the flJCIl .••·•••·illg ,"lJOtretinoscope: ,.(a)lf ther'c is a sptierical error only:

a)The movement of the red reflex is repeatedly observed withincreasing the power of the spherical lenses (convex or concave)placed in front of the observed eye until the red reflex movementis abolished and its brightness is at maximum and followed by acomplete darkness.

b)At the neutral point the observed eye is rendered myopic of IDwith trial lenses when the obserVer's working distance is I 111.

(b)lf thcre is a rcgular astigmatism:a)We get the neutral point of lower me.ridian which is corrected first.b)Then we correct the other meridian by either:

a- Higher spherical lenses.b- Cylindrical lenses with the axis along the band reached.

2- With the str.eak retinoscope: IPrindples: ,

(a e source· 0 Ig t· usea IS made to produce a inear imageinS ea 0 a clrcu ar image);" With a band of light appearing in

the pupillary aperture (streak reflex, Fig. 14.l6a).(b 1e strea rej7.ex moves wit 1 or against the band of light outsid~

t e u iI' Depending on the refractive state of the observed eyeand the type of the mirror or mirror effect used (Fig. 14.16b andc).

(c) The Irst meriaian is neutralized: At which point the streakdisappears and the pupil becomes completely filled with light(Fig. 14.l6d) or completely dark:(a) If.. all 11 erit it", .•.•' ar.· sim· ar~v lIelitraTi ell· There is no

astigmatism.(b) a and s7ll1ped reflex appears on any other meridian:

Astigmatism is present.

'Oua) ext ct~jJthr.()ug7ttlle a 'is of astigmatism A sharply

defined reflex band is seen which moves exactly parallel to theband of light outside the pupil, either with or against.

b) It es no pass exactly through tlte axis of astigmatism(Fig. 14.16e) :a- The reflex becomes more poorly defined and tends to

remain fixed in the astigmatic meridian producing a breakin the alignment between the reflex in the pupil and theband outside it and tending to be in a position intermediatebetween that of the latter and the true axis of astigmatism.

b- The axis even in the case of low astigmatic error, cnn thusbe determined by rotating the streak until it moves parallelwith the reflex.

c- .The astigmatic meridian is then sim~larly neutralized.

fC-"n.fo,,- ,lrj)

. FromRl:l''''~' Pupil

Fig.14.16:lmagc sccn withstrcal< "ctinoscopc: (a)Streak reflex; (b) Witlt mOl'emel1t;(c)Against movement; (d)Neutralpoillt; (e)Streak is /lot ill the axis of astigmatism.

The advantages of the streak retinoscope:(a) It allows the examiner to resolve quickly and more easily the

problem of the orientation of the principal meridians of therefraction.

(b )Easier recognition of reflexes in practice than with any otherretinoscopes.

I leu les III e ec IOn 0

i-If tzere is irregular scissors red reflex.: It iseliminated by either:

(a) The lens which causes thetwo bands of light to move (meat) in thecentre of the pupil.

(b) Al tering the gaze of the observed eye.2-lf there is k.eratocoilUS with spinning red reflex:

(a) We work at the central part of the red reflex.(b) The stenopaeic slit may be used:

a) It is placed in the trial frame and rotated till the patient sees best.b) This meridian is corrected first with spherical lenses.

c) Then the meridian at right angle is also corrected.3- If there is a spherical aberration by the crystalline lens with a dilated,

pupil: We work at the central part of the red reflex.ecKi tlie neutra .poin . After detection of the neutral point it can be

checked by:1- If the power of the lens in the trial frame is increased by O.5D The

movement of the red reflex will be reversed.2- If tlte observer moves his head backwards:' The red reflex moves

against the movement of the mirror.3- Iftlte observer moves his headforwards:'The red reflex moves with the

movement of the mirror.NB: The name "neutral point" is better than the name point of

reversal because if reversal of the movement of the red reflex istaken' this would mean overcorrection.

8) The recording of the retinoscopic result.";;1- This is usually done in the form of a cross which indicates the

ilcutralization point or thc two main mcridians and also their orientationFig. 14.17).

2- The right eye is recorded on the len half of the page and the left eye on theright half.

3- Examples of the retinoscopic results are presented in Fig. 14.17.

L

+aoo -"r-_wo

+1.00 _1.00

+J.oo +-4.00

Fig.14.17:Retinoscopy results(Rt eye on left half of page and Lt eye on the right half of page):(a)Right hypermetropia and left myopia;(b}Right simple hypermetropic asti~matism. and let-simplemyopic astigmatism;(c)Right compound hypermetropic astigmatism (with the rule)andleft compound myopic astigmatism (against the rule);(d)Right mixed astigmatism and leftoblique astigmatism;(c)Symmctrical astigmatism;(t)Asymmctrical astigmatism.

9) The calculation ofthejinal refractio : This is obtained by deducing:1-The dioptric va ue at the observer's working distance· 1D for 1m or

1.5D for 2/3m.2- The effect of ciliary muscle if paralysed with a mydriatic cycloplegic

during retinoscopy: 0.5-1 D.(4)Trial Hlder mydriatic: A trial of lenses under the mydriatic given is important

after retif'oscopy: 1) It detects improvement in vision.2) It detects the axis of the cylinder in cases of astigmatism.

(5) Post mydriatic test:I) The patient is asked to return for the post mydriatic test after:

1- 24-48 hours if a mydriatic was used.2- Three weeks if atropine was used.

2) The clinical tes(s.f(Jrfar vision:t - he test types l;Ised are

(a) Snellen's test types or Landolt's broken rings for adults.(b) Animal pictures for children (but if the child is uncooperative, suitable

lenses based on the retinoscopy test are prescribed).2- The te t chart should be:

(a) Four sided test types which is placed on a stand and can be changed byrevolving ..

(b) Properly illuminated.(c) Placed at a distance of 3 m from a plane mirror, which ref1ects the

illumination with the test types, to make it easy for the examiner tostand by the test types and point to them (without the need for- anassistant to point to the test types if they are at 6 m).

3) The trial frame should be:1- Adjusted on the patient's face in front of his eyes.2- Accurately fitted on the bridge of the nose and round the ears.J- Not tilted or exerting pressure.

4) THetrial lenses:1- The right eye is tested first while the left eye is covered by an opaque disc

and vice versa.2- The lowest concave or the highest convex lens which gives the patient the

best vision, taking the retinoscopy record as a basis, is placed in the frame.3- The astigmatic correction if needed is added and the cylinder is rotated

until the axis is adjusted and the letters appear well defined.DYNAMIC RETINOSCOPY:

Definition: It is an objective measurement of the refractive state of the eye with thepatient's eyes fixed at a near distance while he is actively accommodating andconverging (unlike static retinoscopy with the patient's eyes fixed at infinity orwith a cycloplegic drug to relax the accommodation).

Uses and methods:(l)Defermination of the near point of accommodation:

I) A self luminous retinoscope is used for dynamic retinoscopy while thepatient is wearing the correcting lenses for distance as has beendetermined by the static retinoscopy.

2) The patient fixes and accommodates binocularly upon a target in front ofthe retinoscope (as the patient's finger held by the examiner).

3)The target is brought nearer and nearer to his eyes until the band of lightand shadow in his pupil is reversed despite his strongest effort ofaccommodation for the distance of the target.

4) The surgeon then moves closer to the patient until the reversal movementstops which measures the near point of accommodation (if it occurs at 33cm from the eye, the total accommodation is 3 D).

(2)Objecfive measurement of refraction when eye isfocussedfor near vision:I)The patient wears the correcting lenses for distance and he is asked to fix

binocularly and focus the target placed at his working distance (say 33cm).

2) With a plane mirror effect and a patient with full accommodative power, awith movement is obtained.

3)This is neutralized with the addition of convex lenses (+0.50D or +0.75 D)to the trial frame to give the low neutral point.

4) Further convex lenses are then added with gradual relaxation ofaccommodation till the shadow is reversed marking the high neutral point(unlike in static retinoscopy where a rapid reversal of the shadow isobtained).

5) This high neutral point represents an objective measurement of refractionwhen the eye is focussed for near vision which is an objective finding ofthe negative relative accommodation.

NB: Static and dynamic retinoscopy finding-.s:(1) Dynamic retinoscopy findings are taken if both static and (~vnal1lic

retinoscopy findings are similar.(2) Postcycloplegic test is essential if the static alul dynamic retinoscopy

findings are not similar.

5OPHTHALMOSCOPY--~(1) D RECT OP ALMOSCO Y

DEFINITION: The method by which we see an erect magnified image of fundususing direct ophthalmoscope.

THE DIRECT OPHTHALMOSCOPE: It is the instrument used for fundusexamination by the direct method.

Corneal fo( bulb' re lec/ion~a[]e

Fig. 15.1: Dir"cct ophthalmoscopy.COMPONENTS OF THE DIRECT OPfITHALMOSCOPE:(1.)Thedirect ophthalmoscope consists of:

l)Elecfric bulb· Halogen bulbs are used now as a source of light due to theiradvantages:I-Intense ~rightness and whiteness (higher luminance).2:-Greater blue portion of the spectrum to get a greater scattering and

fluorescence of transparent media.3-A bright red-free light in the presence of green filters (red-free filters) can be

cast on the fundus which is useful [or:-(a)Observation of the nerve fibre layer for defects as in early glaucoma.(b)Enhancement of the contrast of the blood vessels against the retinal

background.2)A system of lenses: Which focus the light source onto a plane mirror.

3)A plane mirror: 1 Where a real image of the bulb filament is formed(Fig. 15.1).2-Then the mirror reflects the emitted light in a diverging beam

which is used to illuminate the retina of the patient's eye.3-The mirror contains a hole through which the observer

can view the illuminated retina of the patient's eye.4-The image of the bulb is formed just below the mirror hole,

so its corneal reflection does not lie in the visual axis or theobserver to avoid dazzling.

5-Light reflected from illuminated retina of patient's eye passesback through hole in the mirror then into the observer's eye.

Accessories of the direct ophthalmoscope:1)A line figure' As an astigmatic dial for accurate focussing.2)Fixation star' A dot or a star shaped figure to determine eccentric fixation.3) Slit diaphragm. For observation of elevated retinalles~ons.4)Pinhole or half-circle diaphragm: 0 reduce reflections, by limiting the

. illumination beam.5)Red free filter(green filter): For a green light to detect certain retinal

conditions(in NFL or BVs).6)Bluefilter: 0 enhance the visibility of fluorescein staining of the cornea.7)Polarizedfilters: To reduce reflections from the cornea and retina

NB:The direct ophthalmoscope contains no prism.

(a)Collsfricfed pupil. (b) Dilafed pupiLFig.lS.4: l<:ffect of the pupil size on the field of view ah.

~:(a)Larger disfallce -7smalIer field of view. (b)Sl1laller distallce -7larger field of view.

Fig.15.5:Effcct of distancc betwcen observed eyc and observer on fieJd of vi.ew.HEFIELD OF VIEW (FIELD OF VISION): .Definition:Extent of the fundus of observed eye that can be seen at one moment.Construction of the field of view: The image A' B' of sight-hole AB (or observer's

pupil, which ever is smaller) is constructed using two rays (Fig. 15.2):(I)A ray through the nodal point N.(2)A ray parallel to the visual axis, is refracted by the eye to pass through its

posterior focal point.

Factorsaffecting the field of view:(1) The state of refraction (axial length of the eyeJ The field of view is largest in

H, intermediate in E, and smallest in M (Fig. 15.3).(2) The size of the pupil of the observed eye: he field of view is considerably

enlarged when the pupil of the observed eye is dilated by a mydriatic (Fig.15.4 ).

(3) The size of the sight-hole in the mirror of the ophthalmoscope or the size ofthe observer's pupil Whichever is small limits the size of the field of view .

(4) The distance between the observed eye and the ob erver: s this distancedecreases, the field of view becomes larger (Fig. 15.5).

(5) Total internal reflection of light at the periphery. of h rystalline lens: Thisleads to a dark shadow which is seen when the peripheral parts of the retinaare examined.

--..::: .. -_ .- .•....

orititlllli.ghl source sou.tce image

Fig.15. 7:Effect of concave mirror on field of illumination:(a)Liglt' SOllrce at princilwl FocusF;(b)Lig!lt sOllrcefar from prillcipalfoclls F;(c)Lig!lt sOllrce nearer 'hall prillcipalfocus F.THE FIELD OF ILLUMINATION:

ExtentAlways greater than the field of view in direct ophthalmoscopy because thesize of the light source(the size of illumination of the observed fundus) isgreater than the size of the observer'spupil.

Factors affecting the field of illumination: .(l) The type of the mirror used:

1) ane mirror·The ret1ected rays by the plane mir~or are divergent and socome to a focus behind even the retina or the myope (Fig. 15.6) and so thefield of illumination is largest in H, i'ntermediate in E, and smallest in M.

2) Concave mirror: 'The resulted field of illumination depends on theposition of the source of light in relation to the concave mirror (with a

(2)(3)(4)

-r+\Y_"!_~ --..... ... ..- - .N

(a)J

CONSl(A) C

fOI

(11

focal length of 20 cm Jlsually) as follows:1- With the lig t source at the princl 'Q7]jJcusF of the concave mirror:

The ret1ected rays by the concave mirror are parallel and so come to afocus on the retina of E (Fig. 15.7a) and so the field of iIlumination islargest in Hand M and smallest in E.

2- With t e light sourceJar from the rinci a focus F of the concavemirror, The reflected rays by the concave mirror are convergent andso come to a focus in front of even the retina of the H (Fig. 15.7b) andso the field of illumination is smallest in H, intermediate in E, andlargest in M (this is the position commonly used).

3- With the ight source nearer t wn the princip.al focus F of the concavemirror: he reflected rays by the concave mirror are divergent (aswith the plane mirror) and come to rocus behind even the retina or M(Fig. 15.7c) and so the field of illumination is largest in H,intermediate in E, and smaIlest in M.

(2) The refi-active state of t eye: As described above.(3) The position afthe light source: ~s described above.(4) The intensity of illumination of the fundus: Varies inversely with the size of

field of illumination.

f1.)Emmetropic observed eye. (b)Hypermetropic observed eye. (c)Myopic observed eye.Fig. 15.8: Construction of image of the observed retina in the observer's eye:

CONSTRUCTION OF THE IMAGE OF THE OBSERVED RETINA:(A) Construction of the position and size of image of the observed retina

formed in the emmetropic observer's eye:(1) The image xy of illuminated retina XY of observed eye (is formed atfar point

of observed eye) is constructed first by two rays (Fig. 15.8):I) A ray from X parallel to the optic axis passes through anterior focal point

F of observed eye.2) A ray from X passes through nodal point N of observed eye undeviated.

(2) Th image xy is formed at the far point of the observed eye as/ollows:1) n E observed eyer The two emerging rays are parallel and so if produced

backwards foml a virtual image behind its retina at infinity.2) In H observed eye:The 2 emerging rays are divergent and so if produced

backwards,a virtual erect magnified image is formed behind retina.3) In M observed eye: he two emerging rays are convergent and so form a

real inverted image in front of the eye.

(3) 'he image X'Y' is then constrf:1cled(assuming that the anteriorji)cal point (~lthe observed eye F coincides with the anterior focal point of the observer'seye Fo) by the following two rays:1)A ray from the top of image xy passing through observer's nodal point No.2) Extension of the ray xF to meet the ray xNo at X' .

J'IB: The ubserved retina. principal plane, nodal point and anterior focal poin't arerepresented in Figs. 15.8, 15.9 and 15.10 as Il, P, Nand F respectively whilethe observer's retina, principal plane, nodal point and anterior focal pointare rerrcsented as Ro,Po,No and Fo.

(4) The resulted image X7'formea in observer's retina is characterized by:1)The image X'Y' on the observer's retina of the illuminated retina XY of

the observed e.ye is inverted: So seen as erect whether the observed eye isE, H or M.

2)The size and position or the image X'Y' on the observer's retina varieswith refractive state of the observed eye as follows:1- The size of the image: Is smallest in H, intermediate in E and largest in

M observed eye.2- The position of the image in relation to the observer's retina: Is behind

in H, on in E and in front of the observer's eye in M observed eye(sothe view of an emmetropic observer is clear when the observed eye is Eai1d blurred when the observed eye is H or M).

\OB) The ability of the emmetropic observer to focus the beam of light, reflectedfrom the observed retina, on his retina:(1)1/1 E observed eye: TI e rays of light leaving E observed eye are parallel and

focussed on E observer's retina without accommodative effort or the use ofa correcting lens (Fig. 15.8a).

(2)1/1 H observed eye. he rays of light leaving H observed eye are divergentand focussed behind E observer's retina (Fig. 15.8b) and so to bring the raysof light to a focus on the retina of E observer, he either:

I-Accommodates to get the focussed image on his retina but the image sizestill smaller than in E observed eye (Fig. 15.9a), or

2-Uses a correcting convex lens to get the focussed image on his retinawith the same image size as in E observed eye (Fig. 15.9b).

(3)1n M observed eye: he rays of light leaving M observed eye are convergentand focussed in front of E observer's retina (Fig. 15.8c). and so to bring therays of light to a focus on the retina of E observer, he uses a correctingconcave lens (Fig. 15.9c). .

(C) Construction of the focussed image of an ametropic observed retina on theemmetropic observer's retina by accommodation or by correcting lens:(l)Focussed image of H observed eye by accommodation of the observer: rhe

anterior focal point of the observer F approaches his eye and so the ray xFenters the observer's eye in a convergent direction and so the resultedimage of the observed retina is focussed on the observer's retina but is c;'

Fi{!.15(fl

l\1A\(1

x:~~:::-.- ..I

3- So M will be 15X,in E eye with a power of +600, less than 15 X,in H eyewith less than +60D and more than 15 X,in M eye with more than 600.

2) From the reduced eye.1- When tile anterior focal point F of the observed eye coincides with the

anterior focal point Fo of the observer's eye:

(a)In E observed eye: a)Formilla: I M = 6 = D IWhere V=Near point=250inm.

U =15mm from the reduced 'eye.b) Construction (Fig. 15.10):

a- xy is the projected virtual image ofXY by theobserver at the nearest point of distinct vision i.e.at 25 cm (250 mm).b- X'Y' = XY.

, }5L V 250 mmc)CalculatLOn: M = Xy = U = 15 mm = 16.5.

(b)1n H observed eye:M is less than in E (narrower X'NoY' angle) and theimage xy is virtual.

(c)Jn M observed eye: M is greater than in E( wider X'NoY' angle) andthe image x is real.Therefore the widei the angle subtended by the observed retina Xl' atthe nodal point of the observer's eye, the greater the magnification.

2- When Fo of the observer is within F of the observed eye: M is greatestin H, intermediate in E, and least in M.

3- Practically, correcting lens is slightly far from F of observed eye: M isthe same as when F of observed eye coincides with F0 of observer's eye.

(2) In astigmatic observed eye:There are two different prin~ipal meridians V andH( with two different anterior principal foci Fy & FH of greater and of leastmeridians respectively, Fig. 6.21) with wider angle at Fy than FH , and so Mis greater in the meridian of the greatest refraction and vice versa.

NB: Therefore in I'eguhu' astigmatism with the rule the optic disc is oval vertically indirect ophthalmoscopy.

R P X Po RoFz 0 .:----ir

---1,

y

-u-·'N. L

x'

Dire!(

CLINICAL APPLICATIONS OF DIRECT OPHTHALMOSCOPY:The method of doing fundus examination:

(1)The patient looks to his nose for the observer to see the optic disc and largerblood vessels.

(2)Tthen he looks straight forward for the observer to see the macula.(3)Then he looks to various directions for the observer to see the whole fundus.(4)Then the observer rack up lenses in the direct ophthalmoscope to see different

levels of the vitreous.Direct ophthalmoscopy by ametropic observer: There are two possibilities:

(l)The observer can remove his spectac/es:'I see the fundus of the patient by alens in the direct ophthalmoscope (of algebraic sum of his own refractiveerror and the patient's refractive error).

(2) The observer can use hisspectac/es: But his field of view will be restricted asthe sight hole of the minor will be further from his eye.

Direct ophthalmoscopy of an ametropic patients:(i)H. fundus: Small image size and a wide field of view and so H fundus can

be scanned quickly.(2)M./undus: ,arger image size with a reduced field of view and so a highly M

fundus is difficult to be seen ( but the posterior pole of a highly M fundus isseen if the patient keeps his glasses on).

(3)Astig!11Qlic/undu~ The correcting lenses in the direct ophthalmoscope arespherical lenses. only and so high astigmatism leads to distortion of theimage and the optic disc appears oval. .

FallaCies in the refractive state of the observed eye by direct ophthalmoscopy:(l)Equal H and AI in numerical value in the observed e e and observer's eye (

and vice versa) do not hav(! their far points coincide:1) +40 observed eye is seen by -3.50 Dobserver'seye.2) -4D observed eye in seen by +4.50D observer's eye).

(2) 711edistance between the observed eye and the observer's eye is practically.more than 30 mm' 0 the anterior focal point of the observed eye and of theobserver's eye are separated.

(3)The Dbserved e 'e and the observer's eye are using their accommodation:Causes a more separation of the anterior focal point of the observed eye andof the observ~r's eye.

(4)The lens 111 the ophthalmoscope is not in CQ tact with the cornea of theobserved eye ( but at least 1.5 cm in front (~rit : So the lens correcting H islower than H error and lens correcting M is higher than the M error).

(5) Hi/hl!I' 'eji·active error ~ eads to a greater faulty difference of the power ofthe correcting lens.

(6)Hi h minus lenses: ead toa greater magnification (as in the eyepiece of theGalilean telescope).

(7) Greafer magnification with reducedjield olvisian:Disadvantage in high M.

(8) The degree of astigmatism is noi absolutely correct Because it is estimated·by taking the difference in the readings between the two principal meridians.

Difficulties of direct ophthalmoscopy with constricted pupil of observed eye:fl)Reflexesfrom the cornea:

1)Are due to corneal image of light source which will prevent viewing themacula clearly.

2)Avoided by looking obliquely through the pupil to focus the optic discthen moving the observer's head and ophthalmos~ope outwards ( equal to1.5 disc diameter) to see the macula (as the axis of the observer must notlie in line with the corneal reflex of the bulb image) ..

(2)Reflexesfrom the lens. These reflexes also prevent seeing the macula clearly.(3) Constriction of the pupil of theobserved eye on exposure to light of the

direct ophthalmoscope: This makes viewing the macula more difficult.Location of a vitreous opacity by direct ophthalmoscopy:

(1) By Newton's formula:Newton's formula;, It states that 1112= 1'112in any system as the.eye.Construction (Fig. 15.11):

12=The distance between the object (vitreous opa'city 0) and the secondfocal point F2.

II=The distance between the image I (of the vitreous opacity 0) and thefirst focal point F 1.

fl = The anterior focal length.f2=The posterior focallength.

XY=The strongest convex lens in the sight-hole of the directophthalmoscope which is placed at F 1 (to give a sharp image of thevitreous opacity 0).

C~lculations·1) Calculations for an E observed eye:

f1 = 15 mm and f2 = 20 mm (from the reduced eye).So, 1112= 15 x 20

11 =The focal length of the strongest convex lens placed at F 1 whichgives the sharp image of 0 and will have F}l for its focal length.

JSo I J = in metres., power of the convex lens

Example: If the power of the convex lens is + I0 0:1000 300

II = 10 = 100 IIIIII and so 12= J 00 = 3 Illlll.

So the distance between the retina and vitreous opacity will be 3 mmin E observed eye. .

2) Calculations jor H or M observed eye: By either'1- Usi·ng.the calculations for an ametropic observed eye:

The(.

(a) When. the observed eye is H: The retina will be nearer to theopacity than in E eye and so for each 3 0 H we deduce 1 mm fromthe calculation for E observed eye.

(b) When the observed eye is M: The retina will be farther from theopacity than in E eye and so for each 3 D M we add 1 mm to thecalculation for an E observed eye.

2- Using the amount of refractive error and numerical value f tIe,focus'sing lens in D:(a) When the o~servea eye i If: From the amount of H in D deduced

from the numerical value of correcting lens we can get II in mm.Example: If an opacity in the vitreous of H eye of +2 D is

foclIsscd by + 12 D IClls: 1112= fl f2 = IS X, 20 and II = - /~~~

1000 300=-10 = 100 mm. So, 12= 100 =3 111m.

(b) When the observea ~ye is M: rom the amount of M in D added tothe numerical value of the correcting lens, we can get II in mm.

(2)By parallactic displacemen :To differentiate between the level of twoopacities in the fundus (one is in front of the other) with slight movement ofthe observer's head and ophthalmoscope:1)A nearer opacity moves in the same direction of the observer's head

movement.2) A farther opacity moves in the opposite direction of the observer's head

movement.(3)!l1easurement offimdus lesions in standard unit.'s:

1 ) Lateral measurements: 1 Disc diameter = 1.5 mm.2) Depth measurcments: 1- 3 D = I mill in cmmetropic eye.

2- 3 D = 2 mm in aphakic eye.The advantages of direct ophthalmoscopy:

(J) Brilliant illumination:1) a 0 en light is used in many modern direct ophthalmoscopes for:

1- Even illumination. 2- Sharply limited. 3- Of high specific iritensity.2) A red free filter is ED to get a green light which is important for:,

J -It maKes red. elements velY dark:: With a more contrast for examinationof retinal vessels, showing the microaneurysms as black dots against agreen background and pinp9int hael1lorrhages are seen ~learly).

2- TIp shorter wavelengt71s are scattered more in t11e super ielal retinallayer : To see early retinal oedema and defects in the nei-ve fibre layeras in early glaucoma.

3) A polarized light is used: To reduce the glare from the cornea.(2)Advantages of direct ophthalmoscopy in a dark room:'

1) Relaxation of accommodation of both the observed and observer's eyes.

2)Dilatation of the pupil of the observed eye.(3) The size and position of image:

I) The large magnification in direct ophthalmoscopy is useful to visualize theminute lesions.

2) The fundus image is erect.(4) Examination of anterior segment of eye (used as a :'le(rilluminatin~ loupe):

1) The lens can be inspected through + 10 to + 12 D lens in the sight-hole forlens opacities.

2)The cornea by + 16 D lens to see the red reflex with detection of cornealopacities and KPs.

(5) The direct ophthalmoscope is portable: As it is small and light.

ObsorvereFig. 15.12: Indirect ophthalmoscopy. Fig. 15.13: Binocular viewing system.

(2) RECT OPHTHAI:MOSCOPY1) BINOCULAR INDIRECT OPHTHALMOSCOPY(BIO)

DEFINITION: It is the method by which we render the observed eye artificiallyhighly myopic by a high convex lens which forms a real inverted image of the fundusof the observed eye at a distance in front of the observed eye (between the convexlens and the observer, Fig. 15.12).METHODS OF BINOCULAR INDIRECT OPHTHALMOSCOPY:

(1) Binocular indirect ophthalmoscope(BIO):Components of BfO:

1) The concfensing lens( powerful convex lens) IS held in front of theobserved eye:1- TFzeobserver holds t e conaensing lens at arm's length: The distance

between the observer and the observed eye is about 75 em.2- Tne usual powers useci are: + 13 D, +20 D and +28 D, each of which

differsin magnification, field of view and stereopsis as in table IV.3- Design Oflenses:

(a) Aspheric design: With the surface of the steeper curvature facingthe examiner to decrease the distortion of image.

(b) Doublet lens: May fUlther reduce distortion but increases thereflections.

(c) Antireflective coating: To minimize reflections.

(d) Coloured lenses: Yellow-tinted volk lenses reduce unwantedinfrared radiation on the patient's retina and eliminate blue light toreduce scattering.

(e) A scale on the lens surface: for fundus measurements.Table IV:Optical characteristics of condensing lenses used in BIO.

Tlte power of the cOlUlem-ill ..£._I_e'_ls__Optical Characteristics + 13 0 +20 0 +28 0

.11 Magnitication: 5 3 2(b) Estimated field of view: 30° 50° 60°(c) Approximate stereopsis: as normal 3!_4 normal 1/2 norr:!~

2) The illumination is provided by an electric lamp mounted on thE(observer's head:!1- T,le illun:zinating light beam: Passes through the condensing lens into

the observed eye.2- TI e light ref ectedfrom the observe eye: Is refracted by the condensing

lens to form a real inverted image between the condensing lens and theobserver (Fig. 15.12).

3- ~ Ie observer views this real image of the observed retina which is:(a) Both vertically and laterally invelted (upside down and back to

front).(b). ituated at or near the second principal focus of the condensing

lens(Fig. 15.12),i.e. approximately 8 em in front of + 13 D lens (so the observer views

the image from a distal1ce of 40-50 cm because he holds thecondensing lens at arm's length).

4-· e leyelo the observer's eyet Is the same as that of the observed eye.3) inoc ar viewing systen : Is mounted on the observer's head ::md

contains the binocular prismatic eyepieces with prism binoculars (Fig.15.!3):1- \-vo convex enses '£ I and L2:

(a)Thc power of each convex lens is +2 D to help the observer to viewhe fundus:a) Without exerting hi accommodation(e pecially if the patient is

aphakic or highly H).) If he is a presbyopic observer.

(b) -Iowevcr, the observer must wear his spectacle correction if:a) His near correction is more than +2 0 becaL se of underlying

presbyopia or H.b) He has any significant refractive error.

2- Two re} ecling prisms -' and P2:(a)To provide increased light to the observer's eye by total internal

reflection as the. angle of incidence (45°) is great.er than the criticalglass/air angle (41°).

(b)To achieve binocularity by reducing the observer's pupillary distancefrom 60 mm to 15 mm approximately with the aid of reflectingmirrors M 1 and M2.

4) ccessories of BIO:I-Heatfilter: To absorb the infrared radiation.2-Fibreoptic systems: To provide cooler light.3-Reflecting mirrors: To selectively reflect infrared light out and visible

light for viewing.Advantages of BIO.

1) Better visualization of peri pheral retinal tears.2) Easier manipulations.3) Better illumination system.4) Binocul~r stereoscopic view.

(2)The simple system of illumination:By a concave mirror for light reflection from an external source of light with acondensing lens,with the principles of indirted ophthalmoscope( obsolete now).

ME H P

~4 ~(a)Luminous ophthalmoscope. (b)Reflected lighe/rom a cOllcave mirror.

Fig. 15.14: Field of illumination in indirect ophthalmoscopyFIELD OF ILLUMINATION:

Construction of the field of illumination: P = The principal plane of the observedeye and S = the pupil of the observed eye:(1) Self luminous indirect ophthalmoscopes:

1 The illumination: Is provided by an electric lamp mounted on theobserver's hea"d.

2) . luminating light beam is rendered convergent: B the condensinglens.

3) se convergent rays become more convergent: By. e refracting systemof the observed eye and so they meet at a point in the vitreous (Fig.15.14a).

4) he light then Iver s" ain: To strike the retina.(2)Re]lected light( Of the original source of light)' from a concave mirror(of

25cm focal length): Is in a convergent direction, so that the rays meet in frontof the milTor (Fig. 15.14b) and then diverge again to be refracted by thecondensing lens (as in indirect ophthalmoscope).

Factors affecting the field of illumination:(1) The refractive· state of the eye: It is smallest in H, intermediate in E and

largest in M.(2) The pupil size of the observed eye: t is enlarged when the pupil of the

observed eye is dilated.(3) The intensity of illumination of fundus: It is smaller with high intensity of

fundus illumination.

I"(ofrontthe

(a)Lens with {a~!Jeaperture. (b) Lens with small aperture.Fig.15.15:The field of view. Fig.15.16:Effect of size of condensing lens on field of view.FIELD OF VIEW (FIELD OF VISION):

Construction of the field of view:(l)The condensing lens is held infront of the observed eYf!at such a distance

that the observed pupil and the observer's pupil are conjugate foci: Onlythose rays of light which leave the observed eye ( S = The pupillary plane ofthe observed eye) via the area of image 01 of the observer's pupil 0 can,after refraction by the condensing lens, enter the observer's pupil and seenby him (Fig. 15.15).

(2) The meeting of the convergent light rays by the condensing lens in theobserved eye: Is at a more anterior level with a wider angle in the field ofview than in the. field of illumination which explains why the field of view isgreater than the field of illumination (see Figs. 15.14 and 15.15).

Factors affecting the field of view:(1) the refractive state of the eye:t is smallest in 1-1, intermediate 111 E and

largest in M.(2) The size of the observer's pupil: t is limited by small size of observer's pupil.(3) The condensing lens: he field of view increased by:

1) A larger size (aperture) of the condensing lens (Fig. 15.16).2) ncreased power of the condensing lens:

1- +13 D lens: Leads to 30° field of view.2- +20 D lens: Leads to 50° field of view.3- +28 D lens: Leads to 60° field of view.

CONSTRUCTION OF POSITION AND SIZE OF IMAGE OF OBSERVEDRETINA IN OBSERVER'S EYE:ls done by either:

(1) With the principal focus F1 of the lens coincides with anterior focal pointFa of observed eye (Fig.15.17):

ternFig.

1) The image X'y' is constructed by the fallowing two rays:1- A ray from the top of xy passes through F I of the lens and is refracted

parallel to the optic axis.2- A ray from the top of xy passes through nodal point 0 of lens undeviated.

R P:r---r~-- .•X~··,x···--· FI .' -..._ i\

)''-- -'I N

A1o F['y'

'~-li

(a)E observed eye. (b)H observed eye. (c)M observed e:,'~Fig.15.17:Image position of observed retina w,hen Fa of lens coincides with Fl of observed eye.

2) The position and size of image of the observed retina:1- 'he'position of image of the observed retina depends on thestuie of I

refraction of observed eye:(a)In observed eye: he image X'Y' of the observed retina XY lies at

the second principal fOCLISF2 of the lens, irrespective of the positionof the lens in relation to the observed eye because the lens alwaysreceives parallel rays in emmetropia (Fig. 15.l7a).

(bJ n H 0 serveo eye: The image X'Y' of the observed retina XY liesoutside the second principal focus F2 of the lens as the two raysbefore passing through the lens are divergent (Fig. 15.17b).

(c)ln o5served eye: The image X'Y' of tile observed retina XY liesinside the second principal focus F2 of the lens as the two raysemerging from the myopic eye are convergent(Fig. 15.17c).

2-The size of the image of the observed retina: fthe principal focus of thelens is at the nodal point of observed eye, the image size is constant

irrespective of the refractive state of the observed eye.2) With the principal focus Fj, of the lens coincides with the nodal point N of

the observed eye:The position and size of image are the same as when F 1 ofthe condensing lens coincides with Fa of the observed eye.

,- -'. q~F1 y'

. x'8

MAGNIFICATION OF IMAGE OF THEOBSERVED RETINA:Construction: The position and size of the image X'Y' of emmetropic observed

retina XY in the observer's eye (Fig. 15.18) is constructed in the same way as inFig. 15.17a.

Calculation of the image size:(1)The magnification for E observed eye:

1) Linea.r magnification:_ Size of image X'y.' _ OF2

M - Size of object XY NYWhere, M = The linear magnification.

OF2 = The focal length of the condensing lens(l000/13=75 mmfor +13.0 lens).

NY =The distance bctween thc nodal point and the cl11l11ctropicobserved r~tina (15 111m from the reduced eye).75

So, M=J.5 = 5

So, M of + J 3 D lens is 5xfor E observed ~ye.So, M of +20 D is 3x

SO. M of+28 D is 2x.2) Angular magnification: :fhe angular magnification can also be calculated

and its value is the same as that of linear magnification.(2)Magnijication of an ametropic obser.ved eye:

1) n observed eye-r It is more (due to shorter NY distance) than in Eobserved eye.

2) In M observed eye: It is less (due to longer NY distance) than in Eobserved eye.

( a) (b) (c)Fig. 15.20: Effect of the distance between the condensing lens and the observed eye:

(a)Fa of lens coincides witll FI of observed eye; (b)Fa of lells is olltsideflof observed eye; (C)F:lof lens is inside FI of observed eye.

Factors affecting the size of image of the observed retina:(1)The istance be!llleen the condensing lens and the observed eye (Fig. 15.20 :

1) f the principal focus Fl of the lens coincides with the anterior focal /point a of the observed. eye: In Fig. 15.20a, a ray parallel to the opticaxis passes through F I of the lens and is refracted through the lens parallelto the optic axis and so the image size of the observed retina is the sameirrespective of the refractive state of the observed eye.

XI--X'x; .-'-

,y'--

2) the principal focus Fl of the lens is outside - Ie anterior focal pointa of the observed ey~: In Fig. 15.20b, a ray parallel to the optic axis

passes through Fa of the observed eye (which is outside Fl of the lens)and is refracted through the lens convergent to the optic axis and so theimage size of the observed eye is smallest in H, intermediate in E andlarg;est in M.

3) r the principal focus FI of the lens is inside the anterior focal point Fa.o e 0 served eye:I'In Fig. 15.20c, a ray parallel to the optic axis passesthrough Fa of the observed eye (which is inside F I of the lens) and isrefracted through the lens divergent to the optic axis and so the image sizeof the observed retina is largest in H, intermediate in E and smallest in M.

(2) The refractive state of the observed eye (Fig 15.20 :1) In E observed ey The image size is constant irrespective to the position

of th len in relation to the observed eye.2) In observed eye. The image becomes smaller when the principal focus

of the lens is outside the anterior focal point of the observed eye andbecomes larger when the principal focus of the lens is inside the anteriorfocal point of the observed eye.

3) In obsel"Ved eye" The reverse of what occurs for H observed eye.4) In asti matico observed eye: ::.fhere is unequal magnification in the two

principal meridians and so the optic disc appears oval in astigmatism.(3)The power of the condensing lens used able IV): The image becomes

smaller in size if the power of the lens is increased:1) -f D lens Leads to 5)(, magnification.2) +20 D lens' Leads to 3 X magnification.3) +2 ens: Leads to 2 X magnification.

CLINICAL APPLICATIONS OF BIO~ .The methods of BIO:

(l)Selfluminous indirect binocular ophthalmoscope1)Tlie examine-r lo()ks through the eyepieces of (he indirect binocular

op fhalmoscope mounted on his head: To see the red reflex of thepatient's eye.

2) The condensing lens: .1- It is held in the right hand of the examiner between the index finger and

the thumb while the other fingers support the hand against the patient'shead.

2- The examiner must notice that the position of the image of the patient'sfundus is between him and the condensing lens (he should not try tofocus the red reflex in the patient's pupil otherwise he will be unable tosee the fundus).

r2JA concave mirrorf(Jr reflection/rom an external source qflig t:Obsolete.The examiner's state of refraction:

(l)An ametropic examiner should wear his con-ectirig spectacles.(2)A presbyopic examiner should wear his correcting glasses for near.

Scheme- of fundus examination:(l)Examination of the optic disc The patient looks towards his nose for

examiner to see optic disc.(2)Examination of the macula.

1)Patient should look at examiner's forehead for examiner to see the macula.2)To shift from the optic disc to the macula, the examiner's head is moved

outwards to the temporal side of the patient due to inverted fundus image(macula lies nasal to the optic disc).

(3)Examination of the rest of the fundus:1)The patient looks in various directions, for example he looks down for

examination of thelower part of the fundus( remember that the image is inverted).

2)A retinal detachment chart is placed upside down( on the patient's chest iflying down in·bed). "

3)Examination in a dark room7same advantages as direct ophthalmoscopy.Troublesome reflexes on doing 810:

Optical principles: Three troublesome reflexes are produced by sou'rce of light:(1) he patient's cornea acts as a convex mirror: Thus it produces a virtual

erect diminished image of the light source which may cover the pupil andprevent anything behind to be seen.

(2) The anterior surface of the condensing lens (towards the examiner) ,acts as a convex mirror: JThus it produces a virtual erect diminishedimage of the light source behind the condensing lens (i.e. between thecondensing lens and the patient).

(3) The posterior surface of the condensing lens (towards the patient) acts,as a concave mirror: ,Thus it produces a real invelied image of the lightsource in front of the condensing lens (between the lens and the examiner).

Prevention oJ the reflexes:(1) fie reflex rom the patient's cornea can be avoided by either::

1) A sideways movement of the condensing lens while the light source isstationary: The image of the patient's fundus moves in the samedirection as the lens which is greater than the lens movement (becausethe image is formed nearer to the principal focus of the lens).

or 2) Movement of the examiner's head and keeping the lens stationaly: Theimage of the patient's fundus moves in the opposite direction of themovement of the examiner's head.

(2)TJie two reflexes from the anterior and poste.·ior surfaces of the,condensing lens can be avoided by slight tilting of the condensing lenstowards or away from the patient: This will move the two images of thelight source (the two reflexes) in opposite directions and so the fundus could

could be scen in.thc clcar spacc bctwccn the two imagcs.ParaHactic displaceme t with binocular indirect ophthalr:noscopy:

Value:To differentiate between level of two opacities near each other in fundus.Principle:

(l)A and B are two opacities in the fundus for example A on the edge of theoptic disc and B at the bottom of optic glaucomatous cup (Fig. 15.19).

(2) Wnen condensing lens is shifted slightly so that its optical centre movesfrom 01 to 02 image of A and Bwill move from A 1 to A2 and B 1 to B2.

Advantages of BIO:(l)A good illumination More useful in examining opacities in ocular media.(2) Good illumination with a wide field of view: More useful in examining

patients with retinal detachment specially if there is an extensive subretinalfluid with underlying malignancy.

(3)lndentation of retinal periphery.·For iewing peripheral retinal lesionsspecially retinal tears.

(4) Ophthalmoscope is used at a distance: 0 preserve a sterile operative field.(5) The low magnification and good illumination are useful in:

I)To examine a highly M fundus as image produced is smaJIer and brighter.2) Mass screening of the fundus to avoid missing pathological lesions.

(6) Binocular stereoscopic view. .(7)Refractive error has slight influence than in direct ophthalmoscopy.(8) There is a teaching mirro .

·2) MONOCULAR INDIRECT OPHTHALMOSCOPYDEFINITION: Hand-held monocular indirect ophthalmoscope with a natural pupil.COMPONENTS:

( 1) The ililimination system: 1)Light source.2)lris diaphragm:Control area of fundus illumination.

(2) The viewing ::>ystem:. 1)The ophthalmoscope lens: ith less diameter and smaller field of view.2) The monocular compound microscope:

I-Eye piece which can be moved back and forth to focus the instrument.2-The fundus image is inverted, or rather re-inverted, and so is seen erect.

ADVANTAGES OF MONOCULAR INDIRECT OPHTHALMOSCOPE:(1)View through small pupils.(2)Reduces reflections and light scattering by cataracts with wider pupils.(3 )Viewing through a peripheral iris coloboma if central pupil is undi lated.(4) It allows small details to be seen larger.

NB: The heam of a direct oohthalmoscope with a ~alogen or ~ibreoptic light source: Canbe llsed with the condensing [ens for monocular indirect ophthalmoscopy.

A T {THE VERIFICATION OF THE REFRACTION OF THE EYE

(1) A EXA 0(A) GENERAL HISTORY:

(1) Visual disturbances.(2) Headact-le.

(B) CLINICAL OCULAR EXAMINATION:(1) External examination:

l)General inspectio : This is done in a good diffuse light.2)Binocular magnifier(loupe): ts Magnification is2 x., with a stereoscopic

effect andit determines the depth ot opacities.

Fig.16.1 :Cornealloupe. Fig.16.2:Parallactic displacement by concave mirror'.·3) Uniocular (corneal) loup :It has the same principle of placing the object

inside F of 13D convex lens witb a small light source (as a small electrictorch) in oblique illumination (Fig. 16.1):1- Done in a dark room with a small light source S , 60cm in front bu to

one side of the patient(a)The light is concentrated upon the cornea E by the strong convex

lens C to form a minute image which can be moved over thecornea by slight lateral movement of the lens.

(b )Light is focussed upon the iris or the lens by moving the lensslightly nearer to the eye.

2- The observer 0 sees magnified image lby looking through uniocularcorneal loupe L held other hand and his eye should be close to loupe.

3- The degree of magnification is usually .10)\ and so fine details oflesions as KPs can be seen.

4) Slit-lamp biomicroscopy: Chapter 23.(2) Internal examination:

1) The red reflex examination:J - By a plane mirror at a distance of one metre for

(a) Retinoscopy: Chapter 14.(b)Detection and location offine opacities in different ocular

meaia. The illuminated area of the fundus is less with plane mirror

and so the shadow of the opacity is bigger.2- By the distant direct method using a concave mirror at a distancel

of 22 centimetres fo :(a)Detect opaci(ies in the refractive-media by:

a e mobTi of tile 0 acities:a-Floating opacities in aqueous or vitreous move with

movement of the patient's eye.b-Fixed opacities in cornea or lens cannot move with

movement of the patient's eye.b) ara ac Ie (lisp accmcnt: To dcterminc the exact position of

opacities as follows:a-In Fig. 16.2, 4 is the centre of rotation of eye and 1, 2, 3,

4 and.5 are sites of opacities at di fferent levels in eye.b- When the eye is rotated a small amount, all the opacities

except 4 will move.c-The movement is greater the further the opacity is from

the centre of rotation.d-All movements will be referred to the edge of the pupil,

to an observer situated at A, all the opacities will appearas a single spot in the centre of the pupillary reflex.

e-Ifthe patient's eye is rotated in the opposite direction orif the observer shifts his position (his head with theconcave mirror) to B :

(i)Opacity 2 in the papillary area will remainstationary in the centre of the pupil.

(ii)Opacity 1 in front of the pypillary area will move inthe same direction.

(iii)Opacities 3, 4 and 5 behind papillary area will movein the opposite direction with 5 lost behind the iris.

c) orneal re ex: he centre of rotation of the cornea is 8mmbehind the anterior surface of the cornea and is covered bythe corneal reflex and so:

a-Opacities in front of corneal reflex move in the samedirection as the patient's eye.

b-Opacities behind corneal reflex move in the oppositedirection of the patient's eye.

(6 Detect the eage of a dislocated lens or a notch in congenitalcoloboma of the lens -The edge of the lens is seen as a darkcrescent in the pupillary area(as all light reflected from funduswhich falls upon the extreme edge of the lens is totally reflectedwithin the lens and none of it leaves the patient's eye and so noneof it can enter the observer's eye.

(A) C

(1

(c)Detect a change in the colour, of the red rejle»a- 'fey re reflex: n retinal detachment or in an intraocular tumour.b- aCK spo S In me red ref ex!: Due to opacities or foreign bodies in

different media.c- IX: sen re. reflex In total retinal· detachment, intensive vitreous

haemorrhage, mature cataract and total corneal opacity.(d)Conjirmation of the resuLtsfound by the external examination- A black

spot on the iris may allow a red reflex through it and so show itself to bea hole.

2) Slit lamp biomicrosco .3) Indirect ophthalmosco .4) Direct ophthalmoscopy.

(3) Functional examination: Vision, field of vision and colour vision are tested.1) The vision is tested: 1 For distance and for near.

2- Uniocularly and binocularly.2) Thefield of vision:' I detennined.3) The cololir vision: Js tested.

(4) Investigations of binocular muscle anomalies:Heterophoria and heterotropia.(5) Verification of accommodation and its disturbances.(6) Verification of convergence and its anomalies.

(2) INA . OF THE STATE OF R FRACTIO(A) OBJECTIVE EXAMINATION:

(1)The distant direct method using a concave mirror at 22 centimetres:1)In emmetropi : No fundus detai Is are seen.2) In hypermetropi .Vessels seem to move in same direction as that of mirror.3) In myopia The vessels seem to move in opposite direction.4) In astigmatism. The vessels of one meridian only are seen.

(2) Ophthalmoscopy:1)indirect ophthalmoscopy:

1- In emmetropia' No change in the shape and size of the inverted image ofthe optic disc when the convex lens is withdrawn from the patient's eye.

2-1n hypermetropia:tThe image becomes smaller but the shape is the same.3- In myopi~: The image becomes larger but the shape is the same.4- In astigmatism'.The disc appears oval in shape and its shape changes on

withdrawing the lens:(a)In simple astigmaTism' One diameter decreases while the other diameter

is not changed.(6) 11 compound astigmatisn. Both diameters increase or decrease

unequally.(c)in mixe astigmatism: ne diameter increases and the other decreases.

2) Direct ophthalmoscopy:1- Nature of any re ractive error.:Equals the lens power which get fundus details.

·2- Typical fundus changes i'Tmarked ametropia:As in myopia can be seen.(2) Retinoscopy.(3)-Keratometry (Chapter 29):Only of value when used with other tests.(4)Objective refractometry: Chapter 27.

(B) SUBJECTIVE EXAMINATION:sYl (1)Subjective refractometry: Chapter 27.

/r (2) Other subjective methods: To indicate the presence of astigmatism and checkits degree and axis:i) Fogging method:

I-Vision is blurred 6/18 on myopic side to relax accommodation by either:(a)+0.25 OS at a time is placed before the eye to be tested, while the other

eye is closed, until the patient can just read 6/12 on the myopic side:The patient must close his eyes while the lenses are being changed toavoid stimulation of accommodation; or

(b)+ 1.00 DS, more than the findings of retinoscopy, keratometry orrefractometry, is placed before the eye to be tested while the other eye iscovered, then reduciJig 0.25 OS at a time until patient reads 6112 line.

2- Then -0.25 DC is tried at different axes to obtain improvement of visionand then another -0.25 DC in the same axis is tried and so on until nofurther improvement:(a) When a convex-s-phere or cylinder is added:Test type is better or worse.(b) When a concave sphere or cylinder is added:Read more letters or not.

3- The vision should be brought back to 6/18 by adding a small convex spherelens and then test for the best axis by rotating the cylinder lens few degrees.

4- Then add -0.25 OS at a time till vision is 6/5.5- Then +0.25 DC or -0.25 DC is tried for more letters to be seen.

'r1 6- Then all the fogging method is repeated for the other eye in the same way.5Yl s;:- 2) Astigmatic dial and fan:

/'

~ i-Astigmatic dial chart (CIQck chart).(a) Fogging is done first: To 6/18.(b )The astigmatic dial: Is formed of radiating lines in different meridians

from 0° to 360° with a central movable V (Fig. 16.3) which determinesthe axis of astigmatism as follows:

a)The patient determines the clearer arm of the arms of V.andthen the V is rotated in the direction of that clearer arm till thetwo arms appear to be equally grey.

b)Figure on chart at apex of V denotes the axis of the concave cylinder.d)The strength of the concave cylinder is found by gradual increase of

the concave cylinder until the other principal radial axis is of thesame clarity as the first.

e) Further defogging with concave spheres is done to clear distant visionfor the tesltypes.

Fig. 16.3: Astigmatic dial chart. Fig.16.4: Astigmatic fan.2-Astigmatic fan (V chart): With same principle and value as dial chart:

(a )Fogging is done first: To 6/18.(b)Astigmatic fan:Radiating lines in all meridians(O° to 180°, Fig. 16.4):

a)lf the vertical lines are clearer, the V meridian is more ametropic withvertical diffusion ellipses on the retina and so the axis of astigmatismis in the H meridian.

b)Concave cylinders with axes at right angles to clearer vertical lines(i.e. in H meridian) are added until all the lines are equally clear.

3) Jackson's cross cylinder (Fig. 16.5):Characters:Sphero-cylindrical lens with a cylindrical and a spherical surface:

1-The power of the cylinder is twice the power of the sphere and ofopposite sign and the result is the same as superimposing two cylindricallenses of equal power but opposite sign with their axes at right anglesand so its spherical equivalent is zero.

2- The cross cylinder lens is mounted on a handle which is at 45° to theaxes marked on the cross cylinder lens (which are the axes of no powerof the two cylinders).~

3- The power of each cyl inder iies at 90" to the marked axis and coincideswith the marked axis (of no power) of other cylinder (of opposite sign).

Methods of use: ..I-To check the axis of the c linder prescribed:

(a)Cross cylinder is held before the eye with its handle in line withthe axis of trial cylinder.

. (b)Cross cylinder is then turned over a quick turn(flipped over) andthe patient indicates the position of better vision.

(c)Cross cylinder is held in preferred posi'tion and axis of trial cylinder isrotated slightly towards axis of same sign on the cross cylinder.

(d)The process is repeated until the trial cylinder is in the correct axis for, the eye, at which time rotation of the cross cylinder will offer equal

visual alternations to the patient.2-To check the power of the trial cylinder.

(a) Cross cylinder is held first with one axis and then the otheroverlying the trial cylinder.

(b)This, increasing and then decreasing the power of the trial cylinder.3- 0 veri y that no cylini:lrical correction is necessary for the patient ·f

no cylinder was detectea on retinoscopy: No more improvement ofvision when the cross cylinder is applied and rotated with its axis at 90°and 180° and then at 45° and 135°.

Precaution :I-Cross cylinder is of value with a cycloplegic( but is used when the ciliary

muscle is not paralyzed if the patient relaxes his accommodation to avoidaccommodative effort1.

2-Patient 'must have clearest vision possible before cross cylinder is used.

·OO.50DC X+0.50

. +0.50 DC -0.60

(aJO! -0.50 DS/+/.OODC.· (b)Axe.\· a.\·Oil lells witlt powers ill iJ~illcipall1laidialls.Fig.16.5:The cross cylinder.

(C)METHODS FOR ASSESSMENT OF MAXIMUM VISUAL ACUITY:

=========~~ CF --6/12

Low hypermetropia

========:3H- a/18 -o/a

(~171eoreticallv.()ne ray (~rIiKltt. (b)PmcliCfllly,pellcil (if IIgltt mys.Fig.16.6:0ptical principles of pinhole test.

(1) Pinhole test:Indicatio . To determine whether reduced visual acuity is due to l:efractive error

rather than ocular pathology, amblyopia or neurological disease as follows:I) In refractive errors the pinhole test improves the reduced vision up to 6/9.2) In all other conditions, the pinhole test cannot improve the reduced vision.

Principle:1) Small pinhole allows only one ray of light from each point on an object to

pass through to the screen (Fig. 16.6a): Thus a clear image is formedregardless of the position of the screen.

2) The ideal. pinhole leads to the formation of a clear retinal imageirrespective of the refractive state of the eye: As the ray passes through the.axis of the dioptric system of the eye.

3) In clinical practice, pinholes allow a narrow pencil of light to pass throughthem (Fig. 16.6b):

1- In.low degrees of refractive error:(a)Pinhole improves the clarity of the retinal image with 6/9 vision.lb)Pinhole aperture is 1.2mm for refractive errors from -4 OS to +4 OS.

2- In high degrees of refractive error:Less improvement in VA.(2) Stenopaeic slit:

Indication: It is used to determine the two principal meridians in astigmatism.Principle.

1)The slit aperture acts as an elongated pinhole:For light in the axis of theslit only to enter the eye.

2)80, when the slit lies in one principal axis of the astigmatic eye: Thesecond line focus is eliminated and the blur of Sturm's conoidreduced,allowing a clear retinal image to be formed.

Method:1) The slit is first rotated to a position in which the best vision is obtained.2) Spherical lenses are added to give further improveme.nt in visual acuity.3) The slit is then rotated through 90° and adjust the spherical lens power to

give the best vision.4) The cylindrical correction required by the eye equals the algebraic

difference between the two spherical corrections used, and its axis is thatS~ of the original direction of the slit.'13) Duochrome test:

Principle It is based on chromatic aberration ( Fig. 8.1) in which light rays atthe violet end of the spectrum are deviated more than those which come fromthe red end and so a hypermetropic eye can read blue letters better while amyopic eye can read red letters better.

J..1ethod:1)Thc paticnt looks at uppcr red and lowcr blue horizontal p~\llcls ~ltthe

anterior surface of a box, with black letters on each panel, larger at the lefthand end than at the right hand end.

2)Letters on both panels appear equally clear if error of refraction is corrected.Indications:

1) To avoid overcorrection of myopia: ·If the letters on red panel areclearer on blue green panel.

2) To compare final refraction of two eyes:J3lue green panel is better used.3) To ensure the cylindrical correction in astigmatism: If the red letters are

clearer in a special meridian, it is sti II myopic.Allied tests

1) Duochrome test may be incorporated in Snellen's test type chart.2) Red and green colours may be incorporated in astigmatic fan.3) Cobalt blue glass is used as the duochrome test:lt allows only red and

blue rays to pass and so red point with blue circles in M or blue point withred circles in 1-1are seen.

--- --- ~--

(a) The di5 . (b) Normal imaKe. (c) Image ill keratocollUS.?'tt Fig. 16.7: Placido disc.

(4) Placido disc and electric keratoscope:Principl : It is bassed on the study of the first Purkinje image for anterior

corneal curvature.Indications'

1) Confirmation of corneal astigmatism and assessment of its axis.2) Demonstration of lenticular astigmatism if an astigmatic eye has normal

corneal configuration.3) Postoperative examination of corneal grafts.4) Detection of corneal scars and ulcers.

Methods1)Placido disc: .

1-This is a flat disc bearing concentric black and white rings (Fig. 16.7a).2- A convex lens is mounted in an aperture in the centre of the disc.3- The examiner looks through the central aperture and observes the image

of the disc formed by reflection from the patient's cornea.4- Best results are obtained by ensuring that the disc is brightly illuminated

by adjusting the light behind the patient's head, leaving the patient's eyein shadow.

5- The nature of the image of the disc renected from the cornea provides anidea about the regularity or distortion of corneal curvature which is seenby the observer as:(a) Normal concentric circular rings:If no astigmatism (Fig. 16.6b).(b)Abnonnal elliptical rings.~ In regular astigmatism.(c) A bnormaldistorted rings: In irregular astigmatism as keratoconus

(Fig.16.6c).2) Electric keratoscope:'

1- It gives a brilliant and clearly defined reflected image from the cornea.2- Used to interpret entire corneal patterns by directing the patient's gaze

along any meridian.

(2)Tor(3) PIa

CAUSm(1) Ne'(2) Ole(3) Fra

DEFINI1LENSM

(1) Gla!1)

AP ER 7SPECTACLES

THE MAKING AND FITTING OF SPECTACLES(1) ECTACLE FRAME

CHARACTERS OF AN IDEAL FRAME:(1) Holds both lenses in plane perpendicular to the direction of regard:

1) For distance Vertical pla~e.2) For reading 1- Angled downwards 10-15°.

2- Slightly lowered.3- Converges slightly.

(2) Practically held at 12-14 mm:TheoreticaIly it is better to he held at 15.7 mm.(3) Frame cells should be relatively large: As large lenses permit larger field.(4)Rigid, strong, light, fits securely, rests lightly and non irritant to skin points

on which it rests.TYPES OF FRAMES:

(1) Spectacle frames.(2) Nose frames: In presbyopes.(3) Monocles: For rapid reference in presbyopes.(4) Lorgnettes:Held in the hand for rapid reference as monocles.

MA TERIALS OF FRAMES:(1) Metal: 1) Stainless' steel.

2) Gold.3) Aluminum.4) Nickel.

(2)Tortoise shell.(3) Plastic.

CAUSES OF SKIN IRRITATION FROM FRAMES:(1) Newly spectacle frames bear heavily on the skin.(2) Old spectacles with dirty or rough surface.(3) Frame material specially with nickel or plastic.

(2) CTAC~ i C NSES1) SINGLE VISION LENSES

DEFINITION: These are lenses with one focal power.LENS MATERIAL:

(1) Glass lenses:1) Crown glass 1- Hard with refractive index 1.52.

2- 92% light transmission(so only 8% light reflections, 4% ateach surface).

2) Flint glass. 1- Refractive index 1.62-1.70.2- Thinner(edge thickness 30% less)but heavier than crown glass.3- Has a slight yellow colour.4- Useful especially for bi focals and achromatic lenses.

3) Ultrathin flint glassI-Refractive index 1.8-1.9.2- Thinner, lighter and with more resistance than the 1.70 flint glass but with

less light transmission (due to higher RI with more reflectivity).3- Still heavier than plastic lenses (due to higher Rl than plastic lenses).4- Useful in aphakic lenses as magnification is reduced from 30% to 18% (but

binocular ision is still not possible in most specta,cle corrected unilateralaphakia).

(2) Plastic lenses:Advantages.

I.)Light transmission is more than glass due to lower Rl with less reflectivity.2)Lighter (1/2 weight of glass lens). .3)Can be coated to reduce Iight transmission.4)Can be dyed to reduce reflections.5)Can be used as an achromatic lens.6)Do not fog with changes in temperature.7)Sa fcr (but not unbrcakablc).

Disadvantage :I)Scratch easily (more with old plastic lenses).2)Wrap (deformed) on heating or under pressure during processing.3)Edge thickness of strong concave lenses is more than glass lenses (due to

less refractive index).4)\Vann rapidly than glass lens.

Types1) Old plastic lenses:-Methylmethacrylate with a Rl of 1.49.2) ew plastic lenses:rr'hermosetting resin with higher Rl of 1.50-1.60.

NBl :Scratch-rcsistant coating and antifog coatinl! of lenscs:Are availabk.NB2:Antifog coating of Icnscs:Makes tlte lens surface more wettable witlt formation

of a titill film of water (instead of minute droplets) with good light transmissionand it evaporates wlten tile lells warms up.

LENS CHARACTERS:(1) Shape: I) Round.

2) Round oval.3) Long oval.4) Pantoscopic (an oval with a flattened top).

(2)Size: Plastic lenses can attain large sizes with fewer aberrations than glasslenses ( needed for children for larger field).

(3) Weight and thickness: 1) Flint glass is heavier than crown glass but thinner.2) Plastic lenses are lighter than flint and crown glass.

LENS FORMS:(1) The ideal lens:A lens which corrects all aberrations but this is impossible.(2) The best forms lenses: It is a lens which reduces the aberrations.(3) The standard lens forms :A limited number of standard(shaped) forms with:

Surfaces A standard surface (base surface) with a known base curve and acombining surface.

The radius afcurvature afeach surface(r) Is calculated from the formula:D = n2 - n1 / r (Chapter 4) to obtain a given surface power.

The base curve: ositive or negative power of standard surface (base surface):1)A negative base curve is used· or positive lenses from +7 to 0 DS and a

positive base curve for negative lenses from -20 to 0 DS (there is nostandard lenses for more than +70S or -200S).

2) The best practical base curves are: .1- -6 D next to the eye for +7 D to 0 lenses.2- +6 D fulihest from the eye for 0 to -6D lenses.3- + 1.2S D furthest from the eye for -6 D to -10 D lenses.4- Plano furthest from the eye for -10 0 to -20 0 lenses.

l(a)Svmmetrical. (b)A!'Jymmetrical. (C)PftllW. (d)periscopic. (e)Deep menisclis.

Fig.!7.1: Standard forms of spherical lenses.The standard forms of lenses in practice'1) phericallenses (Fig. 17.1)

1-Symmetrical: A biconvex or biconcave lens in which the two surfacesare ground similarly.

2- symmetrical: A lens in which the two surfaces are ground differently.3- '101101: The whole of the curvature is placed on one side while the other

side is plano.4- Periscopic: A lens with a base curve -1.2SD.5- Deep meniscus: A lens with a base curve -6.00D.

2)Toric lens (Fig. 17.2)f. It is a meniscus lens with:1- ne toric surfac :A curved cylindrical surface ground on the spherical

surface of one side.2- he stanaard surface 10n the other side with a base curve -60.

(a)PLanocon vex. (b)P/anoconcave. ((~Con vex men isclls. (d) Concave men isclis.ig.17.2:Toric lens. Fig. 17.3:Lcnticular lenses.

3)Lenticu ar lens1- A high powered lens which is ground only in the centre, but the field of

vision is reduced.2- It is used in aphakia and in extreme myopia (i.e. beyond +7D and -20D).

4) s he ical (a lanatic) lenstfhe peripheral palis are of less curvature thancentral parts( Fig.8.3).

SPECIFICAT10 ( OTATION) OF LENSES IN PRESCRIPTIONS:(1)A spherical lens alone:A lens of ID is written as:

1) + 1.00 DS (dioptre sphere) for convex lens.2) -1 DS for a concave lens.

(2)A cylindrical lens alone:The dioptric power and the orientation of the axislllIlst be specified: A lens of -1 D,placed with its axis( of no power) veliical iswritten as -1 DC axis90° (DC = dioptre cyl inder).

(3)A combined spherical and cylindrical lens:As +2.00 DS and + 1.00 DC axis+2.00 DS

90" is written as: + 1.00 DC axis 90° .Nill :Arrow indicates the axis of cylinder in addition to the ngure: For more assurance.NB2:Cylindricallens is placed in front of spherical lens: To aI/ow tlte axis line to be see/l.

THE SPHERICAL EQUIVALENT OF OPTICAL PRESCRIPTIONS:./ (1)Adding half of the dioptric power of the cylinder to the sphere:For fitting of

soft contact lenses to correct spherical errors with a small degree of-6 DS

astigmatism: Therefore -1 DC will be -6.5 DS.

(2) Reducing the correcting cylinder a given amount and algebraically addinghalf of the dioptric reduction of the cylinder to the sphere:For a largeastigmatic error in which full correction may not provide comfortable vision, ina patient who has not previously worn glasses:

. +3.00 DSTherefore to reduce -2.00 DC from a full correctIOn of -7.00 DC axis 90° ,

. . . +2.00 DSthe sphencal eqUIvalent will be -5.00 DC axis 90°

TRA SPOSITION OF LENSES TO EQUIVALENT FORMS:Definition:The changing of a lens from a standard form to another equivalent form.Types of transposition:

(1)Simple transpositionDefinitio : It is the alteration of a standard form into another equivalent form

of standard lenses.~\pesJl)Simple transposition of spheres:

nitiol1' It is the alteration of the lens form of spherical lenses.resul :Algebraic sum of surface powers of the original spherical lens

(transposition of +2DS sphere into a periscopic form i.e. with

base curve of 1.25D,bccomcs -1.25DS and +3.25DS,( Fig.I?1 d).imple trans osition of cylinders:

mtlOn It is the alteration of the standard form of cylindrical lenses.'Su t A sphere of equal power and of same sign as original cylinder of

equal power but of opposite sign to the original cylinder andhaving an axis at right angles to the previous axis.

cations:1-To keep the axis of the cylinder in the two eyes nearly in the same

direction.2- To make the axis of the cylinder vertical in order to diminish the

prismatic effect of the lower part of the lens if the axis of thecylinder is horizontal(e.g. +1.00 DC axis' IS0° will be +1.00 DSand -1.00 DC axis 90°.

3 Simple transposition of spheres and cylinoers:@!!!~~lticio~n.s alteration of standard form of a combination of spherical and

cylindrical lenses and the result is a sphere of a power equal to••the algebraic sum of the original sphere and cylinder powers and

a cylinder of equal power to the original cylinder but of oppositesign and having an axis at right angles to the previous axis.

,catIOns an exam les:1-To keep the axis of the cylinder in the two eyes nearly in the same

direction which is better to be a vertical direction( e.g. If right eye -1.00 DS with +2.00 DC axis IS0° and left eye -0.50DS with -1.00DCaxis 90°,the right eye will be + 1.00 DS with -2.00 DC axis 90° .

2- To keep the lenses as light as possible( e.g. to add + 1.50 DS as apresbyopic correction to -0.50 DS with - 1.00DC axis IS0°, simpletransposition will be + 1.00 DC axis 90).

3- Cross cylinder may be better than a spherocylinder as it is lighterwith a wider field( e.g +3.00DS with -4.00DC axis IS0° will be+3.00DC axis 90° with -1.00DC axislSOO).

4-To compare the present refraction with a previous prescription.(2)Toric transposition-

Definitiofl: It is the alteration of a combination of spherical and cylindricallenses into a toric lens for ideal correction of astigmatism (depends onthe same principles as simple transposition).hod: 0 transpose +3.00 DS with +1.00 DC axis 90° to a toric fOnllula

with a base curve -6:1) Firs transpose tfIe glven prescription: To one having a cylinder of

the same sign as the base curve which is to be used:+3.00 DS +0.400 OS

So +1.00 DC axis 90° becomes -1.00 DC axis ISC)".2) Then the toricformula:

Sph'ere - base curve powerbase curve (with its axis at right angles to that of the cylinder) /

base curve + cylinder (with its axis at right angles to that of the base curve)1- Nomerator:Sarface power of the spherical surface.2- Denominator:Surface power and axis of the other principal

meridian of the toric surface.VERTEX FOCAL LENGTHS (FIG.17.4):

(1)The principal points of a thick lens lie:1-Wi thin the lens in biconvex or biconcave lenses.2-0n the curved surface in planoconvex or concave lenses3-0utside the lens in convex or concave meniscus lenses.

(2) In practice, measurements given by instruments such as the focimeter are:I) The anterior vertex focal length (AVFL): It is the distance from the centre of

the anterior surface (anterior vertex) of the lens to FI in convex lenses or toF2 in concave lenses.

2) The posterior vertex focal length (P VFL): It is the distance from the centre ofthe posterior surface (posterior vertex) or thc lens to F2 in convex lenses or toF I in concave lenses.

NB:A VFL and PVFL are measured by the focimeter: Are not equal and differsfrom Fj and F2 of the eye.

P, P,P,P,

F,'iF I ~,

0( AVFL PVFLPVFI E'- )r, 0(

/," I,

(a)Convex meniscus. (b)Concave meniscus.FIG.17.4:Vertex focal lengths.

BACK VERTEX POWER:Definition:Power of posterior surface of lens in D , i.e. reciprocal of PVFL in m.Factors affecting the back vertex power:

I)The lens desig :Shape, size, thickness and form of combination of thick lenses.2) The distance at which the spectacles are worn (the position ofspectacles in

relation to FI a the eye): s an effect on the image size,image movement andback vertex poweres( Table V).

~Bl :Spectacles are graded by backvertex power: As their PVFL corrects optical defects in eye.NB2:Back vertex power differs from the equivalent power (true focal.power) of the lens

(calculated from the two surface powers plus a conection for vergence change due tolens thickness): Which calise error in dispensing high-powered spectacle lelises or highlycurved contact lenses amI so there are mathematical tables to adjust this discrepancy.

NB3:Difference in image size increases with increases in the bad<- vertex power of the lens and.•so in anisometropia discomfort occurs: Due td unequal refractive error of the two eyes.

Table V:Effect of distance at which spectacles·are worn on back vertexpower and on sizc(RSM) and movement of retinal image.Image size Image movement Back vertex powerLellS form

1) Convex lens:l)AtFI:

2) Outside FI:3) Inside Fl:

(2) Concavelens:I)AtFI:

1- Same. size in axial H.2- Larger size in refractive H.Larger size.Smaller size.

Moves forward~.Moves backwards.

Increased.Decreased.

1- Same size in axial M.2- S~naller size in refractive M.

re:'e of)[ to

2) Outside F I: Smaller size.3) Inside F] : Larger size.

M<;>vesbackwards.Moves forwards.

Decreased.Increased.

BACK VERTEX DISTANCE (BVD):Definition: It is the distance between the posterior vertex (centre of the posterior

surface) of the trial lens in the back cell of trial frame and the cornea(in mm).Measurement of the back vertex distance:

(J)A millimetre scale or a screw on the side arm of the trial frame pointintowards the temple

(2)Disc with a stenopaeic slit through which a I illimetre rule is passed: Until ittouches the closed upper lid (with a correction factor to be added to allow forthe lid thickness).

(3) Wessely keratometer.(4)Zeiss parallax vertexometer(5)Exophthalmometer.

Effects of back vertex distance in relation to anterior focal point of the eye:(1) On the back vertex power Crable V).(2) On the size of retinal image (Table V and Fig. 17.6).(3) On the movement of retinal image (Table V and Figs. 17.6 a & b).

Clinical applications:(I)A stronger convex lens or a weaker concave lens is required in the spectacle

plane (13 mm from cornea) than in FI of the eye (15.7 mm from cornea).(2)A hypermetrope or a presbyope gets his glasses far away from his eyes to get

a larger fundus image while a myope gets his glasses nearer to his eyes toget the same effect.

(3 )The back vertex distance should be measured for prescriptions over 5Dwhich materially affects the optical condition as in:

I) Aphakia,high myopia and contact lenses.2) Spectacles worn at a different distance as in a high bridge nose.

SPECTACLE MAGNIFICATION:Definitions:(I)Spectacle magnification(SM):

'e of>1' to

. . corrected ametropic image sizeIt IS the ratIO . . .uncorrected ametropic Image SIze

(2)Relative spectacle magnIfication (RSM) .corrected ametropic image size

. It is the ratioemmetropic image size

Fig.17.5:Variation in size of retinal image with differcnt positions of spcctaclcs:(a)Axial H with cOllvex lens at Fa; (b)Axial M with concave lens at Fa;(c) Fa,'(c) Axial H with convex lens inside Fll (CL); (d Axial M witlt concave lens inside

Fa (CL); (e)Refractive H(apltakia) with CL at Fh;(f)Refractive M with concave lens at Fm.Clinical application: Clinically it is more useful to use RSM than SM:

(1)1/1 axial ametropia:l)If the correcting lens (convex or concave) is placed at Fl of the eye:

Image size is the same as in E (Figs.17.5a and 17.5b) and so RSM is unity.2) Uthe correcting lens (convex or concave) is placed nearer than Fl:.

i-Convex lens:The image size is decreased than in E(Fig.17 .5c) and soRSM is less than unity.

2-Convex lens The image size is increased than in E(Fig.17.5d) and soRSM is greater than unity (thus contact lenses in M havea magnifying effect).

(2)1n refractive ametropia In contrast to axial ametropia the image size inrefractive ametropia differs from the E image even when the correctinglens is at the anterior focal point of the eye:l)If the correcting lens is placed at Fl of the eye: I

i-Convex lens:The Image size is increased than in E(Fig. 17.5e) and. RSM is greater than unity.

2-Concave lens: The image size is decreased than in E (Fig. 17.5t) and

RSM is less than unity.2)lf the correcting lens (convex or concave) is worm nea-rer to the eye .

than F(: The image size approaches the E image size and so RSMapproaches unity.

3)Spectac e correction in aphakia (refractive H)' produces:1-RSM of 1.36 when placed at F1 of the aphakic eye (Fig. l7.6a).2-Spectacles are usually worn 12-14 mm from the cornea and the RSM

at this position is 1.33.3-When a contact lens is used, RSM is reduced to 1.10 (Fig. 17 .6b) .

••...•••.• .•.F ,,,~

F tph .•..•.••.•.•••.

(a) With spectacles at Fl. (b)With a contact lens.Fig. 17. 6:Relative spectacle magnification in corrected ophakia:

LENS CENTRES (CENTERING OF LENSES):(l)The geometrical centre of a lens: It is the point in the middle of the lens.(2)The optical centre of a thin lens: It is a point on the principal axis through which

all rays undergo no deviation (it should correspond to the visual axis of the eye,Chapter 6) and can be detected by:

1) Fusing 2 images formed by 2 lens surfaces when a distant light spot isviewed through the lens.

2) Image formed when two lines, crossed at 90°, are viewed through the lens(Chapter 17).

3) Focimeter (Chapler 30).THE DECENTRING OF LENSES:

Methods:(1 )The frame may be displaced by lengthening or shortening nose-piece.(2)The lens may be displaced, which is more cosmetic ant so is preferred.

Effects: The lens becomes a prismosphere as if a prism is incorporated in the lens:(1) The strength of the prism vary with the amount of decentration and

with the lens diopters (there is a prismatic efrect of I 6 per I D 1'01'

every 1 cm of decentration).(2) Decentring a convex lens acts as if a prism is incorporated with its

base towards the direction of decentration while of concave lens hasthe opposite effect(Fig.l7.7).

(3) Decentration is an alternati ve to a prismatic effect by a prism in orgrinding the lens ..

Advantages: It is easier and cheaper than incorporating a prism in a lens.Disadvantages: (1 )The weight of the lenses increases.

(2)Chromatic and distortion efTects (decentration of> 4 mln).

Indications:(1)Decentring for near work:

1) Optical centres for reading should be about 6.5 mm below the horizontal.2) Head is lowered 20-300.3) Eyes are rotated downwards 150.4) Amount of convergence is about 2.5 mm from the mid-line(Fig.17 .8).

(2)Decentring to adapt a pair of spectacles to an asymmetrical face.(3)Decentring in heterophoria or in a deficiency or an excess of convergence.

TYPl(1)

Fi2:.17.7:DecenterinQ: of lenses bv displacement. FiQ:.17.8'?1)~cel1tering of lenses tor near work.ORTHOSCOPIC LENSES:

Definition: Are prisl110spheres which relieve the same amount of accommodationand convergence. .

Example: If +2D sphere lenses are required for near work~ prismospheres of +2Dsphere with prisms (bases-in) to produce 2 ma of convergence.

Indications: Useful in non-presbyopic persons to increase VA for very fine workby increasing the size of the image as in operating ophthalmicsurgeons( as the simple binocular magnifying glasses lead to strainas the accommodation is relieved while convergence is not).NB:Orthoscooic lenses cannot be used in nresbvones: As tlte COllverf!ellce

remains cOllstant while accommodation (limillishes with age.2) MULTIFOCAL LENSES

DEFINITION: These are lenses with more than one focal power.TYPES: (1) Bifocal lenses: With two focal powers.

(2) Trifocal lenses: With three focal powers.(3) Varifocal lenses (progressive addition lenses): With gradual change of

power: 1) Over the. whole lens, or2) Over a region intermediate between areas of uniform powers.

1- BIFOCAL LENSESDEFINITION: Are lenses with two focal powers, one for distant vision and the other

for near vision.

INDICATIONS:(1) Presbyopes:With distant ametropia.(2) Myopes with: 1) Esophoria for near.

2) Poor accommodation.(3)Aphakia: Young aphakics can tolerate them (but not old aphakics).(4) Hypermetropia with: Esotropia for near.

CONTRAINDICATIONS:(1) Unstable patients; As intolerance often occurs.(2) Handicapped patients as lirilping:To keep eyes opposite lens optical centre.(3) Marked astigmatism and anisometropia: Due to the prismatic effect of

lenses.(4)Certain occupations:

1) Sports as golf because a full field of distant vision is required.2) Sailors and builders due to loss of balance.

TYPES:(1) Franklin bifocals (split bifocals): Two separate lenses in the same frame

(obsolete, Fig. 17.9). \(2) Cemented wafer bifocals: A supplementary lens (wafer) is cemented on the

surf8cc of the m8in lens by Canada b81sClm with the same refractive indexglass( obsolete. Fig.17.1 0).

Fig. 17. 9:Franklin (split) bifocal lens. . Fig. 17.lO:Cementcd wafer bifocal lens.(3) Fused bifocals (invisible bifocals):

Ch9racfers: Are made by combining a lens of crown glass with a segment offl int glass of higher refractive power:

1)The difference in the refractive index leads to the increase in refractivity.2)The spherical surface of a crown glass lens is hollowed out to receive a

segment of flint glass and the two are fused together with heat at 6000 C.Types o.ffused bifocals (Fig. 17.11):

1) Rounrl segment fused bifocals(collvcntional type. The edge of thesegment for near forms a part of a circle( which is liable to optical defects).

2) Flat topped "0 segment" fused bifocals 0 segment for near is 22 X,16mm and is flat topped to get rid of the part with most optical defects.

3) Flat topped "rectangular segmcnt" fused bifocals:! The rectangularsegment for near is 22 X, 9 111m which permits distant vision below it.

4) Curve topped fused bifocals: The lower segment is as D segment but itsupper border is curved with its convexity upwards.

Disadvantages iffused bifocals:More in conventional round segment type:1) Chromatic aberration due to difference in dispersion of two types of glass.2) Less definition on reading due to contact surface of two types of glass.

3) Restricted size and so a limited field for near, as the segment diameter isless than 24 mm.

4) Thicker and heavier than other types.

CH(:

i-Rounded. 2-Flat tOlJ(D). 3-Flat top(reclallgular). 4-Curve lopped.Fig. 17.11:Fuscd (invisiblc)bifocallcnscs.

(4)Solid bifocals (one-piece bifocals):Characters These are made of one piece of glass or plastic:

1) Two distinct curves are ground upon one spherical surface which is thesegment side while any cylinder is ground on the other side (Fig. 17.12).

2) Segment side is usually posterior except in higher ametropia.Advantages overjilsed bifuc.;als are

1) No chromatism.2) Better definition through the reading portion.3) Thinner and lighter.4) Larger field for near, as the segluent diameter is 22-45 mm.

Fig.17.12:Solid lcns. Fig.17.13:Exccutivc I. Fig.17.14:Prism-controllcd I. Fig.17.15:Up-cun'c I.Special types of solid bifoca s:

1) Executive bifocals:I-As Franklin bifocals, but is one piece of glass or plastic(Fig.I7.I3).2-The optic:ll centres of both portions are situated at the dividing line

which extends across the front surface of the lens and so there is a widefield for near and there is no prismatic jump.

2) Centre-controlled solid bifocals:nn whidl there is a selection of the sitesof optical centres.

(5) Other types of bifocals:l)Prism-controlled b(focal : Contains a prism to counteract their intrinsic

prismatic effect (Fig.I7 .14).2) Up-curve bifocals: ][0 look up from near work (Fig. 17.15).3)Monocentric bifocals· In which the optical centres of the two portions

coincide at the interface and so both centres are away from their visual points.4)Rising-front bifocals:Spectacles can be raised or lowered by an adjustable

step in the nose-piece:1- In the first position the reading segment is almost straight in front of pupil:

To read without having to hold the written matter below the horizontal.2-1n second position, reading segment is below visual axis: In walking about.

5) Hook-fronts Are separate pairs of lenses placed in front of the distant lenses.CHARACTERISTICS OF AN IDEAL BIFOCAL LENS:

(1) Optical requirements:1) Clear vision with no aberrations by the two segments of a bifocal len : By

bending to a toric form (but oblique astigmatism is unavoidable on eccentricviewing through the lower segment).

2) No sudden change at junction of2 segments to avoid prismatic jumping by:I-Monocentric bifocal ( optical centre of distant segment coincides with that

of the near portion).2-0ptical centres of distant and near segments are at or near the junetion of

two segments.3- Prism base-up in the reading segment.

3) The centring of the two segments should be separately controllable:I-Distance visual point (DVP)'~ point in spectacle lens through which the

-----visual axis pass in distant vision,it coincides with the optical centre and isabove the top of the lower segment.

2- The near visual point ( VP):Through which visli~l axis pass on reading,at a point usually 8 mm below and 2 111mnasal to the DVP (Fig. 17.16).

(2) The needs of the individual patient:1)A typist or supermarket cashier: larger near segment.2) Outdoor person for a widefield of distance vision: smaller near segment.

N::B:Emmcrrooic ocrsons al'c oftcn well-suited try half(pantoscooid glasses forpreshyopic correction alone, while a low myope may choose a spectacle lens withthe lower portion cut away, enabling him to read with the naked eye.

(3) Other requirements: Bifocals should be light and the lenses invisible.

I Ijt"1,201m

Fil!. 17.16:Thc geometry of bifocals. Fill. 17.17: Prismatic cffcct at NVP.OPTICAL DEFECTS OF BIFOCALS:(1)Aberrations of spectacle lenses: Chapter 8.(2)Other optical defects of bifocals:

1jPrismatic effect at the near visual point (Fig. 17.17):rentice ru e; P = D X h; Chapter 6 and Fig. 6.13.

causes: 1- A prism base-up in convex distant lenses and base-down inconcave distant lenses.

2- A prism base-down in the reading segment in conventional types.3- Incorporated cylinder in the bifocals.

Corrcctio •1- Base-up prism in reading segment to cancel its prism base-down ~ffect.2- The optical centre of reading segment is brought Close to the NVP as in:

(a) Flat topped fused bifocals.(b) Curve topped fused bifocals.(c) Executive solid bifocals.

3- The optical centre of the distant segment IS lowered (decentration) toapproach the NVP.

4- Equalisation of the prismatic effect in both eyes by differem: segmentdiameters in both.

prisnJ of the edgeof tlte segment

Fig.17.18:Prismatic jump. l?ig.17.19:Elimination of prismatic jump.2)Prismatic jumping at the top of the reading segment:

Definitio : It is abrupt displacement of image with change of vision due toprismatic effect at the top of the lower segment with prism base-downeffect (Fig. 17.18). .

Disadvantages: More in high-powered bifocals and with extra ocular muscleimbalance.

orrections:1- Executive solid bi focals.2- Flat topped or curve topped fused bifocals.

3-Prism-controlled bifocals (Fig. 17.19).4- Monocentric bifocals.

3) Oblique astigmatism at the near visual point:Cause On reading, the near visual axis is oblique to the optic axis of the

near portion of the lens.Correction: -lard to correct but most patients can overcome its effects.

4)Limitatiol1 of the visualfield: oth for near and for distance are limited and soif a large field for certain activities use: 1- Executive solid bifocals, or

2- Flat topped fused bifocals.

2- TRIFOCAL LENSESDEFINITION: Are lenses in which a second intennediate addition is introduced in a

bifocal lens to fill in the region where clear vision cannot be obtained througheither the distance portion or the near portion.

INDICATIONS: For certain activities in advanced presbyopes when amplitude ofaccommodation has declined to the point where intermediate distances are blurred.

ADVANTAGES: (1) Larger field of vision.. (2) No prismatic jump effect.

(3) Clear vision at intennediate distances.DISADV ANTAGES:Not suitable for:

( 1) Anisometropes.(2) Prism use for near work.(3) Very small segments as in fishing spectacles.

(ll)Fused trifoca[(D-se!!ment) lens. fb)Solid trifocallells.Fig. 17.20: Trifocal lenses:

TYPES:(1) Fused trifocals:

1) Theflat topped (D segment) fused trifocals (common type, Fig.17.20a):1-Distant portion of crown glass with refractive index 1.52.2- Intermediate portion of light flint glass with refractive index 1.57.3- Reading portion of dense flint glass with refractive index j .62.

2) The intermediate portion.·r s refractive index is intermediate between that ofdistant and reading portions (so the addition is half that of near portion).

3) The advantages of fused trifo.cals are:'1- Larger field of vision during near work (as the top of the intermediate

portion is higher).2- Clear distant images by the intermediate segment (as it is half the power

of the near segment).(2)Solid trifocals:Executive solid trifocals (common,. j 7.20b) and there is no

prismatic jump.(3) Combined fused and solid trifocals: The intermediate addition is near the top

of the lens (Fig. 17.20c).3- VARIFOCAL LENSES

(PROGRESSIVE ADDITON LENSES - PALs)DEFINITION:A lens with progressive change of power from top to bottom for

Intermediate distances from infinity to workingdistanccs.PROGRESSIVE LENS POWER:

(1) Concave surface: For distance sphere and cylinder lens prescription.(2) Convex surface: 4 optical zones usualIy: 1) Spherical distance zone.

2) Transition zone (corridor).3) Spherical reading zone.4) Peripheral distortion zones.

ADVANTAGES: (1) No abrupt change in power.(2) No prismatic effect.(3) No apparent lens segment.

DISADV ANTAGES:(1) Restricted visual field.(2) Gross distortion on looking to the side (especially if there is high degree or

astigmatism at an oblique axis) due to aberrations by the lateral parts of thelenses (as their curvatures are nonspherical).

(3) They are not suitable when there is a large cylinder in the prescription.(4) Ditlicult veritication of multifocal spectacles: \

1) By neutralization method: On centering over maximal add reading portion.2) By focimeter:But the correct position can be obtained from:

1- The regularity of the circles of dots.2- If axis of cylinder in reading segment is same as in distance pOliion.

DESIGNS:(1)Hard lens design:With large area of sharp definition but with greater degree of

astigmatism on looking downwards and nasalIy or dowrlwards and temporally.(2)Soft lens design:With minimal degree of astigmatism but with small area of

sharp definition( but is preferred).TYPES:

(1) Varilux fused varifocal lens (Fig. 17.21.a):1) Upper portion a: Of uniform power is for distant vision.2) Intermediate zone b: A 12 mm vertical segment of increasing power.3) Lower portion c: Of uniform power for reading (about 30° below a head gaze).

(:l)Var.i!ux fused lens. (b)Omnifocallens.Fig. 17.21: Varifocal (progressive addition) lenses.

(2)Omnifocal varifocallens (Fig. 17.21b): .1) It has increased power in the vertical and horizontal directions simultaneously.2) Add +0.50D to the subjective reading addition as the maximum addition is

(1) SP1)

(2) RJ(3)H

rnr2'

(4)S]12

(5) D

IN(1;

(~o(1

found only near the lower rim of this lens i.e. below the near visual point.SPECTACLES FOR SPECIAL PURPOSES

(1) SPORT SPECTACLES:1) Spectacles for shooting or playing billiards: Decentred vertically (adjustable

angle joints to tilt lenses upwards) to get no prismatic effect on upwards gaze.2) Fishing spectacles: Small bifocal segments fitted below and to the side to

allow full distant vision~(2) RECUMBENT SPECTACLES: With right angled prism (to read in comfort).(3)HEMIANOPIC SPECTACLES: Make the wearer aware'?fmovements in the

missing part of the field with:1) A right angled prism (8 6.) with its base towards the blind side, or2) A small mirror.

(4)SHlELDED (SOLUMBRA) SPECTACLES:1) With a flange attached to the upper rims to eliminate glare from above.2) Side-shields to lessen lateral glare (for patients with early cataracts on reading).

(5) DIVER'S SPECTACLES:Two mcniscus plano-Icnses with a biconcave lens ofair inbetween are used for vision under watcr , to compensate for corncalrefractivity which is abolished when immersed in Water.

(6) CRUTCH SPECTACLES: With a metal extension running inwards from the rim:1) Ptosis spectacles: For a drooping upper lid.2) Entropion spectacles: To evert the lower lid'.

PROTECTIVE SPECTACLESMECHANISM: (1) Diminishing the total intensity of light.

(2) Cutting off a noxious part of spectrum as long infrared, visibleor short ultraviolet rays.

INDICATIONS:(1) To lessen discomfort of glare: By cutting visible luminous rays:

I) 0 protect healthy eye from 1- Strong sun.2- Large mirror surface as sea or desert.

2) To protect diseased eye as in: 1- Prolonged mydriasis or c.ycloplegia.2- Albinos.3- Recent ocular surgery as cataract extraction.4- Photophobia.

(2) To protect the eyes: From the harmful effects of ultraviolet and infrared rays:1) From strong sun.2) From large mirror surfaces as the sea.3) In some industries as electric welding, cinema studios,ultraviolet light clinics

and glass industries.(3) For cosmetic purposes: Chiefly brownish pink and pink lenses.

OPTICAL PRINCIPLES: .(l)ABSORBING GLASSES:

Characters of abso bing lenses:

1)Absorb the unwanted rays: By chemical substances.2) They are prepared by combining the glass with some chemical substances

or substances with special absorbing properties: By metals as copper,gold, manganese, iron, cobalt, chromium and others (these metals are allcoloured and the lenses are therefore tinted).

3) Actual colours of the tinted glasses, whether pink, green, brown, yellow orneutral grey have no relation to the absorption q( any particular11)Q))elength·High ultraviolet absorption is a feature of all types of opticaltinted glasses,but selective infrared absorbing glasses tend to be green andselective ultraviolet absorbing glasses tend to be brown or pink

Methods of making the absorbing lens:1) Jncorporating the chemical substances in the glass lens (wlid tinl.~):Rarely

used as a convex lens will be deeply tinted at the thicker centre, while aconcave lens at the thicker periphery.

2) Swjace coating of plastic or glass lens with deposition of the chemicalsubstances on the lens surface electrotitically: By vacuum coating to form ahard fine film which resists scratching:-

1- Coated strong concave lenses: for high myopes, anisometropes andcertain bifocals especially if flint glass is used, to be thinner.

2- G ra ient lenses: Witl denser coating in the upper part of the lens foroutdoor activities.

3) Isochromatic lenses: By grinding of a planoconvex or a plano-concave lensand then a slab of a chemically tinted glass is cemented onto the lens orinserted into the substance of a split iens (but size and weight are more).

4) Surface dying of plastic lenses: With a choice of colours and usually ha~eultraviolet inhibitors in the resin (with or without using a surface dye) butcannot absorb all the infrared rays.

Types of absorbing lenses.1) Neutral density lenses: :redl,lce the visual spectrum uniformly with 90%

light transmission.2) Lightly tinted lenses:/Reduce glare and absorb some ultraviolet rays with

40-80% light transmission and are used chiefly for cosmetic purposes.3) Dark lenses (sunglasses): 'Transmit 20-40% of visible light:

1- S andaro absorbing lenses· Not suitable for high powered lenses as:(a) +ve lenses are darker at centre and -ve lenses are darker at periphery.(b) Higher power lenses are darker.

2- Uniform density lensest Uniform density in strong prescriptions by:(a) Coating of both lens surfaces with:

a) Absorbing coating of the front of the lens.b) Antireflection coating on the ocular surface <;>fthe lens.

(b)Surface dyed p'lastic lenses.4) Ultraviolet ahsorbing enses: ·Ultraviolet spectacles, contact lenses and

ncesJper,e all

intraocular lenses can be provided with ultraviolet inhibitors within the lensmaterial or by surface coatings to avoid the harmful effect of ultravioletrays on the macula (as in aphakic patients) and on the lens.

5) Photochromatic lens!'

I-It alters its colour on exposure to ultraviolet rays due to incorporationof submicroscopic crystals of substances as silver halides i.e. darkensin sunlight and lightens in shade (but it interferes slightly withrelative perception of colour).

2-New glasses will half-darken in 10 seconds and half-fade in 100 seconds.Types. Single vision, bifocal or multifocallenses.

atenal: Crown, flint and plastic lenses(photochromatic plastic lensescontain a surface layer covered by a hard antircl1ection coatingand it is not efficient as glass photochromatics.

Colour Photogrey, photobrown, ph.otopink, photogreen and photoblue.noications: All conditions of illul1lin'-\tion (full degree of dal'kening may

take about 1/2 hour).6) Polaroid lens:

C aracter:· A film (a matrix of ,ninute dichroic crystals which polarizeslight in one direction) is mounted between two pieces of glassas in ordinary laminated lens.

Aovantages' 1- Arrest horizontally polarized light and so lessens glare.2- Tone down the ordinary light in addition.

noications 1- Motorists.2- Fishermen.

(1)ANTIREFLECTION (REFLECTION-REDUCING) GLASSES:Characters:

1) Reflect unwanted rays on a mirror surface with a very thine 10-15 m)1)coating of layers of a metal as gold, silver or platinum to be transparent toluminous rays to a large extent.

2) The delicate lamina is then protected by incorporating it in the substance ofa split lens where it is cemented or fused.

3) These lenses are more effective when applied to flint glass and can beapplied to plastic lenses.

4) The best example is Schreiner's mirrored lenses.Principler.

l)I5estructive intelferen .I-Light waves reflected fr0111the interface between a single lens coating

and the glass will be I/~ WL out of phase with those rel1ected 1'1'0111theouter surface of coating, and the two light waves will cancel each otherwith no reflection.

2-In Fig.I7.22, a ingle lens coating of 1/4 WL of incident light I with

worularticaland

relyIe a

1calma

ensor

lveJut

destructive interference( as glass interface reflection G willcancel air interface reflection A).

Fig.17.22:Single lens coating.2) Theoretically, this occurs if the following 2 factors are fulfilled:

I-If the optical thiciq,less of the thin transparent single coating on the lensis 1/4 wavelength of light.

2-lfthe index of refraction of the coating is properly selected.3) Practical I): ReOections are 8% in clear crown glass lenses and only I % in

a single lens coating.Light reflections of clear crown glass in air:

Amount of reflection,s : 8% of light ( 4% at each surface) are reflected backtowards the eye.

Types of reflections •.:1) Reflected ghost image: Bright light source is reflected from the ocular, surface to the front surface of weak minus lenses (-0.50 to -1.50 D).

2) Bright spot: Bright sources (as street or head lights or windows in adark room) are imaged by reflection from cornea which is then imagedby either the front or back surface of spectacle lens acting as a mirror.

3) Ring effect: Multiple reflections within the lens in strong minus lenses.4) Reflections of light sources behind the patient: Reflections by either

or both lens surfaces into the eye, usually the back surface in plus lenses.Prevention of reflections: 1) Antireflection or lightly tinted lenses.

2) Lightly dyed plastic lenses.3) Bending the frame inwards.

(3)SPECIAL INDUSTRIAL PROTECTIVE GLASSES:1)Spectacl~s with side-pieces (goggles)~To protect eyes from mechanical injury.2)Non-splintering lens:

yes:1- lass lenses:

(a)I..riplex lens: In-between wo segments of glass there is a cemented plateof cellulose acetate or an adhesive with no sealing at edges to be thinner.

(b)Case-hardened or f, ugnened lense : Heated lens to 6000 C in an ovenand rapidly cooled to harden the outer shell quickly than the inner mass.

2-Plastic lens : These lenses does not splinter with a high safety factor and isindicated for: 1- Motorists.

2- Aviators.3-Children.4- Who olav games wearing their soectar.IP. •. •. - _ ..•.

REMARKS LEARNED BY EXPERIENCE IN SPECTACLE PRESCRIPTIONS(1) Let the transposition for the optician.(2) Change to another segment style in bifocals if troublesome to patient for years.(3) Adjustment ability to raise or lower bridge of s.pectacle frames may be needed.(4) Large spectacle lenses causes aberrations on looking obliquely and in bifocals.(5) Change glasses for old people if at least three lines more are seen, as he may

return to his old lenses.(6) Reading distinctly and 1'011'0wed by a blur may be due to accommodative

inadequacy or refractive.(7) Avoid bifocals with adds of less than +1.25 D.(8) Avoid high-style frames in bifocals or strong lenses.(9) Strong minus lenses can be made more than one-third thinner with the use of

flint glass lenses.(10) Use the same light tints in new prescription ( oflittle physiological value but also

do not hann ).(11) Dark or tinted glasses are comfortable but not necessary (the patient may 1ike to

hide and yet see).(12) Full astigmatic correction for children, less for adults and beware of changing

the axis in old patients.METHODS OF VERIFICATION (CHECKING) OF SPECTACLES

(1) VERIFICATION OF SPECTACLE LENSES:1) Detection method of lenses and prisms: By lens image formed when two

lines, crossed at 90°:

(a)Convex lens (against movement). (b)Concave lens (with movement).Fig.17.23:Detcction of spherical lenses.

l-Sphericallenses:(a)No distortion of the cross: :Even with rotation of the spherical lens.(b)Movement of fhe cro s if the lens is moved from ide to side uud up .

and down (Fig. 17.23)a)1n opposite direction to the lens (against movement) if lens is convex .

. b)In same direc' n as the lens (with movement), if the lens is concave.

2-Cylindricallenses:(a) Distortion of the cross:,Unless its axis coincides with the cross lines.(b)Scissors movement on lens movement,: As the crossed lines are

displaced (Fig. 17.24).(c) The principal meridian is identified, and aligned with the cross:Then ,

each meridian can be examined as for a spherical lens.3- The optical centre of a lens:

(<!) The lens is moved until one cross line is undisplaced.(b)A line is then drawn on the lens surface, superimposed on the undispiaced

cross line.(c) The process is then repeated for the cross line at 90°.(d) The f\oint where lines drawn on lens intersect is the optical centre of lens.

Fig.17.24:Detectiof.l of cylindrical lenses. Fig.17.25:Detection of a prism.4-Prisms ·Pri~m displaces one line of cross towards apex of prism. (Fig.I7.25).

NB:The cross lines are olaced at the furthest convenient distance and tn:lens is held well away from thc cyc. .

2) Checking of the strength of lenses and prisms:1- The neutralization method:

(a)The power of a lens or a prism by the detection method:a)Sphericallenses:Lenses of opposite type and known power are

superimposed upon the unknown lens until a combination is found l:

which gives no movement of the image orthe cross lines (e.g. a +3.00DS lens neutralizes a -3.00 DS lens).

b) ylin rica lenses: Each meridian is neutralized separately.c)F!risms: By a prism with its base facing the apex of the unknown prism.

(b)The supedmposition of the neutralizing lens on the unknownspectacle lens must be:a)The neutralizing ens mustoe placed in contact with either.'

a-The back surface of the spectacle lens (back vertex power); orb-The anterior surface if the spectacle lens is highly curved (accurate if

less than 2D).b) he lenses s 10uld be 1e d in contact: With their optical centres opposite

each other.(c) The lens is then laid down on the protractor(Fig.17.26) with its.

horizontal diameter along the zero line for:a) Marking and reading of the axis of cylinder.b) Measurement of the geometrical centre.

f.ll.3)

c) Verification of any deccntring.NB:The neutralization is a method only for an approximation if

more accurate method is not available.2- Geneva lens measure (lens clock, Fig.] 7.27):

Use. It measures the curvature of lens surfaces of crown glass in anymeridian to get the power.

Disadvantages:(a) It does not measure the back vertex power.(b) Correction factor must be applied for flint glass lenses or plastic lenses.

Principle and method:'(a)The lens clock has three prongs:

a)The central prong is movable and connected to an indicator.b)The other two prongs are fixed. '

(b) The clock is calibrated for an index of refraction of 1.53:Approximately that of crown glass.

(c)The curvature is indicated on the dial in D when the movable pronga((j/lslS ilse/flo Ihe c/lrvalure q(lhe sllI/ace (~llhe unknown lens:

a)Both surfaces of the lens are measured and their algebraic sumgives the lens power.

b)The strongest and the weakest readings of the surface on whichthe cylinder is ground give the cylinder power.

3- Thefocimeter (lensmeter : Chapter 30.

~,

S111.own

f.17.26:Lens protractor. F.17.27:Gerieva lens measure. F.17.28:Glass with 2 Cl"OSS markings.3) Checking of the fitting of spectacle lenses:

1- Checking of the centring of spectacle lenses:Centring of spectacle lenses:

(a) The spectacle frames must be accurately centred, for the optical centreof each spectacle lens to lie upon the patient's visual axis for distanceand for near.

(b)The distance of the visual axis of each eye from the mid-line is moreaccurate than the interpupillary distance'specially with face asymmetry.

(c) Small error especially in segments of bifocals lead to discomfort whichis marked if convex glasses are placed too far in or concave glasses are

placed too far out.The methods of checking of the centring of spectacle lenses are:

(a) Ins ectio :Adjust the trial frame with the mid- pupil(but may be notupon the visual axis). (A)T

(b) Glass with I1vocross markings meetll1g in the centre (or cross wire) lerwith tne trial frame: eli

a) The patient looks straight forward at a light in the distance and rethe observer determines position of the light reflex on cornea. co

b) The frames are adjusted till the cross-lines(Fig.17 .28) meet in the (B)lcentre of t reflection. (1

c) Then the patient looks at a near light for near vision and theframes are adjusted.

(c) Pupillary "distance (ED; rule: Two plano lenses with cross lines.(d) PupillwJ! distance (PD) measure Measure the nasal PD by :

a) A corneal reflection coincidence system: Displaying a brighttarget on the patient's pupils.

b) A split image coil\l:idence system.c) Digital PD metre: With one measurement from 30 cm to infinity

using a microcomputer.2- Checking of the tilting of spectacle lenses:

(a) The spherical power increases slightly.b) A cylindrical effect is added by tilting of the lens, BUT may be useful to:

a- A need of a horizontal cylinder as in aphakia (by straightening of thespectacle lenses).

b- A high myope who cannot afford sphero-cylinders.4) Checking the distance at which the spectacle lenses are worn (back vertex

distance): Chapter 17.5) Other Checking's as lens material, antireflective coating, wrong

prescription or spectacle lens: Are done.(2) CHECKING OF THE FITTING OF SPECTACLE FRAMES:

1) The centring of spectacle frames: As checking the centring of the spectaclelenses.

2) Other measurements: Bitemporal distance (temple width), height and contourof bridge of nose and distance from the spectacle plane to top of the ear.

3) Checking the criteria of an ideal frame: Is discussed.

eRA R 18CONTACT LENSES

OPTICAL PRINCIPLES OF CONTACT LENSES(A)THE BASIC OPTICAL PRINCIPLE OF CONTACT LENSES:The contact

lens corrects ametropia by replacing the anterior surface of the cornea as iteliminates the corneal refraction at its anterior surface ( which is the primary-refractive surface of the eye) becausee the Rl of contact lens( 1.495), of thecornea(1.376) and of the lacrimal (fluid) lens in-between (1.338) are almost alike.

(B)THE OPTICAL SYS'TEM OF A CONTACT LENS:(1)The resulted combined optical system consists of two lenses (Fig. 18.1):

l)The lacrimallen :Surfaces: 1- Anterior convex surface.

2- Posterior concave surface.Importance; It abolishes the refraction at the anterior surface of the cornea

even if irregular.2) The contact fen :

urface : 1- Anterior convex surface.2- Posterior concave surface.

ImportanceA -It determines the curvature of the anterior surface of the lacrimal lens.2-Its anterior surface (the main refractive surface of the eye ) can be

modified by:(a)Adding -ve or +ve power to neutralize the remaining spherical

error of the eye.(b)Adding no powers in afocal lenses.

3-Same posterior curvature of the contact lens and that of the anteriorsurface of the cornea leads to:(a)Increased refractive power of the eye ,if the anterior curvature of the

lens is greater than that of the cornea.(b)Decreased refractive power of the eye, if the anterior curvature or

the lens is lesser than that of the cornea.. .(2)The power of the resulted combined optical system:

1)The power of the contact fens alone: I ~it touches the apex of the cornea withno lacrimal lens.

2) The power of both contact and lacrimal lenses: lfthe lens is focal andcannot touch the cornea.

(3)The refractive power of the eye with a contact lens: The contact lens correctionat the corneal plane is:

vl)Closer to the far point in H : So a lens of greater power is required thanspectacle lens con·ection.

2) Far from the far point in M So a lens of less power is required thanspectacle lens correction.

l'1I_N

(2) Keratometry: For base curve.(3) Focimetery: For lens power.

TYPES OF CONTACT LENSES(1) hard(conventional) contact lenses:

1) hard corneal lenses.2) hard scler:]1 ienses.

(2) rigid gas perrr:eable contact lenses.(3) soft (hydrophilic) contact lenses.

(1) HARD CONTACT LENSES1) HARD CORNEAL CONTACT LENSES

LENS MATERIAL:Polymethylmethacrylate(PMMA) is the standard material with:1)Refractive index 1.495.2)ChemicaUy inert.3)No clinical toxicity.4)Light weight5)Machined easily.

TilE PHYSIOLOGY OF liARD CORNEAL CONTACT LENSES:(1)The contact lens rests on the tear layer:With dissolved oxygen for aerobic

metabolism of the cornea.(2) Hard contact lenses specialty scleral lenses can lead to corneal oedema by:

1) Interference with the tear flow under the contact lens: With decreasedmetabolism with a resulting deficiency in glycogen stores .

2) Lens trauma to the corneal epithelium: hich will deplete glycogen stores,3) Corneal anaesthesia:ln hard contact lens wearers(it returns to normal when

the lens is removed).ADVANTAGES OVER SPECTACLES:

(1)Optical: I) Better VA (specially in high myopia, high regular astigmatism andirregular astigmatism).

2) Increased field of vision.3) Less magnification (essential for aphakia and anisometropia to

correct aniseikonia).4) Less prismatic effect ( i.e. no ring scotoma).

(2)Non-optical: 1) The lens does not become misty easily (in rain, snow and dust).2) Less danger from cuts (than with broken spectacles).3) More cosmetic and psychological advantages.

DISADVANTAGES:(1) Costly.(2) Requires time for patient's instructions.(3) Complications of the lens use.

INDICATIONS:(1) Refractive lenses:

J) J1 metropia:

1- As mmetrical ametropia (irregular astigmatism): r(a) Keratoconus.(b) Corneal surface irregularities.

2- Symmetrical ametropia:(a) High myopia.(b) High hypermetropia.(c) High regular astigmatism.(d) Aphakia.

~: Arrest of mvopia may occur with hard cont,act lens wear: Due to flattening oftlte corneal apex which is the basis of the practice of orthokeratology (i.e.intelltional flattening of the corneal apex).

2) Anisometropia: - Curvature anisometropia2- Axial anisometropia.3- Monocular aphakia.4- Monocular Keratoconus.

3) With low vision aid' As a part of Galilean telescopic system or a reversedone (Chapter 21).

(2) Occupational lenses:1) In which spectacles are not allowed alS: 1- Actors. 2- Aviators.2) In which the ~pectacles are troublesome as 1- Exposure to rain and mist. 2-

Sports.(3) Diagnostic lenses:

1) Indirect ophthalmoscopy.2) Slit lamp biomicroscopy of the fundus.3) Gonioscopy.4) Electroretinography.5) Localization of an intraocular foreign body.6) Orbitonometry.

NB: Therapeutic and tinted soft contact lenses: Replace therapeutic and tinted ItardCOiltact lellses Ilowadays.

CONTRAINDICATIONS:(1) Local ocular contraindications:

1)Lacrimal anomalies 1- Excessive lacrimation or epiphora.2- Excessive dryness as in keratoconjunctivitis sicca.

2) Lid anomalies. 1- Chronic blepharitis. '2- Styes, chalazia and tumours of the lid margin.

3) Conjunctival diseases: 1- Conjunctivitis (infective or allergic). 2- Pterygium.3- Tumours.

4) Corneal and uveal diseases: 1- Keratitis and corneal a!1aesthesia.2- Anterior uveitis.

POcllrPmPA

(2) General contraindications:1) Poor psychological motivation of the patient.2)Bad environmental conditions as hot dusty atmosphere and dirty occupation.3) During pregnancy or with use of contraceptive pills (due to corneal oedema).

Fig. 18.2:Parts of a corneal contact lens:PCC (BC)=Post. central curve(base curve). POZ = Post. optical zone; PPC =Post. peripheralcune: PPCIJV= Post. peripheral curve width; FB = Front bevel; OD = Overall diameter; PPD =Post. peripheral diameter; Gel' = Geometrical centre 'thickness.PARAMETERS OF A HARD CORNEAL CONTACT LENS:

(1) Regions of a corneal contact lens (Fig. 18.2):-1) The axial regiOl : - _

I- The axial region of a corneal lensl Is of varying size and of largelyspherical curve. u.1~

2- The central portion of the posterior surface< Is the most steeply curvedposterior optical zone (paZ) with a spherical curvature callecrtfleposteriorcentral curve (PCC) or base cur (BC).

2) The peripheral regio .1- Peripheral part of the posterior surface of a corneal lenst Is less eurved

than the axial region with a posterior peripheral curve (PPC), a posteriorperipheral curve widtll (PPC/W) and a posterior intermediate curve orcurves (PIC). ~W L,;)~

2- Edge: with a front bevel (FB) of the anterior surface at the periphery.(2)Curves of a corneal contact lens:

1) Continuous curve lens: Which flattens in a non-spherical fashion from theaxis to the periphery.

2) Bicurve lens It has a single peripheral curve and a central curve.3) Tricurve lens: It has two peripheral curves and a central curve.4) Mllfticllrve fen: It has more than two peripheral curves and a central curve.

(3)Suriaces of a corneal contact lens:1) The posterior surfac~: It must conform with the shape of the cornea.2) The anterior surface:

I-Its form depends on ametropia which determines the curvature of the axialregion of the lens.

2- An intermediate zone is present on the anterior surface in lenticular lenses.(4)Edge:Smooth, rounded, not too blunt to be felt by the lid and not too sharp to

ilTitate the cornea.NB:Edl!e lift (Z factor): Is the difference b.etween the most oerioheral band and the

posterior central curve (PCC) and is slightly more in hard than in rigid lenses becauseof the greater need to ensure tear flow.

(5) Diameter: 8.5-9.5 min. usually.(6)Thickness: U.I-0.2 mm.(7) Power: ± 25 D are commonly available.

THE METHODS OF FITTING OF A CORNEAL CONTACT LENS:~) Trial lenses alone:

(1) Trial lenses are fitted successively: From a standard set of hard contactlenses until one is obtained which fulfills the criteria of an ideal lens.

(2)Then the refraction is carried out· With the optimally fitting trial lens (ideallens) in place.

(3)A final lens is then ordered which is reassessed: To improve its fit or opticalfunction.

i?) Keratometry alone: Fittings are based on the the posterior central curve (basecurve) as follows:-(1)On-Kjittin . The posterior central curve (PCC) of the lens is made the same

as that of the flattest meridian of the optic zone of the cornea.(2)Steeper than K fitting. A toric zone something between the flattest and theIi /steepest meridians of the cornea if there is corneal astigmatism.

\' (3)Flatter than K fitting: If there is a spherical cornea to encourage the tear(/ flow.

NB! :The keratometer measures only the curvature of the aoicai zone of the cornea(central 3 mm only) .

NI32:Mean-K fittin!!: PCC is the mean of the 2 keratometric re(l{lillt!s or nearer theflatter one.

NB3:Fittinl!s based on the fluorescein test:(1) Alignment fitting: Like on-K fitting.(2) Aoex-clear tittin!!: Like steeoer than Kfitting.(3) Flat fitting: Like flatter than K fitting ..

NB4:A well fitted lens: Allows corneal steeping of +0.75D or less and 110 distortiollOf the keratomer mires.

(C) Keratometry followed by trial lenses: Keratometry is used at first and thentrial lenses to get the ideal lens and finally refraction with the ideal lens to getthe finallens:-

v(1)Swface anaesthesia: with benoxinate drops 0.4%.(2) Trial lens insertion.:)) Diamete1f: From corneal diameter.

v ./2) Posterior central curve':From keratometric readings.3) Power at the corneal plane' From special tables.

(3) Patient waitfor 112-1hour:For reflex lacrimation and discomfort to subside.(4)Evaluation of the trial lens fitting:The criteria and results oflens fit are done.

<:I 183

EVALUATION OF THE TRIAL LENS FITTING:(1)Criteria of lens fit:

.J)Lens centration on the cornea:<5ross decentration leads to poor opticalfunction through the edge of the lens and discomfort.

2)Lens relation to the eye lids:1-The upper lid covers 2-3 mm of the lens, while the lower lid

exerts no support to the lens except when in contact with its lowermar in (but not cover the lower margin of the lens to avoidoolin of tears between the lens and cornea).

2-Corneal minus contact lenses may ride high under upper lid due toincreased thickness towards the periphery, while corneal pluscontact lenses may ride downwards due to gravity and lid actions.

3)Lens movement: ~/l-Ro~ation of lens without translation:A degree of lens mobility.

,.I ~-Excursion lag:Itens moves 1-2mm to nasal limbus on abduction.v\;) ~ YJ-Lens movements with blinking:

(a)Movcmenls of a well filled lens: Lens first moves downwards with thedescent of upper lid, then it moves rapidly upwards to a level above itsstatic position as the upper lid ascends and finally it settles down againslowly to its initial position.

(b)Movements of an illfitted lens:a Excessive 1110 f fry of 1,e lens: ue to flat or small lens

(which rides high, tends to centre slowly with a centraltouch or may be displaced in the fornix).

rUle 1110 i1ity oJt Ie fen '0 Due to steep or large lens whichrides low or moves little with a'ring of contact.

c mmohilit of the [ells: Due to steep lens with an imprint of its edgeon the cornea.

NB:1f astigmatism is ~ith the rule (flattest K is horizo,ntan:1"llere IS more veruca! "mobility while if agaillst the rule there is more horizontal mobility of the fens.

4)Fluorescein test to evaluate the lens fit:I-ProcedureoThe tear film is coloured by 1% fluorescein to examine

lens position, lens movement and fluorescein pattern betweenthe contact lens and the cornea by:/(a)Naked eye inspection with light filtered through cobalt blue glass.v(b)Slit lamp with a cobalt blue filter and a narrow beam.

2-The normal fluorescein pattern:(a)A central apical clearance with increase in tear pool(intense green).(b)A nalTowing of the lacrimal thickness (dark green with thinning)

in the region of the intermediate zone or curve.(c)Then a widening again (intense green) under the peripheral portion

of the lens. .

3-Evaluation of the tear flow under the lens:/A progressive dilution of thefluorescein in the lens-cornea interspace indicates a good flow of tearsunder the lens(more evident .in astigmatic cornea due to a good flow oftears at the steeper meridian).

(2)Results of lens evaluation:1) Unsatis(qctOlY t:Triallens should be replaced by another one with different

parameters.2)Sat;,~(actol)J fit with an ideal lens:

I-Re:fi:action is carried out to get final lens especially in high refractive errors.2-Lacrimal power change ofO.25D for every 0.05 mm change in pee of trial

lens,and more minus is needed with a steeper base curve( and the revese).NB:Patient's prescription:Should include the lens parameters (diameter, PCC and power).

PATIENT'S INSTRUCTIONS FOR HARD CORNEAL CONTACT LENSES:(1)Wearing schedule: 2-3 hours twice/day and then progressively extending to

all-day wear.Y)Manipulations of the lenses:

I)Insertiol1 of the lenses:1-The moistened lens is placed on the tip of index finger and brought to eye

while the lids are retracted and the eye gazes steadily at approaching lens.2-When the lens is in position, upper lid is released first and then lower lid.

2)Removal of the lens:1-The eye is opened wide so that lid margins are beyond the edge of the lens.2-The index finger is placed at the outer canthus possibly touching the sclera

and with firm pressure backwards stretches the skin and therewith the lidslaterally. '

3-The taut lids will expel the lens (dislocate into a waiting hand).3) Centring of the lens:lf displaced in a conjunctival fornix,it can be centred by :c/l-Five fingers method: 'The lens is felt by all the fiv~ fingers of one hand

through the closed lids and then it is moved back to the cornea./2-Single finger method:'The lens is felt by the middle finger through the

closed lids and then the lens is moved back to the cornea while lookingtowards it

3-Mirror method:The lens is felt directly and pushed towards the corneawhile gazing towards it with the upper lid lifted and lower lid depressed.

(3)Cleaning and storage of the lens: By solutions with preservatives andantibacterial activity as benzalkonium chlQ!ige 0.02%, disodium edentate 0.1%,chlorhexedine and thiomersal.NB:Heat would dam~ge hard contact lenscs:So heat disinfection is contraindicated.

(4)Wetting the lens: By a wetting solution which contain polyvinyl alcohol ormethyl cellulose.

PERIODIC FOLLOW UP:Done 1w , 2w , 1m , 3m and 6m after fitting for:(l)Reassessment of the following items during the early period ofsupervision:

1- Optical eiIect of the lens. v

2- Lens fitness. ./3- Condition of the eye. v'4- Patient's instructions. v'

(2)Modification of the corneal contact lens or a new fitting: f necessary.SPECIALIZED FITTINGS:

(1)Astigmatism:1) Corneal astigmatism:

I-Moderate (1.5-2.50 D): I

(a) Lens fitting on-K or slightly'steeper is often adequate.(b) A small lens diameter with a small POZ·may help.

2- High (more than 2.50 D):(a) A toric posterior curve lens.(b) A bitoric lens (with a toric posterior and anterior curves)~

2) Residual astigmatism without corneal astigmatism Chapter 18.(2) Prismatic contact lenses:

1)Prism ba/las : ~Characters: :

1- A hard contact lens with a prism added to it during its manufacture(usually about 1.5 lJ).

2- The lens tends to orient itself with the prism down as it adds weight tothe lower po!1:ion of the lens but actually the pri·sm ballast is inclined toride about 10° nasally (Fig. 18.3a).

ndic.ations: 1-To orient the axis of the cylinder correctly in toric anteriorsurface lenses.

rp.':> 2-To orient the reading segment of bifocal contact lenses.

<....;. \ 3-To help centring of a high riding lens.2) Lens truncation (truncated lens):

hm'actcr~ It is a hard contact lens wilh cutting off of one edge which isthicker and thus heavier and so acts much like a prism ballast (Fig. 18.3b).

Indications:As prism ballast but used to modify an existing lens rather thanan initial lens design.

o(a)Prism b.allast (b) Lells trullcatiol1.Fig.18.3:Prisulatic contact lenses.

(a) Single cui plus lens. fb) Lenticular pillS lel1s,Fig.18.4:Types of lenticular contact lenses.

(3)Aphakic contact lenses (Fig. 1.8.4):Types: )Single-cut lens-: With a uniform anterior surface curvature but it is

heavy with a thick centre and tends to ride downwards(for low aphakicpowers and narrow palpebral fissures).

2)Lenticular lens: Easier to handle, less likely to pop out and the carrierperiphery is of minus power (so its concavity allows the edge of thelens to tuck in and be held by the upper lid).

NB:Single-cut and lenticular lenses are available for soft contact lenses.Tinting of aphakic contact lens: With a grey tint to reduce the glare and the light

entering the eye .. Over plus contact lens:On which minus power spectacles are used(reversed

telescopic arrangement), if residual aniseikonia in monocular aphakia (10%)is noted by the patient(Chapt20).

(4)Contact lenses for keratoconus:J) A special hard contact lens 's used with:

1- A steep central portion to vault the corneal apex.2- A smaller diameter if the cone is not too eccentric.

2) Fitting depends mainly on:1-Good centration.2- Fluorescein pattern:

(a)Three point touch (apical light touch and two mid-periphery contact).(b)Adequate degree of edge lift.

NB: Keratometry is not satisiactory in KeratoCOnus usuallV: AS me mires a!'usually distorted.

3)Follow up to perform optical keratoplasty if: 1- The cone enlarges.2- Lens intolerance occurs.

coC

(5) Bifocal contact lenses:Principle:

1) Simultaneous vision (bivision):Retina receives optical images of bothdistance and near vision and patient can ignore one of no interest mentally.

2) Alternating vision:'Different portions of the lens have different functions.3) Both simultaneous and alternating vision: With a different extent.

Types of bifocal contact lenses:l)Annular (concentric) bifocals(Fig.18.Sa):lt is the most cOlnmonly used:

I-Near portion is usually an annular zone around entire central area of lens.2-Simultaneous vision ( with some alternating vision, if the outer portion is

for near as the lower lid pushes the lens up on looking down bringing theperiphery opposite the pupil.

2) Segment bifocals:'1- It is usually crescentic and rarely executive in shape (Fig. 18.5b).2- The lens is oriented downwards by prism ballast or truncation.3- Alternating vision(with some simultaneous vision except in small

pupils and high lower lids).

Cd1.-Crescentic. 2- Executive.

(a) Annular contact lens. (b) Segment contact lenses.Fig. 18.5: Types of bifocal contact lenses:

3) Diffractive bifocals:1- A series of concentric rings at the posterior surface of the lens which

acts by diffraction (Chapter 19).2- It functions by simultaneous vision.

NB:AIl these types of bifocals arc available for all types of contact lenses andfor intraocular lenses.

COMPLICATIONS OF HARD CORNEAL CONTACT LENSES:(A) DISCOMFORT:

(1) Normal or adaptive phenomena: These are mild .\ymptoms in the initialstage: 1)Foreign body sensation.

2) Mild tearing.3) Some edge reflections.4) Trembling of image.5) Glare. ~ [\:706) Photophobia.

(2) Symptoms due to oor fit:1) Too loosefi~: 1- Trembling of image.

2- Sliding of the lens olT the cornea.3- Falling of the lens offth,e eye.4- Symptoms of corneal abrasions.5- Excessive awareness of the lens presence.

2) Too tight fit: . i- Corneal oedema with veiling of vision.. 2- Spectacle blur.

(B) ACTUAL COMPLICATIONS:(1) Medical complications:

1) Corneal abrasions_'auscs Mechanical trauma by:

I-Wiens fitting2-High degree of astigmatism. 3- Riding high lens edge.4-Lens inseltion.5-Drying of the cornea.6-Foreign body under the lens.7-Lens defect as poor polishing of the lens or of lens edge.

Clinical feature: The corneal epithelial defect may be:1 rcuate urve wlth a ring 0]contact: Due to tight fitting.2 Lin 1': ' the least meridian in high degree astigmatism.3- I a or irre ular: Due t~ foreign body under the lens.4 entra Wlt a centra touc : Due to too steep fitting. or too loose

fitting.5- rtlca: Due to too loose fitting or foreign body.6 almng a an 0 c oc ue to drying of the cornea with poor tear

interchange.2) Chronic corneal erosions!:

aus : Chronic corneal irritation due to ill fitted lenses.Clinical feature :

1- 0 s m toms W en teens ,in worn: Due to lens protection from therubbing lids.

2- a ent perlO W en t 1e lens is removed Due to corneal anaesthesia(lasts ~ew hours).

3- vel' wear syn rome: ew hours after removal of the lens and mayawaken the patient from sleep with:

(a) Symptoms: a) Pain.b) Watering.c) Foreign body sensation.

(b) Signs: a) Large erosion.b) Corneal oedema.

4 ecurrences: Are common with a continuous disability if the lens isreinserted.

3) Corneal oedema (Sattler's veil):'Pathogenesis 1- Mechanical trauma to the cornea.

2- Hypoxia of the epithelium.3- Negative hydrostatic pressure under the lens.4- Drying of the cornea.S- Anomalies of the tear film particularly its lipid layer.

Caus : Common with ill fitted lenses (especially scleral contact lenses).Symptoms:

I-Start 1-6 hours after wearing lens as mistiness and veiling of vision.2- Persist for some time after removal of the lens with a spectacle blur

from the induced myopia by the swollen cornea with increasedrefractive power.

Signs: 1 ornell oe(. e11l11.Localised usually.2- ornelll veslcles:Burst if the lens is not removed.

Diagnosis1 7nvlsl : By keratometry as an increase in corneal

curvature.

2- 105'1 e cornea oedema:(a)By the. slit lamp (by sclerotic scatter or by retro-illumination).(b)Dimple veiling:ls present sometimes as a corneal staining due to

trapping of small air bubbles under the lens which is transient butit may indicate too steep fitting with poor tear interchange.

4) Desquamation of corneal epithelium:· .I-In the initial period of adaptation or,2-Due to a trauma usually with lens edge.

5)Co1'neal vascularization and opacification:, Especially in aphakia and inkeratoconus.

6)Displacement of the lens off the cornea. Into the upper fornix,subconjunctivally or rolling of lens off the eye(due to loose fitting) ..

7)Breaking and loss of the lens: May,occur.(2) Optical complica ions:

1) Residual astigmatism: Incidcnc : Of more than O.SOD especially with spherical soft contact lens.

0,I \\ I, 0,

I \\ I\ I

(a) Oval contact lens. (b) Rectangular contact lens. (c) Triangular contact lens.Fig. UL6:Dcviccs for prevention of lens rotation.

2 enses: a) Lens with a hard centre and a soil edge.(b) Rigid lens with a soft periphery.(c) Soft lens with a hard lens fitted over its centre.

NB: Toric soft lenses and toric gas permeable lenses arc available.3- 111 rIca speclac e correction: f the above lines fail.

2) Spectacle blur:Causes Ill-lens fitting specially a steep fit lens with corneal

oedema,distortion or increased thickness of the cornea.Symptom :Transient blurred vision (1-2 hours) with a change from contact

lenses to spectacles.Diagnosis: By the keratometer.Correction By giving the proper lens fitting.

3) Increased light intolerancetCauses: - More light allowed by the contact lens than spectacles.

2- Irritation of the corneal nerve endings due to ill fitted lens.ymptom~. Photophobia and glare.

Correctio : I-Proper lens fitting.2-Tinted contact lenses(in albinism,aniridia and iris coloboma).

4) Induced imbalance:Causes This is due to decentration of the lens (less than with spectacles as

the lens is nearer to F of the eye) :1 ertrca (,ecelltratlOll : a) Tight lid which takes the lens up.

(b)Lax lid in which the lens sags down.(c)Heavy lens and so it sags down.(d) Thick lens edge.

2 Horizontal decelltration Usually with astigmatism against the rule.Diagnosi :1- Blurring of vision, diplopia or polyopia.

2- Flare or ghost images due to edge reflections on lookingthrough lens periphery.

Correctio : Change to a small thin lens.5) Post wear e.fJect~ 1- Blurring of vision.

2- Lacrimation.3- Photophobia.

6) Optical complications ofbifocal contact lenses:I-Smaller field for distance specially the lower field·.2-Jumping up of objects due to the prismatic effect of the near segment.3-The need of raising up the head and lowering the eyes in order to read.

2) HARD SCLERAL CONTACT LENSESDEFINITION: A scleral contact lens is a large lens covering the entire cornea and a

portion of the sclera.INDICATIONS OF SCLERAL LENSES:(1) Cosmetic shell: A plastic shell for a grossly di0jigured eye in which the scleral

portion is clear or white and corneal portion is coloured with the iris pattern.(2) Occupational: An under water scleral lens which incorporates an air space

with a flat front allowing the swimmer good vision both in and out of water.TYPES OF SCLERAL LENSES:

(1) Sealed lenses: Which are non-ventilated (rarely used now).(2) Fenestrated lenses: With one or more perforations in the scleral portion .

. (3) Channelled lenses: With grooves along posterior surface of the scleral portion.PARTS OF A SCLERAL LENS:

(1) Corneal (central optical) portion.(2) Scleral (haptic) portion.(3) Transitional zone: Between the corneal and scleral portions in modenl1enses.

SCLERAL LENS MATERIAL:(1) Plastic moulded lenses.(2) By cast ings fi'om the individl~al patient's eye.

THE CRITF,RIA OF AN IDEAL FIT OF SCLERAL LENSES:(1) The v'cleral part Ires in even contact with the globe: To within 2-3 mm of the

Embus.(2) Clearance: A minimal clearance of 0.1 mm at the apical region and a greater

clearance with no sharp transition between the corneal and scleral portions.(3) Some relative movements of the scleral lens: Is important for exchange of fluid

under the lens. .THE FITTING OF A SCLERAL LENS:

(1) The lens may be inserted dlT or with fluid: A drop of 1% fluorescein is addedto evaluate the lens fit.

(2) Then the.fit is eva/ualed by: .l)N~ked e re illSRcctiol1 in white light: for testing position and mobility of

lens.2)FluoreL e'n test: '0 examine the fluorescein pattern by the slit lamp with the

cobalt blue filter:1- Clearance at the limbal region should be adequate with 2-3 mm width of

fluorescein ring.2- Digital pressure on the lens gives some indication of fluid flow under lens.

(3) Any areas requiring modification are marked: After removal of tile lens.(4) Once the physica fit is satisfactory:

1) The refraction is done.2) The centre of the pupil is marked.3) The lens is adjusted for optical power and haptic thickness.

(2) RIGID GAS PKRMEABLE (RGP) CONTACT LENSESI) DAILLY 'NEAR RGP CONTACTLENSES

LENS lVIATERIAL: A gas (oxygen) permeable material such as:(1) Cellulose acetate bv~yrate (eAB).(2) Polymethyl methac/)JLate - Silicone copolymers (PA1MA-S).

(3)Silicone/acrylate (S/A).ADVANTAGES:

(1) Gas permeable with good oxygen transmission.(2) Very thin (80-120 m~). .(3)Very small (7-9.5 mm diameter) and so the lens covers 2/5 of the cornea only.(4) Flexible and increases the tear flow between it and the cornea.(5) Washed with ordinary water.(6)Easy to manipulate.(7)lfthe lens is scratched it can be repaired.(8) The posterior surface and the edge of the lens are less annoying.

DISADVANTAGES:(1) Lens spoilage: Lens flattening, lens flexure, lens warping or cracking with

, unstable vision.(2) Lens adherence: With extended wear lenses.

FITTING METHODS:(1)Triallenses: A specific manufacturer's gas permeable trial lens set is used.(2) Diameter: Is larger than in hard lenses due to their inherent nexibility.(3) Optical zone: Is larger than in hard lenses due to their larger size.(4) Fluorescein pattern: The ideal fluorescein pattern is an alignment fit in which

the posterior curve of the lens is parallel to the corneal curvature.(5) Lens fitting: l)For spherical error: On-K or slightly flatter than K.

2)For astigmatism: lightly steeper than the flattest K.3)For keratoconus: tart with the flatter K reading and study the

fluorescein pattern and then replace with a steeper lens till theproper lens is reached.

(6)Wearing schedule: l)Daily wear lenses: ~s in hard contact lenses.2)Extended wear lenses: Better daily also.

2) OVERNIGHT RGP CONTACT LENSESLENS CONFIGURATION: Four zones with a deep tear layer (Fig.18.7):

Fig. 18.7:Zones of overnight wear RGP contact lens.(l)Base curve: Is the cause of corneal changes ..(2) Reverse/relief curve: Is the steepest zone(reservoir zone).(3)Alignment curve(cone angle):For lens ccntration and lens movement:

3- Loose map:Dark blue map is the flattest(39D) .. 2) To compare corneal topography before and after overnight lens wear.

FITTING PROCEDURE:(l)Triallenses:Overnight lens wear for 7 days with daily wear ofI the len gradually.(2)Retainerlens: After 10 nights oflens wear with stable vision all the time usually.

COMPLICATIONS: Corneal ulceration and scarring in 0.13 % of casas.(3) SOFT (HYDROPHILIC) CONTACT LENSES

LENS MATERIALS:(l)Hydro:>,yethylmethaclylate (HEMA):Basic plastic polymerized in the soft lens.(2)Additional or other materials:

1)Ethylene glycol dimethylacrylate(EDMA).2)Polyvinyl pyrolidone(PVP).

(3) Water content: 1) Moderate (35-50%): In daily wear lenses.2) High (50-80%): In extended wear lenses.

ADVANTAGES OF SOFT CONTACT LENSES:(1)The advantages of hard contact lenses: Discussed.(2)The other advantages of soft contact lenses are:

1) Well tolerated: - Pliable and elastic when hydrated.2- Shaped to the cornea with no friction.3- Stable with no enzymatic action.

2)Hydrophilic: 1- The lens contains 35-80% water.2- Permeable to gases, water and water dissolved substances

(more with thin lenses) ..3)Soft. With less damage to the eye.4)Optical suitability: Is more. .5)Complications Are less.

DISADVANTAGES o.F SOFT CONTACT LENSES:(1)The same disadvantages of hard contact lenses: Discussed.(2)The other disadvantages of soft contact lenses are:

1) Salt deposits Spoil the lens (especially calcium deposits).2)Mechanical damage of the lens:(fhe lens splits or breaks and the lens edge

may become frayed.3)Deterioratin of lens material.4)Growth offungus within the ten5)Handling difficultie .

COMPLICATIONS OF SOFT CONTACT LENSES:(1)The same complications of hard contact lenses: Discussed.(2)The other complications of soft contact lenses are:

1) Ocular complications:1- Punctate epitheliopathy: with staining spots, erosions and microcysts.·2- Infective keratitis: (a) Pyogenic organisms especially pyocyaneus.

(b) Acanthamoeba histolytica.

3- Giant papillary conjunctivitis;Cause: ) Allergy to protein deposits on the lens.

(b) Mechanical factors (with prolonged wear) ..Clinically: a)Burning sensation.

(b)Excessive mucous which interferes with vision.(c)Large papillae at upper tarsal conjunctiva(as spring catarrh).

Treatment. (a) Lens removal, enzymatic cleaning or change to a differentsoft lens.

(b) Sodium chromoglycate and corticosteroids.4- Tight lens syndrome .

Definitio . A painful red eye on waking due to dehydrated immobileextended wear lens.

Cause: oor oxygenation of the cornea.Clinica ly: (a) Corneal oedema.

(b) Chemosis.(c) Iridocyclitis.

Treatment: ) Lens removal.(b) Cycloplegics and antibiotics.(c) Change to a looser fitting.

5- Chronic meibomianitis.2) Lens ~poilage (damage) due to:

I-Deposits on the lens (a)Calcium, mucin or pro1tein deposits.(b)Pigment as melanin (endogenous), nicotine or

cosmetics.2- Deterioration of lens material3- Growth of bacteria and fungi.4- Handing problem~.5- Mechanical damage with lens split or frayed lens edge.

FITTING METHODS:Keratometry followed by triall 'mses is the.best( as inhard contact lens) :

(1)Surface anaesthesia: Is not needed.(2) Trial lens insertion: From a standard set of soft contact lenses with the

indicated lens parameters:1)Diameter 1.5-2 mm larger than the corn~al diameter (horizontal visible iris

diameter).2) Base curve: Flatter than the flattest K readings.3) Power: Form spectacle power (using special tables as in hard contact lenses).4) Type of soft lens material High water content lenses for extended wear

(especially for aphakia) are more flexible and so need slightly steeper littingthan low water content lenses.

(3) Patient is allowed to sit for 20-30 minutes: To allow time for hydration.(4) Evaluation of the trial lens fitting:

l)Criteria of lens fit:I-Lens centration. A properly fitted lens will centre well.2-Lens movement:

(a Sati~fact?ry fi : a-On looking up : Lens lag is less than 0.5-1.5 mm.b-On blinking: Lens is carried up about 1mm

before it recentres.(b) nsatisfa 'toryfi :

a)little movement of the lens(tight fitting): With clearer vision afterblinking.

b)Excessive movement of the lens(loose fitting): With distortedvision after blinking.

(c)Retinoscopy reflex. Regular and stable retinoscopy reflex indicates agood lens fit

(d)Keratometry' Undistorted mires indicates a good lens fit.(e) isual acui~, Clear vision with minimal fluctuations after blinking in a

good lens fit.NB: Fluorescein test is not use<;lin soft lens fitting: As fluorescein will d.'SCOLO:

the soft lenses.2)Results of lens evaluatio '

I-Unsatisfactory fitt(a)Eccentric len ,,'Better centration may be done by increasing the lens

diameter.(b)Edge compression: May be diminished by a flatter posterior central

curve of the lens.(c)Tight lens. Should be replaced by one with a smaller diameter or a

flatter PCC or both.(d)Loose lens The reverse of tight lens.

2-Satisfactory fit with an ideallen&,: Refraction is carried out to get final lens.NB:Prescription of soft: contact lenses include: The diameter, PCC, power and water

, content(low or high).(5) Patient's instructions:

1) Wearing schedule:1- Daily wear lenses: Continuous wear during the waking hours.2- Extended wear l~nsesl: But remove the lenses before retiring at night to

avoid complications. ,3- Disposable lenses:/

(a) Worn up to 3 months (better on a daily wear basis).(b) Disinfection with hydrogen peroxide.(c) With less incidence of keratitis and giant papillary conjunctivitis.

2) Lens manipulations1- Lens insertion': The soft contact lens is inserted by either:

(a) The same as for hard corneal contact lens, Or(b)The lens is placed on the sclera below and then moved upwards to

centre over the cornea.2- Lens removal:Done by touching the lens, sliding it inferiorly, compressing

it between thumb and index finger and removing it from the eye.3- Lens centring: 'Done by the mirror method as in hard contact lenses.

3) Lens disinfectzon and storage:1-Thermal disinfection:ln an electric care kit unit(with the case containing

the lenses immersed insaline solution)which is exposed to dry heat at 90°C for 10 minutes, but its disadvantages are:

(a) Adherence of lens deposits to the lens.(b) Not completely bactericidal.(c) Shorten the useful life of soft lenses.

2- Chemical disinfection:fBy chemical disinfectants as:(a)H ro en peroxide 3%· hicl1 must be neutralized before the lens is

inserted in the eye by a chemical solution as sodium pyruv::tte, sodiumbicarbonate or sodium thiosulphate.rBl:Soft lenses must be left at least 4 hours in hvdrogen peroxide 3% solution:

To be effective against aCllntllOl11oebakeratitis.NB2:Hydrogen peroxide may remain in the substance of the soft lens even

after neutralization.NB3:Heat disinfection is unsuitable for tinted soft contact lenses: So hydrogen

peroxide is the best.(b)Preservative so utions containing: isodium acetate 0.01% and dymed.

4)Lens cleaning:I-Daily cleaning with solutions containing surfactant chemicals; As

isopropyl alcohol.2-Weekly cleaning with enzymatic preparations:Tb remove protein

deposition(not used nowadays).5) The use of medications with soft contact lenses.

I-None with lens worn (absorbed by lens)except a tear substitute drops(a)Hypotonic solutions as water:Lens becomes flatter and increases in size.(b)Hypertonic solutions as 5% sodium chloride: The lens becomes steeper

and it shrinks.2- Simple antibiotic or anti-allergic eye drops: Can be used with the soft

contact lenses off the eye.6) Instructions about the causes of soft contact lens darr,age which are:

I-Salt deposition.2-Nails, sprays, powders, ... etc.3-Drying of the lenses.4-Lens loss off the eye.

TYPES AND INDICATIONS OF SOFT CONTACT LENSES:(A) REFRACTIVE SOFT CONTACT LENSES:

(1) Myopia Are best con-ected by soft lenses.(2) Hypermetropia: educe the need for accommodation and convergence and

avoid the prismatic effect of spectacles.(3) Aphakia' Are better than hard lenses for aphakic children and older patients.(4) Anisometropi : Are better than spectacles and than hard lenses if there is no

high degree of astigmatism.(5) Astigmatism" Hard lenses are better than toric soft lenses in astigmatism of

more than 1.5D.(6) Keratoconus, Hard lenses are usually better but toric soft lenses may be

useful in early cases.(7) Presbyope May be better than hard lens bifocals due to less lens mobility.

(B) THERAPEUTIC (BANDAGE) SOFT CONTACT LENSES:Advantage : (1) Highly effective and comfortable.

(2) Good optical performance.(3)Easier in inu:-__

Indications:(1) Irregular astigmatism with fragile epithelium: As in recent corneal grafts.(2)Bullous keratopathy: 1) It relieves pain and discomfort.

2) It assists "endothelial recovery.3) It flatten bullae decreases corneal oedema.

(3)Persistent corneal epithelial defects:To protects the fragile epitheliumfrom the lid trauma.,

(4)Recurrent corneal erosions: ,The lens wear may be a long term.(5) Small corneal perforations and wound leaks: To allow reformation of AC.(6) Indolent cornea ulcers:For resistant superficial sterile ulcers that failed to

heal medically.(7) Alkali burns of the cornea:ln the healing stage(not in the early stage with

marked chemosis).(8) Corneal grafting: It protects the grafted epithelium and permits healing.(9) Drug penetration: :As in tear substitute soaked contact lenses for dry eye.(10) Other indications: As lagophthalmus for corneal protection.

(C) TINTED (COLOURED) SOFT CONTACT LENSES:Regions of a tinted soft contact lens(1)A peripheral rim: A clear transparent zone( 1-2 mm diameter) on the

limbal conjunctiva.(2) A peripheral iris' 1) Coloured opaque.

2) Coloured transparent.3) Clear transparent.

(3)A central pupil: 1) Coloured opaque.2) Co loured transparent.3) Cleartransparent.

Apparent iris colour:Coloured opaque iris produce a change in apparent iriscolour(more in lighter than in brown irises(enhancement of the patient'sown eye coluor is the result of colour mixing).

•._..~~., "

,N.•...,~., _.-/

Fig.18.8:Coloured soft contact lenses: (a)Opaque iris and transparent p upil; (b)Opaque[ris and Pu if; c Trans arent iris and opaque pupil; (d) Transparent iris and pupiL

y es ana indica ions of tinted so con act len es (EI-Rifai's classification):(l)A coloured opaque iris and a clear transparent pupil of 4-6 mm(Fig.18. 'Sa):

1)Peripheral corneal opacities in a seeing eye '2)Iris anomalies(albinism,aniridia or coloboma).3)A beauty aid to change the colour of the eye.

(2)A coloured opaque iris and a coloured opaque (black) pupil (Fig. 18.8b):1)Diffuse or total corneal opacities in blind eye.2)Total occluder for amblyopia treatment.3) To simulate squint or blindness for artistic purposes.

(3)A clear transparent iris ancj a colmired opaque (black) pupil(Fig.18.8c):For inoperable cataract or central corneal opacity in an blind eye.

(4)Coloured transparent iris and pupil (Fig. 188d):To p event photophobia inbright light:1)Hypersensitivity to bright light2) In actors in television and movies.

(5) Optically powered tinted lenses: . coloured opaque iris and a transparentclear pupil or with a coloured transparent iris and pupil (Fig.I8. 7a&d) tocorrect ametropia also.

(6)Pinhole contact lenses:With a coloured opaque iris and a clear transparent.pupil of1 mm dial'neter, to be used with low vision aids (Chapter 20).

(D) OCCUPATIONAL SOFT CONTACT LENSES: Are better than hardcontact lenses especially in sports.

CONTRAINDICATIONS OF SOFT CONTACT LENSES: These are the same asin hard contact lenses. '

CHAPTER 19ARLENSES

(1) APHAKIC IOLl)SINGLE FOCUS IOL

CALCULATION OF IOL POWER:(1)Methods based on patient's optical state:A limited range of implant powers.

l)Standard lens power method:I··Patient is considered to be emmetropic:With normal ocular parameters.2-Average crystalline lens power is +I9.7D(with SD of±1.62D)and its

equivalent IOL power will be:-(a) + 17 D for AC lens.(b) + 19 D for pupil supported lens.(c) +20 D or +21 D for PC lens.

2)Spherical equivalent of the patient's refractive state: Aiming for E ~r slight Min eldery patients for reading.

~B:Powers of IOL in air is 3-4 times(more than 600) more than inside the eye.(2)Biometry for mathematical measurements of the ocular parameters:

."t .-AXIAL UNOTH~

Fig.19.1 :Biomeyry for measurement of the axial length of the eye with a scall.l)Ultrasonic measurement of the axial length of the eye:

The principle: Is a conversion of a measured time interval between echoes toa distance on the basis of the sound velocity in the intervening media:I-The ve ocity of sound wave is:-

(a)1640 m/sec in normal crystalline lens.(b) 1590-1670 m/sec (averag of 1629 m/sec) in cataractous lens.(c)1532 m/sec in aqueous and vitreous.

2-Ifthe instrument is calibrated or a velocity of 1532 m1sec: T e axiallength of the eye will appear shorter because sound waves pass throughthe lens at a higher velocity than the aqueous and vitreous and so thequantity 1629/1532 (the lens thickness) is added.NB:Average values for axial length: Are 23-24 mmfor emmetropic eyes.

The procedure: .(Fig. 19.1):I-The methods of coupling the ultrasonic probe to the cornea under

local anaesthesia:(a fhtj : While the patient is in the supine position.

(b) ) ana/LOn me/1O : ltrasonic probe replaces applanation tonometeron the slit lamp.

2-The features of a correct patter . Are high anterior and posterior lensechoes (AL and PL), a high and steep retinal echo (R), and a lowmembrane reduplication echo.

Errors in ultrasonographic measurement:I-A small error of 1/10 mm: (a)Sound velocity in cataract.

(b)Minor eye movements.2- A large error up to 1 mm: (a)A missed retinal echo.

(b)An air bulk on the membrane.NB:Error of 1 mm in axiallcngth: Results in a postoperative refractive error of 2.50.

2)Keratometric measurement of the refractive power of/he cornea:The average measurements-.of at least three readings in each principal

meridian is recorded to minimize any error.Errors in Keratometric measurement:

1- Mistakes in reading principal meridians.2- Postoperative change in corneal curvature.3- Corneal Rl varies for each keratometer.4- The surgically induced astigmatism.5- The fixation target may not be seen in mature cataract.

3)Measurement of the anterior chamber(AC) depth:Methods:I-Standard AC depth method: The. empirical values of the postoperative

AC depth are: (a) 3.19mm for AC lens.(b) 4.2 mm for PC lens.

2-Pachometry: By pachometric attachment to the slit-l~mp (Chapter 23).3-Ultrasonography: using the A-scan.

Errors in measuring AC depth:I-The thickness of the cornea is discounted in all these methods.2-AC depth is measured from the vertex of the cornea to the vertex of the

lens and not to the first pupil plane of the lens.::.! :The advantages of a planoconvex over a biconvex IOLare:-

(1)First pupil plane coincides with its vertex.(2)Distance between principal planes is constantfor a given lens thickness.

NB2:There is a decrease in the effective planoconvex IOL power withhypermetropia, if the plano side placed anteriorly: Which is partially dueto a posterior shifting of the principal planes.

3-AC depth is decreased progressively with age (0.1 mm/decade).4-AC depth varies with refractive state (0.06 mm deeper for every-l D).5- IOL is as a thin lens but its 2 principal planes do not coincide.

NBI :Error of 0.1 mm in AC depth: Results in a postoperative refractive error of ID.NB2:Calculation of IOL power for 2ry IOL implantation is more accurate due to:

(l)Aplzakic glass power and its vertex distance are known.(2)AL measurement is more accurate due to absence of crystalline lens.

(3)J(eratometry is more accurate as corneal curvature and refractioll are settled.(4)AC depth is well measured postoperatively.

APPLICATIONS OF OCULAR PARAMETERS TO MEASURE IOL POWER:(1)Formulae for calculation of IOLpower for emmetropia:

1)Theoreticalformulae:Based on optical state(without surgeon or IOL characters ):I-Original theoretical formulae based on the. Gauss and Listing theory for

the schematic eye with IOL : P= n/L-d - 1/ liD-dinWhere, P = Power of IOL( to produce emmetropia in aqeous).

n = Rl of aqueous and vitreol,ls(1.336).L = Axial length of the eye.D = Pseudophakic AC depth. .

2-0riginal theoretical Binkhorst formula for AC len:;. .. 1336(4r-L)

modified for PC lens: P = (L _ d) (4r _ d) Where"

P=IOL power in D (to produce E in aqueous).l-=Radius of curvature of ant. surface of cornea in mm.

L=Axiallength of the eye in mm.d=Distance from the anterior surface of the cornea to

anterior surface of IOL at the optic axis in mm(assumed postoperative AC depth which is still anunresolved problem and so values for d are usedwhich are 3.19 mm for AC lens and 4.2 mm for PClens).

3-Recent theoretical formulae as Colenbrander-Hoffer formulain which conection factors are applied.

2)Regression formular1:Are based on the surgeon characterestics,on IOL style and on the original Saundcrs,RetzlafTand KrafTformula ( SRK-I formula ):I-Original SRK-I formula:

P = A - 2.5 L - 0.9 K (for axial lengths of22- 24.5 mm), where,P = IOL power in D (to produce E in aqueous).L = Axial length of the eye.K = Average keratometric reading in D.A = Specific constant for IOL style.

2-Recent SRK-ll formulae': For shorter or longer eyes (as SRK-I formula isnot accurate):(a)Short AL (below 22 mm): Add I to constant A ifit is 21-22mm, add 2 to

constant A if it is 20-21 mm and add 3 to constant A if it is less than 20mm.(b )Long AL (above 24.5 mm): Subtract 0.5 mm from constant A.

3) Combined modifications.-I-Theoretical and regression formulae. With the fudge factor (related to the

surgeon's results).2-Updat regression SRK formulae I and II:SRK-T formula for very long eyes.

(2)Formula for modification of calculated IOL power for emmetropia to produceintentional ametropia: Pam = Pemm ~ Psp / Rf, Where,

Pam = 10L power to produce ametropia.Pemm . = 10L power to produce emmetropia ..

Psp = Final spectacle power produced by Pam·Rf = 0.8 for Pemm > 14 D and 1 for Pemm ::;14 D.

(3)Formula for the expected refraction with a given IOL power:Px = (Po - Pi)R: Where,

Px = Expected refraction in D.Po = loL power in D (to produce E in aqueous).Pi = Actual 10L (implant) power in D.R = Refraction factor converting change in 10L power to change in the

spectacle plane (in SRK-I fom1Ula, R is 1.5 and in SRK-II fonnula,R is 1.25 if Po is more than 14D and is 1 if Po is 14D or less).

(4)Simplified application of ocular parameters to get the correct IOL power:l)NolJ1ograms:Use two parameters (AL of the eye and the power of the cornea).2)Calculators:Ocular parameters, refractive error and the state of the other eye to

give 10L in D in E or ametropia.IOL SELECTION:

(1) Calculation of IOL power: Discussed.(2)Aim of postoperative refractive state of the patient:

1) If the fellow eye is E:. Aim for E(usualIy) or for a low M (which is useful forreading vision).

2) If the fellow eye is highly ametropic: Aim is to prevent postoperativeanisometropia by: 1- Give 3D less if the fellow eye is highly M.

2- Give 3D more if the fellow eye is highly H.3) If the fellow eye is pseudophakic Aim for -2 or -2.5D M (for reading vision).4) Reading vision:lYim for low degree M or implant bifocal or multifocallens.

NB:Pseudophakic accommodation with a good near vision using the dlsmnc~correction (may be nil) due to: (l)Miosis of near view.

(2)Forward movement of JOL by capsular bag.(3)Residualmyopic astigmatism.

(3) Optical quality of IOL:1)Lens power: s multiplied by 0.46 to give refractive power of lens in aqueous.2) Lens design: Must be of good quality with no surface defects.

(4) Magnification of IOL: PC-I0L would theoretically induce no imagemagnification, while roL at the pupillary plane or the prepupillary spaceinduces image magnification of2- 3% practically.

(5)Aim of postoperative optical results of the patient:1)Magnification of 3% in papillary IOL is eliminated, [[the eye is made M of-

1.50D: Aiming for a slight residual M of 1.5 D (this is explained by theprinciple of the Galilean telescope).

2)ln bilateral macular degeneration lOL is preferred than spectacles and thancontact lens because: 1- Spectacles magnifies the central scotoma.

2- Contact lenses are difficult to be seen.3)Magn£fication of combined lOL and spectacles (Galilean telescope principle):

I-Spectacle correction by 1D leads to an increased magnification of 2% inH and the reverse in M.

2-Spectacles or contact lenses may be needed after IOL implantation tocorrect a significant corneal astigmatism or a residual sphericalerrorecorrection of cylinder first and then sphere for distance by coatedlenses against UVR and conection for near If needed).

3-Problems of refraction after IOL implantation are opacification of posteriorcapsule of the lens, too small pupil and reflections from IOL surfaces.NB:Moderate postoperative myopic astigmatism leads to: A useful unaided Ilear

visioll with s IIseflil distallce visioll (as the IIl1corrected pllllkic myopic..astigmatdel'e!ops presbyopic ~Y11lpt011lS!ater tltall 45 years of age).

2) MULTIFOCAL lOLPRINCIPLE: It refracts light from both distance and near vision at the same time, as

some light is not in focus with the light which is in focus(Fig.19.2 , asmultifocaol contact lenses but unlike multifocal spectacles):

(a)For distance vision. (b)For Ilear vision.Fig. 19.2: Refraction through multifocal IOL.

(1)Distant objects:Form the distance focus at the retina and the near focusanteriorly at the midvitreous.

(2)Near objects:Form a sharp near focus on the retina while the distance focusmoves behind the retina. ~c::. l

~ ~TYPES: ~ ~. ~.•.•.o~~~.

l~-<! .•O~ .,q .,Q. ~.

if' ~~ ~"O ~<:'

J iF if ~••;;:- ~e ~ •.f t;:'f; ".! .~(J

e, ,,~

o ~(a) (b) (c) (d)

Fig.19.3:Annular(conccntric)multifocal IOL:(a)With 3 anterior spheric surfaces;(b)With 2 anterior spheric surfaces;(c)With 4 anterior spheric surfaces and posterior

spheric surface;(d)With anterior multiple aspheric and anter.ior aspheric surfaces.

(A)Annular (concentric) multifocal IOL:With a combination of:(l)Anterior spheric refractive surfaces:

1) 3 anterior spheric surfaces: I-Central circular zone:Distance vision.(Fig.19.3a) 2-lntermediate ring-shapedzone:Near vision.

3-Peripheral ring-shaped zone:Distance vision.2)Two anterior spheric surface: I-Central zone:Near vision

(Fig.19.3b) 2-Peripheral zone:Distance vision.(2)Allterior spheric and anterior aspheric surfaces: 4 different zones (Fig.19.3c):

1) Central spheric zone: Distance vision.2) Inner steepening aspheric midzone: Near vision.3) Outer t1attening aspheric midzone: Near vision.4) Peripheral spheric zone: Distance vision.

(3)Anterior multiple aspheric surfaces and posterior spheric swface(Fig. 19.3d):Five annular aspheric progressive zones on the anterior surface (with acontinuous replicative curves in each of the 5 zones and all distances betweennear and far focus are focussed by some part of the IOL).

(B)Diffractive multifocaIIOL(Fig.19.4):Design:(1) Anterior spheric surface with a refractivc power.

(2) Postcrior multiple structured surface with a difTractive power.lvleclzanism: It is based on the Huygens-Fresnel principle which states that

every point of a primary wave front is the source of secondarywave fronts which spread in a spherical direction till a Fresnelzone .plate is produced that can produce optical foci.

Distance and near powers:(1) The distance power:ls the combined optical power of the anterior and

posterior surfaces with a zero diffractive power.(2) The near power:Distance power with the highest diffractivc powcr.

Fig. 19.4: Diffractivc IOL: With spheric anterior surface and diffractive posterior surface.Interpretation: From spreading of wave fronts by 20-30 concentric zoncs of the

posterior TOL surface:(l)Constructive interference (82% of light is in phase and is

transmitted): 1) 41% of light is focussed for near vision.2) 41% of light is focussed for distance vision.

(2)Destructivc interference (18% of light is out of phase): So theremaining 18% of light is lost.

Performanc :A multifocal performance with a diffractive optical effect at allpoints of the IOL and so the multi focal performance is not affected bydecentration or by pupil size, deformation or eccentricity(with possibledistance and near vision if any part of lens optic is present behind pupil.

(B)Segment multifocal IOL:Rare.3) ACCOMMODATIVE lOL

(l)DUAL-OPTIC ACCOMMODATING IOL:Principle:Theoretically each of these 2 IOLs will focus the distant and near objects

on the fovea simultaneously.Optical-designs:

1) Dual flex Two lenses are inserted: .1- A plate haptic IOL is inserted in the capsular bag to make up a two-thirds

of the total lend power.2- A three-piece silicone IOL is inserted in the sulcus to makes the remaining

one-third of the lens power.2) Dual-optic systen1J:

1- This lens was designed with:(a) An exaggerated anterior optical power.(b) A negative posterior diverging lens:

2- This design allows for great amplification effect of small degree ofmovement by increasing the power of the plus component.

3)Piggy back lOL:1- Implantation:( a)A standard IOL is implanted as a first stage.

(b)A new complementary lens is inserted after one month.2- This design creates a multifocal multizones system.3- It is a posterior chamber lOL with a concave posterior surface to avoid

any risk of contact between the two optics.(2)PHYSIOLOGICALLY ACCOMMODATIVE IOL:

Principle: 1)The ciliary body presses on the lens and causes anterior displacementof the vitreous, moving the lens forward.

2)Axial translation of the lens optic resulting from ciliary musclecontraction (the so called shift principle) by either:

I-Axial movement of the lens optic forwards, or2-By using a flexible polymers designed for injection into nearly

intact capsular bag.Optical designs:

l)The C&C -AT45 vis·ion crystalens:1-The 2 main types are:

(a)C&C cJystalens:A three-piece lens with a T-shaped modified platehaptics and polymide loops to fixate in the bag.

(b)AT lens:With a long space in front of silicone lens with a hinge at thejunction of haptic with the optic to facilitate forwards movement of optic.

2-Performance is by contraction of the ciliary body with vaulting of the IOL

by either:( a) Direct action, or.(b) By displacing the vitreous body anteriorly.

3-Placing the hinge bet een haptic and optic makes it easier for lens to move.4-The use of longer haptics increase the amplitude of movement.

2)The human optics leU lensI-It was developed to allow transmission of the contracting forces of ciliary

muscle to lens optic to move anteriorly for pseudophakic accommodation.2-1t is one- piece lens made of hydrophilic acrylic with ultraviolet inhibitor.

3) The human optics AG len'S:I-It is a deformable accommodative IOLwith similar natural properties of the

crystalline lens-7the single piece lens is inserted in the capsular bag and thehaptics are unfolded manually(needs additional surgical skills).

2-lt is a hydrophilic acrylic foldable IOL designed to be three dimensional asthe capsular bag and stretching the sac horizontally and verticallY-7toailow the bag to assume its preoperative shape and to maintall1the functions of ciliary muscle and zonular fibres and so allowsdeformation of the ·lens.

4) The smartlens:I-A novel concept using a new smart tnaterial to be implanted as a

small lens through a small corneal incision.2-It uses a thennodynamic hydrophilic acrylic material (packed as a

solid rod) which is transformed into a soft gel-like material withthe shape of a full-sized biconvex lens that fills the capsule.

5) The injectable IOL(still under research). The lenses is fanned froman injectable chemically modified soluble collagen that isextracted, purified and prossesed from human tissue.

(2) PHAKlC IOLINDICATION:A refractive bifocal IOL can be implanted in AC of the phakic eyes

to correct presbyopia with and without distance refractive errors.CHARACTKRS:

(1) The optic is a hydrophilic acrylic and has 4 optical zones:1) Central zone: I.5mm for distance vision.2) Inner ring zone: 0.55mm for near vision.3) Quter ring zone: 1.45m111for distance vision.

(2) The haptics are made of P~1rv1Awith hydrophilic acrylic footplates.

1) The object is between F 1and the lens and a magnified virtualimage is viewed by the eye.

2) As the object moves nearer to F 1, the virtual image becomeslarger and is further from the eye.

Fig. 20.1:Convex lens used as a magnifying loupe. Fig.20.2:Triplct design.Indications ofmagnifying glasses:

1) Constricted visual field to 10° or less2) With an auxiliary lens for reading sm'all type.3) Difficult prescription of spectacles.4) Extensive visual loss with limited benefit.5) Patients with hand or head tremors as in Parkinsonism.

Types of magnifying glasses'1) Hand-held magnifiers:/

1- Magnifying lens'.orms.A biconvex or planoconvex reading lenses of different sizes and

powers(from +4 D to +20 D usually) and the distance between the eyeand the lens can be varied quickly depending on the reading distanceand the magnification needed.

'SHapes: 1- Circular lens.2- Rectangu Jar lens.

iJ1.a'v-a~f-lt~a~ges: (a) Easy to carry.(b) Easy to use.(c) Increased reading range ..

lsa"d'v-a-n~[a-g-es(a) Small visual field.(b) Distortion of image.

2-Press-on membrane }:fresnellens (stepped lens):orm Thin plastic Dexible membrane (Chapter 6).

vantages. (a) No disadvantage of reduced lens diameter with increasedmagnification.

(b) This lens is very thin (of membrane thickness).2) Stand magnifier :

I-Stand magnifying lens.2~Plastic aspheric stand magnifying lens.3-Focusable stand magnifier.4-Focusable stand magnifier with light source.5-Plano-convex (paper weight) stand magnifier.

3) Neck-held magnifiers:. Are resting on the chest leaving the hands free.4) Spectacle-born high-plus reading lenses:/

Powers +4. D to +20 D.Forms 1Sing e visionlorms,:(a)Monocular reading correction.

(b )Binocular reading correction.2 I oca· orms:

(a)Bifocals with powers up to +32 D. .(b )Deccntration or incorporation of a prism base-in.

(2) Telescopic lenses: 1) Galilean telescopic lens' Chapter 21.2) Astronomical telescopic system' Chapter 21.

(3) Contact lenses:1)For corneal irregularities in: 1- Keratonocus.

2- Irregular astigmatism.3- Corneal scarring.

2) Better than spectacles as in: I Aphakia.2- High myopia.J- S~v~r~ astigmatism.

3) To produce magnification:1- pectacle lens of highly positive power as objective and a contact lens

of highly negative power as negative eyepiece of a Galilean telescope:Advantages'(a) Ma· nified and normal vision in high myopia.

(b) Variations in magnification can be done by changing anyof the following variables: a)Power of contact lens.

b)Powcr of spectacle lens.c)VD between the 2 lenses.

DisadvantageSi:Rarely successful for regular wear and is difficult to fit.2- Reversed telescopic system in which the contact lens becomes high

positive objective lens and the spectacle lens high negative eyepiece: ora reduced ocular image(with less VA) of a larger visual field which isuseful in patients with visual field constriction as in retinitis pigmentosa.

4) Pinhole contact lens:;!\.small clear rounded area at the centre (pinhole) in acoloured contact lens: I-Cornea (a) Scarred cornea from ocular burns.

(b) Diffusc cornca:! opacities.2-Pupil Visually impaired patients with permanent

dilatation of the pupil or distorted pupi l.3-1ris: a) Coloboma of the iris.

(b) Aniridia.4-Media Multiple disseminated opacities of media.

(4) Projection devices:Principles'

l)Projection magnification:A magnified Inverted image is projected ona screen (the object is bctween f and 2F of a convex lens, Fig. 6.6u).

2)Closed circuit television:/But is in limited use.Advantages'1)They are easily adopted by the patient.2)Projection magnifiers form an enlarged image on a translucent screen at a

variable distance.3)High relative distance magnification in addition to the relative size

magnification by projection. -4)Major advantages of CC-TV over other magnification systems:

1- Image with greater brightness and more contrast than the original object.2- Greater magnification range up to 40 times with reduced aberrations.3- A more normal viewing or reading distance.4- Reversed polarity (with white print on black with improved contrast).

Types of projection devices:l)Projection magnifiers~ Same as typical opaque projectors (Chapter 13 &

Fig. 13.4a) and they have broad limitations on magnification, range,illuminl1tion,ima2,c contrast and portability.

2)Compact optical projectors: "ame as microfilm reading unit with a lightsource behind a screen.

3)Closed-circuit television magnitication system ,(CC-TV):They are usefulfor reading and writing as follows:

I-Prototype CC-TV: It forms a variably magnified image(linearmagnification of 1.6 X,to 6.4 X,and of2 X,to 16 X,were used with it).

2-Portable CC-TV: This portable cir it weighs 30 pounds.(5) Non-magnifying ow vL ion aids:

l)Pinhole spectacles: .Indications: I-To improve reading vision in opacities of ocular media.

2··Todetermine the potential vision if retinoscopy is not possible.3-Emergency when spectacles [or near vision is lost or broken.

Forms: I-Stenopaeic hole:For reading usually (no field for distance).2-Stenopaeic glass:Glass with several openings in metal sheet.

2)Reading slit (typoscope):Black device with a rectangular openi-ng to read oneor more lines at a timc in: I-Early cataracts due to increased contrast.

2-Train' ng of centric viewing in macular diseases.(6) Non-optical low vision aids:Are aids other than lenses especially writing aids as

black ink marking pens, large-type books, signature guides and telephone dials.

7Y\~~(1) J p~PRINCIPLES OF GALILEAN TELESCOPIC LENSES (FIG.2l.1) :

J1)Composition: 1- A convex objective lens.2- A concave eyepiece lens.

(2)The distance between the objective and eyepiece lenses:The eyepiece lensf1 (of shOlier F) coincides with f2 of the objective lens and so both lenses areseparated by the difference of their focal lengths.

(3The incident and the emergent rays:1)The image of an object at infinity is formed by the convex lens at its f2.2)This image now becomes the virtual object for the concave eyepiece lens.3)Divergent rays from the virtual object emerge as parallel rays by concave lens.4)Therefore the incident and emergent rays are parallel.

ADVANTAGES:(1 )Erect magnified image( slightly distorted by curvature of field or astigmatism).(2)The system is compact and so can be mounted in a spectacle frame.(3)1t can be adopted for viewing near or distant objects.(4)Cylindrical cOiTection can be incorporated.

DISADV ANTAGES:(1 )Reduced field of view of 1T or less with high magnification.(2)The object to be seen is hold closer to the eye.

MAGNIFICATION:(1)Magnification by increasing the angle subtended by the object at the eye:

3e1)Angular magnification: M = -. Where, .

. al

ae = Angle of emergence.ai = Angle of incidence ..

Fe2)Mathematically: M = - Where,Fo

Fe = The power of the eyepiece lens in D.Fo = The power of the objective lens in D.

3)Angular M by telescope for distant vision with a range ofM from 1.5x to 8x.(2)Convertion into a microscope for near vision (as in the surgical loupe,

Chapter 28 and Fig. 28.2):1)By putting a magnifying lens M2 over the convex objective lens M1 .

2)The effective M = telescopic M X, M of the magnifying lens: ME = M I X, M2.3)Galilean telescopic spectacles are preferred when the maximum reading

distance needed by the patient is attained with the least reading addition.FORM:

(1)The simplest form of telescopic lenses consists of two lenses:AIDJ

l)A minus ocular and a plus objective: Mounted coaxially and separated by adistance equal to the difference of their focalleflgths.

2)Thereforef2 of plus lens coincides withfl ofminus lens which leads to:I-A tolerated flat field free of astigmatism.2-The emerget rays are parallel.3-No focus(afocal) for distance but with a reading addition for near objects.

3) The 2 lenses are.fixed at the end of a cylindrical tube: On one aperture ofthe spectacle frame.

(2)Plastic lenses and plastic housing of the telescope: 0 reduce its weight.(3)A special angled bar: To vary IPD (in the Keeler low vision trial set).

POWER:(1) Objective lens: Usually +20 OS ( with spherical or aspherical curvature ).(2) Eyepiece lens: Usually -20 OS to -40 DS (In addition to the refractive error):

1) For distance vision: Of -3 OS on the reading vision lens.2) For near vision: On the distance correction (but with a restricted lield).3) For both.' Uniocular bifocal lens with much larger reading portion for a

classroom child.10 Foc~1 I'I«no

'1

. Fig.21.1:G~lmcan tclcscope~ Fig.21.2:Astromical telescope.(2) NO leAL TE j'SCOPE

PINCIPLES:(1)Composition. (a) A convex eyepiece of short FI.

(b) A convex objective lens of long F2.(2)The objective lens produces a real inverted image: Of the object at F I of the

eyepiece which coincides with F2 of the objective when the telescope is innormal adjustrr ent and so the final image will be at infinity which is magnifiedby the eyepiece (Fig. 21.2).

MAGNIFICATION:~e

]V! c-=o --: •

::1 {

FoM = f.~~

FORM: Expanded field telescope with reflecting prisms for image inversion(foeusable from 30 em to infinity with an adjustable objective element).

ADV ANTAGE~ Large field and better image than Galilean telescope.DISADVANTAGE: lnverted image.

TE 22MICROSCOPES

VALUE:A magnified view of a near object (unlike a telescope, usually magnifiesdistant objects).

(1) THE SIMPLE MICROSCOPEPRINCIPLES:The image can be located at any position between the minimum

distance of distinct vision(25cm) and infinity depending on whether the objectinside or at F of a biconvex lens:(1)Practically the object is inside F of the lens as in a corneal loupe or

magnifying lens (Chapter 16 & 20):I)A magnified erect image of the object is seen at the minimum distance of

distinct vision(25 cm) of the observer with his eye very close to the lens andobject viewed is very small and at a distance S of a less than F of the lens.

2)Magnification:(l)Angle subtended by image (by microscope)/ Anglesubtended by object situated at S (by eye)

(2)Also M=Apparent size of image/apparent size of object=Distance of image/Distance of objed)=l +S/F.

(2)lf the object is at F of the lens: M=S/F only and so is less by 20% than if theobject at less than F.

USE: In ophthalmic practice: To resolve structural details e.g. the slide viewer.(2) THE COMPOUND MICROSCOPE

(LIGHT MICROSCOPE OR BRIGHT-FIELD MICROSCOPE)COMPONENTS:

(1)The observation system:1) Two convex lenses: 1- The objective lens of 100-300D.

2- The eyepiece lens of 20-50D.2) The condenser lens: Which focusses light rays on the specimen.3) A stage: On which the specimen is placed.

(2)The illumination system: By a light source.

, 0 FaIIII,

':. ......,...-"'----! ~~~~~~~~~~::---J ~~-..,-"

Fig. 22.1 :Optical principles of the compound microscope.OPTICAL PRINCIPLES(FIG.22.1):

(l)Object 0 to be studied:Is at a distance S just outside fo of objective lens OL.(2)Real inverted magnified image I.Is some distance behind objective lens.

(3)Eyepiece lens:The image formed by the objective lens falls at (or close to)its fe and so acts as a loupe and further magnifies the image but it does notincrease the resolving power (resolution).

(4)The final image I: Is vertically and horizontally inverted,virtual and magnified.(5)The maximum magnification is more than 1500 x: M=(I +S/fe)(VlfUl)

where, V I=Distance of image from objective.Ul =Distance of object from objective.

(6)The maximum resolving power: Is about 200 nm (2000 A 0).(7)Porro prisms:l)To get an erect image.

2)To shorten the physical length of the microscope.USES IN OPHTHALMIC PRACTICE:

(l)Jn ophthalmic instruments to provide magn{fied view of the eye as in:1) The keratometer.2) The operating microscope.3) The slit lamp microscope.4) The fundus camera.

(2)Histopath%gical studies: Of ocular specimel'ls.( ~THE SPECULAR MICROSCOPE

VALUE:It examines the endothelial cells by specular reflection from the interfacebetween the endothelial cells and aqueous.

TYPES OF THE SPECULAR MICROSCOPES:(1)THE CONVENTIONAL SPECULAR MICROSCOPE:

Optical principles:It projects a slit beam of light onto the posterior cornealsurface at a nearly normal angle of incidence:

l)];fore than 99% Of light is transmitted through the aqueous due t(a) Corneal transparency.(b) Similar refractive indices of cornea and aqueous.

2)Less than 1% of light is reflected and scattered from:Aqueous interface(0.02%), corneal epithelium, endothelium and stroma.

3)A portion of the reflected and scattered light (from the cornea afteundergoing internal re.flections with overlapping) is collected: 'By theobjective lens and 'an image is produced on a film plane.

Standard equipment:l)Jt is of the contact type.2)Standard equipments include: 1- A digital pachometer.

2- A 35 mm camera.(2)THE WIDE FIELD SPECULAR MICROSCOPE:

Principle:! overcomes interference by the stroma and epithelial surface.Types:

1) on contact type. With a smaller magnification of the endothelial images.2)Scanning slit type:lt records a large field of endothelium but direct viewingof the cells is impossible and an objective is used with 1/2 aperture

215

M2

L5 ~

.djU61~ b I eobjective lensSystem

Fig.22.2:Scanning slit specular microscope. Fig.22.3:Sc·anning mirror specular microscope.for illumination and 1/2 for image formation (Fig. 22.2).

3)Ecanning mirror type:,optical principles(Fig.22.3):

·l-An illuminated slit of light is projected onto the endotheliumutilizing 1/2 objective L3.

2-The slit S1 is scanned by the rotatory motion of the mirror MI.3-The light reflected or scanned by the endothelium is reimaged by

the objective L3 at second slit S2 after the second ret1ection fromthe rotatory mirror MI.

4-Light passing through slit S2 is reflected by a mirror M2 backthrough slit S2 and on to third facet of rotatory mirror M1 for thethird reflection and then final image seen through an eyepiece asa continuous non-flickering image.

Advantages:l-Higher brightness/contrast and illumination.2- Larger field of viewl0-25 tilues.

SPECULAR PHOTOGRAPHY:(1)Specular micrograph:To assess cell appearance and abnormalities as in cornea

guttata or KPs (as some of the overlapping zones interfere with and obscure thecell details).

(2)Cells counts:1)By superimposing a transparent grid on the endothelial cell image.2)By computer analysis of mimber, density, distribution of form and size of cells.

d1>:' WC1rmalcell count in voun':!s exceeds 3000 cells/mm2-7,gradually decreases with age.USES OF THE SPECULAR MICROSCOPE:

(J) Examination and photography of the corneal endot he! i71111 for:1)Scanning of endothelial cell surface and its response to trauma as in

phacoemulsification and corneal grafting.2)Video recording endothelial layer for documentati·on.

(2)Examination and photography of corneal epithelium and stroma.(3) Study of the anterior segment of the eye including' lens, iris and anterior

vitreous face.(4)Measuremen~ of the thickness of the cornea and the depth of AC by an allac/,ed

pachometer.AFOCAL SYSTEMS IN OPHTHALMIC PRACTICE

Definition:These are optical systems which do not form real images at finite distancesbut form virtual images and depends on the angular magnification of thebeams which pass through and not on the vergence of light.

Principle: Two separate optical elements of such a power and in such position thateach cancels the vergence change produced by the other.

Types: (1)Telescopes: 1) Galilean. (non inverting).2) Astronomic (inverting).

(2)Meniscus type of thick lenses.(3)Compound microscope alone and in instruments.

ASPHERIC LENSES IN OPTICAL APPLIANCESDefinition:Aspheric lenses are spectacle lenses, contact lenses or IOLs with

, nonspherical surfaces.Types: (l)Toric lenses (with a toric surface): For correction of astigmatism.

(2)Varifocal (progressive addition) lenses: For correction of presbyopia.(4)Asplreric lenses: For correction of: l)Aphakia.

2)Subnormal vision.3)Prismatic effect(spherical aberrations).

CH PTER23s'v-", SLIT-LAMP BIOMICROSCOPE

COMPONENTS OF THE SLIT LAMP BIOMICROSCOPE:(A) BINOCULAR STEREOMICROSCOPE: Two compound microscopes at an

angle of about 140 to each other (as in the operating microscope) which gives astereos~opic view with magnifications:(1)Optical system(23.1):

l)Objecti 'e lens 0 and eyepiece lens E with relay lens R inbetween:.6bjectis located in the focal plane of the objective lens O.

2)Telescopic or zoom lens system:Between 0 and Rto vary magnificationand to increase the working distance of the compound microscope fromfew mm to 20cm:1-Galilean telescopic system· It consists of a Galilean telescope (Chapter

21 & Fig. 21.1) which receives parallel light from objective 0 andtransmits some parallel light to relay lens R (Fig.23.1).

Fig.23.1 :Binocular stereomicroscope with a Galilean telescopic optical system.2-Zoom lens system(G):

(a) ncompensate s stem Centre element moves with shift of image.(b om ensated s stem: mage is at the same position relative to the

front element: a) Mechanically compensated.b)·Optically compensated.

3) rector (Porro) prisms Pi To erect the intermediate inverted imagesfonned by the relay lens to be observed by the eyepiece E (the final imagehas the same position as the target).

(2)Stops and apertures:Value:To limit the passage of radiant energy through the optical system.Forms: 1)Aperture stop:For resolution of image and depth of field.

2)Field stop:For brightness of the image near the edges of the field.

3)Small glare stops:Eliminate reflected light from sides of tube optics.(3)Magnification.

1)M=5)\-50)\(better 10,16 and 25)\) as resolution is limited with greater M).2)M is changed by lenses of different powers or by zoom lens system.

(4)Back focal distance (object-ta-Iens distance):1)Optimal back focal distance: A range of 9-12 cm( more if slit-lamp IS

mounted for lasers).2)Large back focal distances:For diagnostic or therapeutic maneuvers as

removal of stitches.3)Smaller back focal distances:For bett~r resolution and image brightness.

(5)Lens coatings:1)Single lens coating: 'fo reduce reflections from clear glass lenses from 8%

to 1% (Chapter 17).2)Multiple lens coatings: TO reduce reflections to 0.5% (each layer is

adjusted to different WLs).'in:Uses of multiple lens coatings:

(1)1n slit-lamp.(2)Reflectors to increase reflectance.(3)Narrow band pass filters as in fluorescein angiography.

(6)Depth of field and stereopsis:1- Depth offield By the depth of focus of the slit-lamp and the viewer

accommodation.2- Stereopsis: Seeing an object in depth from a slightly disparate position

with each eye.(B) SLIT ILLUMINATION SYSTEM: To give a bright slit image with a variable

length, width and position(1)Condenser systems:

Value:To gather and project light into the objective lens to achievemaximum image brightness.

Principl:Light source L is imaged in objective lens 0 by condenser systemC and slit aperture S is also imaged by 0 as a final image F at theobserved eye with a bright slit beam(Fig.23.3).

Fig. 23.2: Koeller condenser illumination system.

(2)llIumination:Luminance: Between 200000 and 400000 foot IambertsColour temperature(spectral composition):Rich in the blue end of spectrum.Haiogen lamps: 1)Higher luminance i.e. intense brightness and whiteness.

2)Higher colour temperature (greater blue end of spectrum)-.7greatcr scattcring and Ouorescencc of transparent mcdia.

(3)Ref ectors: 0 concentrate the light source at the centre of curvature of thereflector.

(4)Aspheric and achromatic lenses: To diminish spherical and chromaticaberrations.

(5)Mechanical shutters To provide different sizes and shapes of beams.(6)Filters:l)Absorptionfilters:To change the composition of the light for certain

examinations:1- hie co alt ilter: s used during applanation tonom~try.2-Green reil ree filter: or examination of the vitreous because:

(a)Scattering of light is greatest when the incident light is ofshort WLs.

(b )Reduced background illumination of the fundus( due toreflection of few rays of shorter wavelenghts) to detectposterior vitreous detachment.

2)Neutral density filters To control the intensity of light.(C)SPECIAL MECHANICAL SYSTEM:

(I )Contains microscope and illumination system and keeps patient in position.(2)It has a common focal plane and a common axis of rotation for the

microscope and the light system.(3)There is a long working distance between microscope and observed eye for:

I )Certain maneuvers such as removing a foreign body from the cornea.2)To give a space for certain optical devices as an auxiliary lens.

METHODS OF EXAMINATION BY THE SLIT LAMP:(A)EXAMINATION OF ANTERIOR SEGMENT OF EYE WITH SLIT

LAMP ALONE(FIG.23.3) :(1)Direct illuminatiortBy passing an intense beam of light through semi-

transparent media and observing scattered light against dark background:

~."-'

~ ~ . .

~ . '.

(l.)Direct illumination. (2)Rctroillumination. (3)Sclcrotic illumination. (4)Spccular reflection.Fig23.5:0ptical pathways of methods of slit lamp examination of anterior segment of the eye:

l)Focallllumination. The slit beam is focussed on the ocular structure orthe surgical instrument.

2)Diffuse illumination:The slit beam is slightly out of focus to diffuselyilluminate a large area.

3)lndirect lateral illumination.·To illuminate the ocular structure byreflected light from a tissue just to one side of it(as a beam atpupillary margin to examine the outer rim of the sphincter).

(2)Retroillumination: y using the reflection of light from a diffusing as asecondary source of illumination for examination of a more anteriorstructure (for example, a light from the iris to illuminate the cornea frombehind or fornl the fundus to examine the iris or lens).

(3)Sclerotic scatterI)By lateral displacement of the slit beam for light to fall on the limbus

with internal reflection of light along the cornea.2)Some retroillumination also occurs due to light scattered into the iris

and other more posterior structures.3)To examine the cornea as in: 1- Faint corneal opacities and oedema.

2- Contact lens fitting.(4)Specu la r reflection.

1)By light rays reflected from a mirror like surface as corneal surfaces andanterior lens capsule.

2)Patient's gaze bisects the critical angle between that of illumination andof the microscope.

3)lt is the best way for inspecting the individual cells of the cornealendothelium or epithelium.

(5)Slit illuminatio .To make and examine sections in clear ocular structures.NBl:Corneal opacities or oedema; Is detected by tile reflected liglltfrom it as lij!/f

is not seen in tlte transparent cornea.NB2:With the slit lamp alone it is not possible to see further back into the eye

than the anterior third of the vitreous: because the refractive pOIver of thecornea and lens renders light emerging from tile deeper points oj tlte eyeparallel and so no image is formed wit/lin lite focal range of tlte slit lampmicroscope.

(B)EXAMINATION WITH INSTRUMENTS USED IN CONJUNCTIONWITH SLIT LAMP:

(1)Auxiliary lenses for examination of fundus and vitreous: Chapter (23).(2)Applanation tonometer:'Chapter (23).(3)Pachomete : Chapter (23).(4)Photographic attachment: Chapter (23).(5)A special eyepiece for length and angle measurements:

Uses:'l)Lengtli measurement:For diameter of cornea,pupil or contact lens.2)Angle measurement: the axis of a toric contact lens.

Component:;:A micrometer disc with:

l)A linear scal :Of 15 mm with a scale interval of 0.2 mm.2) n angle scale Of 1800 with a scale interval of 2".

(6)Fluorometer attachmentTo determine the aqueous outflow, after IVinjection of 4 cc of 10% sodium fluorescein, by determination of theconcentration of the dye at short intervals.

(7) Keratome er attachment: ~hapter 29.(8)Ophtha modynamometer:To observe the pulsations of the CRA through its

head ( a fundus contact lens) when the pressure on the cornea is increased byforward pressure on the lens.

(9)Laser interferometer: It measures visual acuity in patients with hazy ocularmedia (Chapter 26).

(10)Goniolenses: Pm indirect gonioscopy (Chapter 24).(ll)Biometric measurement of the ocular p'arameters for IOL calculation:

Chapter 19.AUXILIARY (ACCESSORY) LENSES FOR

EXAMINATION OF FUNDUS AND VITREOUSBASIC PRINCIPLES:

(l)The fundus and the posterior part of the vitreous can be examined by usingan auxiliary lens to abolish the corneal refraction: So the rays coni.ing fromthe fundus are brought within the focussing range of the microscope.

(2)The illumination column of most slit lamps can be tilted (Fig. 23.4): So thatthe axis of the illumination system can be thrown below that of the viewingsystem to get an improved view of the observed fundus with the auxiliary lens(this tilting of the illumination system avoids its overlap with the viewing systemand so light reflected from the cornea does not enter the viewing system).

_ Slillornp

ilJullliOlJlip" SySfCfTl

0) Before tilting ofillwnination cO/llmln. (b)After tilting ofillllminatioll cO/llmll.Fig.23.4:Tilting of the illumination system of the slit lamp.

TYPES OF AUXILIARY LENSES:(l)Planoconcave lenses:

General principl : The lens forms an erect virtual intermediate image of thefundus and so the normal working distance of the slit lamp to the patient isslightly changed but the monocular and the stereoscopic view are limitedbecause the pupil acts as a diaphragm.

Types of planoconcave lenses:l)Hruby len :

Principle.I-It is a movable (non-contact) planoconcave lens of -58.6 D which is

placed with its concave surface Immediately towards the observedeye (Fig. 23.5a).

2-lt forms a virtual erect and intermediate image of illuminated retina,anterior to retina and within the focussing range of the microscope:

(a)Light from the illuminated retina R emerges from the eye parallel(image RI ofR is formed at infinity and is not seen, Fig. 23.5b).

(b)The Hruby lens fonns a virtual erect image RI ofR which lieswithin the focal range of the slit lamp, Fig. 23.5c) and isdiminished in size, and so a wide area of retina to be examinedwithout rotation of the eye (Fig.23.5d).

Working istance of7lie lens The lens held near the observed eye to get theretinal image in the pupillary plane and a special guide ensures that thelens is kept at a distance of 15 mm from the patient's cornea i.e. at F 1of the eye (and so an object located at F2 of the eye appears with amagnification of 1: 1).

Uses: Examination of the axial parts of the fundus'and vitreous (up to 30°vertically and 60° laterally with good dilatation of the pupil).

(a) (b) (c) (d)Fig.23.5:Rhuby lens and its optical principles.

2)Goldmann postenor fundus cantact lens:Pnncip : It is a planoconcave contact lens of -64 D , of a higher

refractive index than the cornea (Fig. 23.8a) and its concave posteriorsurface(with a radius of curvature of 7.6 mm and a diameter of 12 mm)is in contact to the cornea.

ses Examination of the axial parts of the fundus and vitreous (up to 30°from the axis of the eye).

The a vantages 01CJotdman iWJdus contact lens over Hruby lens:I-Lateral and axial magnification is independant on the refractive enor.2-The monocular and binocular field of view are wider.

he advanlages of Hruby lens over Goldmann fundus contact fen :I-Examination of medium lateral periphery of fundus up to 60°.2-Examination of the sensitive patients and shortly after surgery.

3)Goldmann one-mirror contact lens:Prinicpl :

I-It is a planoconcave contact lens which incorporates:(a)Central part which is parallel to anterior surface of lens: For axial

parts of the fundus and vitreous (up to 30° from axis of the eye).(b )Mirror tilted at 62° with anterior surface of lens:For angle of AC.

2-The image of the part under examination is erect and magnified butlaterally reversed (lateral M is 0.9 and axial M is 0.6 for normal eye).

3-Radius of curvature is 7.4 (steeper than the normal cornea).4)Goldmann 3-mirror contact lens:A planoconcave contact lens with a

central part and 3 mirrors with different inclinations (Fig.23.6):I-Central part:For axial parts of the fundus and vitreous (the central 30°).2-Min'or tilted at 73 o:For medium lateral periphery of the fundus (30° _60°).3-Mirror tilted at 6T: For the peripheral parts of the fundus and era serrata.4-Mirror tilted at 59°: For the retrociliary part of fundus and angle of AC.

Fig. 23.6: Goldmann 3-mirror contact lens:(a) Side view of the lens. (b) Anterior surface of the lens. (c) Parts of the lens: 1- Centralpart; 2- Mirror tilted at 73°; 3- Mirror tilted at 67"; 4- Mirror tilted at 59 0. (d) Differentzones of the vitreous and fundus: 1 Axial parts (central 30") of the fundus; 2- Mediumlateral periphery (30 °_60")of the fundus; 3- Peripheral parts of the fundus and ora serrata;4- Retrociliary parts of the fundus amI the angle of ti,e anterior chamber.(2)Planoconvex lenses:

Principle ·It produces a real inverted intermediate image of the fundus as inindirect ophthalmoscopy and so the slit lamp microscope is movedaway from the the patient.

Types of planoconvex lenses:1)El-Bayadi lens: Of +58.6 D (Fig. 23.7).2)Schlegel contact lens,

The advantages of the planoconvex lenses over the planoconcave lenses:I)A very large monocular or stereoscopic field of view.2)Examination of highly myopic eyes where the microscope displacement

away from the eye is required (with myopia of -20 D the displacement is 18mm for the Hruby lens and 7 mm for the Goldmann fundus lens).

3)Lateral and axial magnification are independent of refractive state of the eyein EI-Bayadi planoconvex lens (but not in Schlegel planoconvex lens).

NB:Planoconvex lenses are not used to see the vitreous: Due to abnormalfield curvatures.

Lj~. (~N EL_ g".di I,",

Fig.23~7:EI-Bayadi planoconves lens. Fig.23.8:Volk's + 90 lens.(3)Volk's double aspheric +60 D, +78 0 and +90 0 lenses:

Principle:'A double aspheric biconvex +60D, +78 D or +90 D movable lensproduces a real inverted image of the fundus as in indirectophthalmoscopy (Fig. 23.8).

Uses:Ftallows scanning of the posterior pole, the peripheral retina and vitreous.Working distance of +90D lens. Is 8 mm from the patient's cornea wllh 4-6mm

pupil dilatation (up to 10 mm if widely dilatedpupil) with a maximum field of view of 60°.

Slit lamp 1'l1Ggn[ficationwith +90 D lens:I-Lower M of5-7X:When learning the Yolk +90 D procedure.2-Moderate M of 15 X for: (a) Full field of view of 60°.

(b) Excellent details.3-Higher M of30-40 X: (a)For follow-up of extreme details.

(b)F or photography.

(a) Side view. (b) 2 fellS system(illverted,reversed atUl slightly reduced image).Fig.23.9: Pallfundoscopc:

(4)Rodenstock Superfield Panfundoscopic lens:Principle. It consists of a' meniscus lens applied to the con lea and is

coupled with a spherical lens (i.e. '2 lens system; Fig. 23.9) and itproduces an inverted and reversed, slightly reduced il'lage of thefundus (in a plane within the anterior palt of the spherical lens) whichis enlarged by the magnification of the slit-lamp.

Disadvantage. Wide field of view of the entire retina posterior to theequator in a single field.NB:Contact lenses arc used with slit lamp for gonioscopy and for laser

therapy(Chapter 24&26).APPLANATION TONOMETER

DEFINITION: It Is used to measure the lOP in which an applanation Surf~lCC withoptical doubling system is usually attached to the slit lamp ~d,'!Jaratusor used as a hand-held in trument.

OPTICAL PRINCIPLES:(l)The standard area of contact:Tonometer head is applied to cornea with sufficient

force to produce a standard area of contact of 3.06 mm (Fig. 23.1 Oa) at which:1)Force W applied is directly proportional to the intraocular pressure P.2)Effect of surface tension S and rigidity of the cornea R cancel each other out.3)The replacement ocular volume change caused by the small applanation area

of 3.06 mm is small arid does not alter the intraocular pressure (unlikeindentation tonometer whose high weights result in an increased pressure inabnormal scleral rigidity).NB:Areas of contact: (l)More than 3.06 mm:Comeal rigidity leads to inaccuracy.

(2)Less than 3.06 mm:Surface tension causes error.(2) The applanation head.·Spring loaded lever with adjustable tension on the spring.(3) The doubling unit:Two prisms are mounted in the applanation head with their

bases in opposite, directions (Fig. 23.1 Ob) to act as a doubling unit which bisectsand shifts the image of the applanation area laterally by 3.06 mm. when the innerboundaries of the green half rings are in contact (Fig. 23.'1Oc).

(.f)The llleasuring drulll:ls connected to a rotatory knob and is calibrated from 0 to10 (which is multiplied by 10 to get the lOP in mmHg).

(b)Doubli/lg prism. (c)Correct applanatioll area.Fig.23.10:Applanation tonometer

PROCEDURE:(l)A drop of fluorescein solution into conjunctival sac, after surface anaesthaesia.(2)The measuring drum is turned to graduation line 1 to bring the measuring head

into its frontal end position.(3 )The ann holding the slit lamp is rotated by about 500 to the side, to open the slit

entirely and move in the blue filter.(4 )The position of applanation head is adjusted in height and side to the patient's

cornea and then slowly approach instrument until it touches the patient's cornea.(5)The examiner looks through the right hand or the left hand eyepiece of the slit

lamp microscope (i.e. through the applanation head) and sees the green circle ofcorneal contact to split into two green half circles which are laterally displacedin opposite directions by the 2 prisms.

(6)The applanation head and eyepiece are adjusted until the two green half circles

just overlap one another (i.e. until the inner boundaries of,the two green halfcircles are in contact) and thus the applanation surface is 3.06mm (Fig.23.l Oc).NB:lf adjusted measuring pressure is high:Applaflatioll.area will be large(alld vice versa).

(7)The value read fi-om the measuring drum must be multiplied by 10 to get theintraocular pressure in mm mercury.NB:A blepharostat is used: lfdifficlilties arisefro111111lsteadilless oftlte eye lids.

PACHOMETERDEFINITION: it measure the thickness of the cornea and the depth of AC ( a

portable ultrasound instrument or an optical instrument attached to the slit lamp orto the specular microscope).

TYPES OF PHACOMETERS ATTACHED TO THE SLIT LAMP:Twoattachments with the same principle but differ in the measuring range:(1) In corneal pachometer: Range ofl.l mm.(2) In AC pachometr: Range of 6 mm.

(a) By splittillg incident beam of light. (b) By a .\pedally adapted eyepiece.Fig. 23.11 :Imagc doubling by the pachomcter.

OPTICAL PRINCIPLES:(l)Thc use qfPllrkinje images:

1)Corneai pacholl1cter uses the Purkinje-Sanson images formed by anterior andposterior comeal surfaces (images I and II) to measure the corneal thickness.

2)AC pachometer uses Purkinje-Sanson images formed by the posterior cornealsurface and anterior lens surface (images II and 111)to measure the AC dep b.

(2) The doubling of Purkinje images:l)Th pdnciple of image doubling:The dOllbled image is aligned by the

examiner 30 that the surfaces in question (anterior and posterior cornealsurfaces in corneal pachometry or posterior corneal surface and anterior lenssurface in AC pachometry) coincide and the corneal thickness or the ACdepth can then be direct.ly read off a scale.

2)The method of image doubling may be done by one 0 the followino twomethods:

i-Splitting the if< cident bealn of light by a perspex plate covered by colu-uredcelluloid and have a central cut out area, placed in the slit lamp beam:

Some light of the splitted beam of light proceeds undeviated via the cutout zone (dotted line in Fig. 23.15a) while some tight is laterally deviatedby passing through the perspex plate, and the images formed by the twobeams at the surfaces ( e.g. of comea) are viewed through the slit lampand the plate rotated until the images are superimposed.

2-Specially adapted eyepiece which splits observer's view of the eye:The eyepiece of the slit lamp is substituted by specially adapted eyepiecewhich splits observer's eye view of the eye (Fig. 23.15b) and thenmeasurement is made by rotating a transparent plate so that the twoimages are aligned in such a way that the surfaces( e.g. anterior andposterior corneal surfaces) are in juxtaposition.

PHOTOGRAPHIC ATTACHMENTPHOTOGRAPHIC SYSTEMS FOR THE SLIT LAMP: There are two systems:

1)The separate camera attachment:Components The photographic and microscopic beams arc completely

separated and one of the oculars of the slit-lamp microscope isreplaced by an adaptor holding the camera.

Disadvantage: t is incapable of steriophotography.Advantage: It is simple and inexpensive.

2) The integrated photoadaptor:Componentsl"The camera is mounted on the slit-lamp and a beam splitter is

inserted in the telescopic beam path between themagnification changer and binocular tube.

Advantage: It is capable of stereophotography.Disadvantage: The reduced light available for photography and

observation is a disadvantage which is avoided by the use ofa hinged mirror (but with the mirror, the observation isimpossible during photography).

ILLUMINATION SYSTEM:1) Incandescent lamp: For observation(the filament of the lamp is imaged

into the flash tube with the aid of a double collector).2) A flash tube: For photography.

STEREOPHOTOGRAPHY: In which the stereoscopic image parts are produced intwo separate camera bodies, when both microscope beam paths are used bythe two beam splitters(a single beam splitter was designed for projection ofboth photographic beams onto the two halves of the 35 mm camera toproduce a single frame stereo pictures).

T i 2tGONIOSCOPY (GONIOLENSES)

DEFINITION: It Is used for visualization of the angle of the AC by a goniolens.PRINCIPLES:

(1) Difficulties in the direct visualization of the angle (Chapter 4 & Fig. 4.11):1) Difficulties in viewing the angle:

1-Viewing the angle from the front is difficult due to:(a) Scleral overlap prevents the entrance of rays.(b) Anterior surface of the iris preventsviewing the angle recess.

2- Viewing the angle from the opposite side is difficult due to: Totalinternal reflections(rays emerged from the angle meet anterior surface ofcornea at angle greater than critical angle).

2) Difficulties in illuminating the angle: ue to total internal reflection.t,n~~In orominent cornea. total internal reflection is not comolete: So amde can be

seen from opposite side.NB2:When the eve is immersed in water or when the AC is filled with air: Tile AC

angle becol1le.~visible."(2)Visualization of the angle by a contact goniolens:

l)A contact goniolens will abolish the comeal ret1ection because its refractiveindex together with that of the lacrimal lens between it and the cornea is thesame as that of the cornea, establishing an optical continuity and abolishingthe corneal refraction.

2)Therefore the angle of AC can be illuminated and seen by a contact lensbecause the rays emerging from the angle do not exceed the critical angle( i.e. no total intemal ret1ection,Fig. 24.1).

NB:Fluid as methvl cellulose 2% is used with most ~oniolenses for:(l)Prevention of entery of air Imb/es between tile lens ali(I tile cornea.(2)Luhricatioll tofacilitate rotation of gonio/ens around it's lllltroposterior axis.

A

Fig.24.1: Yiz.ualization of angle of AC bv a goniolcns. Fi{!.24.2:Koe!>pcgoniolens.TYPES OF GONIOSCOPY:(l)INDIRECT GONIOSCOPY:

Definition:ls Indirect viewing of the angle ailer reflection by mirrors or totalreflecting prisms.

Visualization of the angle:1)Image is seen erect magnified but laterally reversed, on opposite side of part

of examined angle and so interpretation is difficult than in direct gonioscopy.

2)Magnification of angle structures by slit-lamp should- be more than 25 times.Types of indirect gOnlo enses:

1 La nostic gonio enses:I-Lenses utilizing mirrors:

(a) 0 mann One-mlrror conrad ens: Chapter 23.(b 'Go mann 3-mirror contact lens: Chapter 23 ..(c) eiSS -mIrror contact ens:

a) It contains 4 mirrors to see the four quadrants of the angle withoutrotating the lens.

b) It has a flatter base than Goldmann contact lenses (with RC of7.85mm instead of 7.4 mm) and so it is applied with just the normaltear film (while Goldmann lenses require methylcellulose). '

c) It is attached to the slit lamp by a special speculum.2- Lenses uti izing tota re ecting prisms (Thorpe"s gonioprisms):With

a four sided reflecting prisms to see the four quadrants of the anglewithout rotating the gonioprism(light strikes the side of the prism at alarger angle than the critical angle, with total internal reflection).

2) era eutlc onio enses:rAs argon laser trabeculoplasty lens, Chapter26.Uses of indirect goniolense :

l)Diagnosti(:: l)Differentiation between OAG and CAG.2)Of traumatic angle recession or angle neovascularization.3)Of FBs, abnormal pigmentation or tumours within angle.

2) Therapeutic: Laser therapy of glaucoma as trabeculoplasty ,iridotomy orcycl ophotocoagulati on.

(2)DIRECT GONIOSCOPY:Definition:ls direct viewing of the angle of AC (without reflecting mirrors or

total reflecting prisms) using a goniolens with a steeper curvature than thenormal cornea which is filled by fluid.

Visualization of the angle: The image is seen erect and is on the same side ofthe part of the angle under examination and a magnifying device is needed asa hand-held microscope or an operating microscope to raise themagnification of the angle structures up to 25 times.

Types of goniolenses:l)Koeppe goniolens (Fig. 24.2): It has a highly convex anterior surface and a

concave posterior surface which rests on the cornea with interspace filledwith saline and the patient m,ust be recumbent.

2) Trul7cated partial Koeppe gonio/ens: with 2 small dimples on its surfacefor stabilizing or manipulating the lens.

3) Worst len~: Which can be sutured to the globe for stabilization and has asmall aperture to allow for introduction of the goniotomy knife.

Uses;. I) Truncated partial Koeppe and Worst lenses:In goniotomy operation.2) Koeppe goniolens: Is rarely used as a diagnostic lens.

ER2FUNDUS CAMERA

(1) FUNDUS PHOTOGRAPHYOPTICAL PRINCIPLES:As the principles of indirect ophthalmoscopy.

Ophlhalmo$copi cLens L

...................................f(l."~".~..T. ~Uoi""' _

s ~~---~"V'Ophthalmoscopic Objective lens i~~'-~Observer

L.ens L 0 0system l

1 ~R 0-:::::: (]o :.-+- -;.-.-:-_-::::::_-_-_-__.~---------fil---u- -rr---i;?~ ·--------\[j-·-n·-u----t/"

Plane of ReflexIntermediate, mirror

(8) real aerial [maqe

E \7.Ophthalmoscopic Ob· r I /J ObserverLens L 'lee Ive ens // .

R @:::::o(]"::::···::::::.c::.:::::::::i~;~~e;-BF .Intermediate, mIrror t1

C!:) real aerial ImageFig.2S.1:Imagc formation by fundus camcr-a:(a)F'irst stagc;(b)Secolld stage;(c)Tlzirtl stage.COM.PONENTS:Two main components, one for observation and photography and

the other for illumination, with only the front lens in common:(1)The front (ophUlalmoscopic) lens: Performs 3 impOJ1ant functions:-

1)1tforms an image of the observer's pupil in the plane of the patient's pupil·So the entrance pupil of the camera is in the plane of the patient's pupil toget the greatest view oftlle patient's retina.

2)1! projects intermediate real inverted image of the retina: nto thephotographic tube of the camera.

3)1tprojects the illuminating beamji"ol11.within the camera ~ystem: nto tl~patient's eye for observation and photography.

(2)Observations system:I) 1mageformation.(3 stages, Fig. 25. I): .

I-First stage:tfhc front lens L receives the rel1ected rays from the patient'sretina R and produces an intermediate image Iat a point inside thephotographic tube T of the camera.

2- Second stage: This intermediate image I passes through the objective lenssystem a located toward the rear of the camera and is projected into theeyepiece of the observation system E.

3- Third stage:· When the camera shutter release button is activated, thereflex mirror M that diverts the image into the eyepiece for observationswings out of the optical pathway, for the image to pass and projected ontothe film F for photography.

2) Magnification:1- The optical system by which the intermediate image is viewed by the

observer or projected onto the photographic film may be in the simplestcase a magnifying glass.

2- For ametropic eyes the magnification and the location of the intermediateimage changes and this can be neutralized by:(a)Adapted fundus camera to a certain range of ametropia.(b)A changer to insert additional lenses for severe ametropia.(c)Optical means to compensate for oblique astigmatism.

3- The fundus camera is used for measurements and correlation betweendistances on the fundus and on the film( e.g. the visual field defects withthe fundus features of the patient).

(3)lIlumination:1) Zones of illumination: distinct zones in patient's pupillary area:

1- First zone for the incident light beam: Which is projected from the lightsource by means of 2 prisms in the camera and into the lower portion of thephotographic tube, where it is directed out of the. camera into the lowerportion of the dilated pupil of the patient.

2-Second zone for the reflected beam: Emerges form the eye through thecentral pupillary area.

2) Corrfiguration of the incident beam: Usually a ring of light surrounding anonilluminated central area ( or rarely two coils of light, one superior and oneinferior to a nonilluminated central area).

3) Illumination system: 1- Low intensity incandescent lamp: For observation.2- Electron flash: For photography.

4) Projection of the illumination beam by: 1[Semitransparent mirror or,2-Transillumination system.

(4) Photography of the fundus: For the purposes of documentation.(2) FLUORESCEIN FUNDUS ANGIOGRAPHY

DEFINITION: Investigation of the circulation in the vessels of the fundus by rapidIV injection of sodium fluorescein and observation of its passagethrough the vessels of the fundus and their fluorescence with the aidof the fundus camera.

COMPONENTS:(A) Electronic component:

(l)Power supply. A higher power generator (up to 30b watt-seconds of energy).(2) Optical filters:

1) Excitation filter (blue filter):For fluorescein dye to fluoresce by blue light.2) Barrier filter (yellow filter): Blocks transmission of all light to the film

except the secondary emitted fluor.escing light.(3)Automated exposure feature.\ . .

1) Colour fundus photography on either: 1- ASA 64 colour film; or2- Polaroid SX- 70 film.

2) Fluorescein angiography: Oil ASA 400 black and white film.(B) Optical component: .

(l)Optical system: s that of the fundus camera.(2)Angle o/view:

l)Common angles of view: 60°, 50° ,4Y, 30° and 20° and can be changed.2)A wider angle of view of OOo:,ls available (more recent camera designs).

(C) Mechanical component:For various knobs and controls, and camera.(D)Accessories:

(l)!vfulurized system to transportjilm through a mm camera body is either:l)Motor drive system- To transport the film through camera continuously.2) Autowind system: {fo transport the film one frame at a time.

(2)Data/timer recording device: To imprint the time and other informations.(3) Polaroid attachment:For instant results on black and white or colour film.

I'm: Indocyanine green angiography can be performed wi·th a modified funduscamera using: (1) Special excitation and barrier filters.

(2) High speed infrared sensitive HIE 135-20 black and whitefilm.STEREO FUNDUS PHOTOGRAPHY

DEFINITION:It is not a true stereo photography because the 2 photographsrequired are taken subsequently rather than simultaneously.

PRINCIPLE AND METHODS:(l)Manual method:Two photographs of identical areas of the fundus are taken:

1)The first through the left-hand margin of the dilated pupillary area.2)The second through the right-hand margin of the dilated pupillary area.

(2)Electronic stereo,-adaptor method.·Using a glass plate which is automaticallytriggered by the power supply unit to rotate between 2 pre-set positions and toyield 2 fundus photographs taken in succession with a stereo-effect.

EXTERNAL EYE PHOTOGRAPHYMETHOD: Photography of the external eye can be done by the fundus camera by

dialing in one of the plus lenses and increasing the camera-to-subject distance toget a red reflex type photograph.

USES:(1)To document the degree of dilatation of the pupil at the time of fundus

photography, and the lenticular and corneal changes( as keratoconus) whichaffects the fundus photographs and corneal changes as keratoconus.

(2)To obtain anterior segment angiograms as in malignant melanoma of the iris.

27REFRACTOMETERS

DEFINITION: Are devices incorporating banks oflenses for measuring the refractivestate of the eye.

BASIC PRINCIPLES:(1)Scheiner double pinhole principle (Fig. 27.1):

1)Dollble pinho e apertures Placed before pupil to isolate 2 small bundles of light.2) An object not conjugate to the retina appears doubled instead of blurred:

1- If the e e is myopic: The rays of the 2 bundles cross each other beforereaching the retina and 2 small spots of light are seen.

2- If the is hypermetropi . The rays of the 2 bundles are intercepted by theretin'a before they meet and 2 small spots of light are seen.

3- By moving the object (mechanically or optically) to the position whereit appears sing e using interchangeable trial lenses. The far point of theeye can be determined by the examiner to get the refractive correction.

4- The refractometers based on this principle are: Topcon eye refractor,Nidek objective automatic refractor and Nikon autorefractometer (in itsdetection system).E,~~eIQ

C·) \31+1=._) ==_-==-=-=_-==-.::..=_-==-=-=~==--=~

E)"~~;:~__====__ ====__ ====_-====_

~~:PIO~( : ) ::: -==- -==- -==- -==-'-' -~========

T"©i _Oplomllir principii

©1~-()::::--==-; Emht.., #

© !-~ MI

©~I----.l.~ HOplomller .

01F

'PlctoCltpion.

o r\-~HI I I I I I I (c)

oS ~I -, 0' <I <I <S

dlopler,

Hyperoplo

CD§=JII -~-==-='== -=== -=Fig. 27.1: Scheiner double pinhole principle.

(2)Optometer principle (Fig. 27.2):1)It permits continuous variation of power in refracting instruments by u ·ng a

single converging lens (instead of the interchangeable trial lenses): Which isplaced with its principal focus F at the spectacle plane (Fig. 27.2a, b).

2) Li ht from a target T on thefar side of the lens enters the eye with vergenceof different amounts (zero, minus or plus), depending on the position of thetarget (Fig. 27.2c):-1- The vergence of the light in the spectacle plane can be changed smoothly

and is directly proportional to the axial displacement of the target.2- This arrangement simulates a spherical trial lens having a smooth variable

power.

3) The refractometers basedon this principle are: Remote controlledconventional refractors and A-O programmed subjective refractor.

(3)Grating focus principle:i)A moving grating of light. Is projected into the eye through a variable power

optometer system.2) The refractors based on this principle are: oherent dioptron and Canon

autoref.(4)Automated Retinoscopy principle: Is used in the illumination system ofNikon

autorefractor.(5) Continuously variable spherocylindrical power:

i) It is based on the fact that any refractive correction can be simulated by thsum of A variable sphere or two variable cross cylinders.

2) The refractometers based on this principle are Humphrey automaticrefractor, Humphrey vision analyser and A-O programmed subjectiverefractor (in which the optometer principle is also applied).

OPERATING PRINCIPLES:(1)OBJECTIVEREFRACTOMETERS:

1) Conventional 0 jective refractometers:i-Manual objective refractometers( Are based on Scheiner principle and

require to a 19h m1tes formed with infrared light on the patient's retina2-Remote-Controlled conventional refractometers: With a combination of:

(a)A remote-controlled refractor which is based on the optometer principleand in some designs it incorporates the Scheiner principle as well.

(b)A vision screener with a port in the rear to allow retinoscopy.2) Automated objec ive refractometers: hese refractometers perform refractive

measurements automatically using infrared light, requiring 0.2-10 seconds onlyfor the actual measurements but with no visual acuity measurement except inthe Humphrey automatic refractor:

i- Coherent dioptron:~ .L (a)It is based on the grating focus principle in which a moving grating of

I l ( light is projected into the eye through a system variable power!y' optometer which utilizes the entire pupil." (b)On the return path, the focus of the grating on the retina is analyzed by

a photodetector behind a stationary grating mask.(c)The instrument finds the best focus in 2 arbitrary meridians, then scans

1800 to locate the principal meridians by detecting a peak signal andfinally measures the refractive correction successively in 6 meridians.

2-Nikon autorefractor: It utilizes 2 principles:(a The principle of retinoscopy in the 'lIumination system: In which

neutralization of the retinoscopic reflex is not performed but the speedof the reflex is determined in each meridian as the instrument rotatesthrough 3600 in 0.5 second by a prism.

(b)Ihe Scheiner princip e in the detection system. in whichphotodetectors (2 for power error and 2 for axis error) are used todetect sphere, cylinder and axis during the rotation of both detectionand illumination systems (through 3600 in 0.5 second by a prism).

3- Canon autoref:olt is based on the grating focus principle andincorporates a beam-splitter arrangement to allow natural viewingconditions.

4-Nidek objective automatic refractor: it is based on Scheiner principle anduses a straightforward Scheiner system to measure throughout a 1800

rotation in 1.5 seconds.5-Humphrey automatic refractor:/

(a)1t is based on the continuously variable spherocylindrical power inwhich the whole pupil is used during the refraction.

(b)The spherocylindrical optics is changed until no further refractive erroris detected.

(c)Sphere finding is done by fogging the visual acuity chart withautomatic tracking of accommodation.

(d)It provides visual acuity measurement (both before and after the'vJ refraction) unlike the above automated objective refractors.

S .y (2)SUBJECTIVE AUTOMATED REFRACTOMETERS: These refractometers use thesubjective responses from the patient to determine the refractive correctionand so require more patient cooperation and more measuring time but havethe advantage of providing subjective refinement and visual acuity:1)Humphrey vision analyzer:

1-The method of refractio : Is based on the continuously variablespherocylindrical power in which pairs of lenses with complicatedoptical surfaces are incorporated into the projection system to generatevariable spherical power or variable cross cylinder power.

2-Lightfrom the targets: Is collimated by collimator lenses and passesthrough the variable power lenses and-then is deflected by mirrors

. .

designed for pupillary distance adjustment and finally is collected by a30-cm concave viewing mirror located approximately 3m from thepatient as phantom images which are seen as circles of light projectedonto the patient's face.

3-The computer Converts the variable power lenses to sphere, cylinderand axis notation.

4-Sphere finding: ~6 determined by fogging or by red-green chart and theinstrument performs the refraction for near also (unlike the objective

refractometers which provide refraction. for near).5-Humphrey over-refraction system:

(a)lt consists f: a) Vision analyzer.

b) Lens analyzer.c) Interface in-between the two.

(b)Valu :Refraction through spectacle lenses, contact lenses orintraocular lenses with more control of the vertex distance andso it is recommended in patients with high refractive error toavoid problems from vertex distance and pantoscopic tilt.

(c)Principle:a)The patient's spectacle lenses, contact lenses or intraocular lenses

are measured on the lens analy'zer which sends the resultsautomatically to the vision analyzer.

b)When the patient arrives at the vision analyzer, the powers of hisglasses are reproduced from memory and he is refracted over hisglasses and the resultant refraction is calculated automatically.

c)The over-refractivn and resultant refraction are both printed out.2)A-O programmed subjective refractor: is based on the optometer

principle with an axially moving cylindrical lens to achieve smoothlyvariable spherocylindrical power over a wide range.

3)Cavitron subjective autorefractor: 1 is based on the optometer principleand has spherical optics only (unlike other subjective refractometers andso it is not expensive) with no refinement of astigmatic correction and soit is considered as a screening refractor only.

COMPARISON OF OBJECTIVE ANDSUBJECTIVEREFRACTOMETERS:(1)Operating skill and patient cooperation: Less needed in objective refractors.(2)Light use: S bjective'refractors use visible light while objective refractors use

invisible infrared light, with the advantage of absence of visual sensations fromthe procedure with no stimuJ~tion of accommodation of tl e patient.

(3)Measurement time:O.2-1 0 seconds only in objective refractors and 2-8 minutesin subjective refractors.

(4 )Corrected visual acuity:Is determined In subjective refractors but not inobjecti ve refractors except the Humphrey automatic refractor.

(5)Results with ocular disease:l)Macular disease: Objective refractors give better results. .2)Cloudy media:Subjective refractors give rough refraction with less than 6/18

VA vhile objective refi"actors do not function properly.(6)Over-refraction: Is prescnt in subjective refractors but usually not in objective

refractors due to: 1)Reflections fi'om glasses or contact lenses.2)Inadequate pupil size with intraocular lenses.

(7)Binocular refraction for distance and 'for near: With the Humphrey subjeclivevision analyzer only.

(8)Optical systern:UsuaHy spherocylindrical in subjective refractors, but sphericalonly in objective refractors.

28OPERATING INSTRUMENTS AND DEVICES

SY'<,/ (1) OPERATING MICROSCOPEDEFINITION:Two compound microscopes mounted at an angle of about 14° to each

other and it gives the observer a binocular stereoscopic view.HE OPTICAL PRINCIPLES: Are the same as that of the compound microscope

(Chapter 22) but in the operating microscope there are two eyepieces and oneobjective (each lens is replaced bya system of lenses to reduce aberrationsspecially spherical and chromatic aberrations and coma, Fig. 28.1):-

magnific~lionch"nger

Fig.28.1:Components of operating microscope:Observation and illumination systems.(1)The observation system:

1)The main binocular stereoscopic microscope:1-Two eyepieces: Eyepiece M of 12.5x usually for surgery and 10x or 20x

with a reticle for photography and is connected to an erect prism box whichcontains a pair of erector (Porro) prisms(the two erect prism boxes areconnected to a 45 inclined eyepiece tube).

2- The microscope body which contains:(a)Maginification changer (as in slit-lamp):

a)Galilean telescopes; orb)Zoom system:Up for highest M and down for least M.

to)A ~agnification indicator window' For the total M of the operatingmicroscope.

(c)A seat to attach a magnifYing glass.(d)Assistant microscope.

3- The objective lens:

(a)F of the objective lens is 15-20 em. .(b)The distance between the patient's eye and surgeon's eye is 35-40 em.(c)The total M of the operating microscope is 6 x to 40 x.(d)The working distance of the microscope is the distance form the

objective lens to the patient's eye and is equal to F of objective lens.(e)The field of view is 5-10 mm in diameter.

4- The beam splitter: The beam splitter is placed between the eyepiece tubeand microscope body and it provides splitting of the light beam throughthe microscope objective into two dimensions, one to the photographicattachment and another one to the teaching head.

2) The assistant binocular stereoscopic microscope: It consists of two eyepiecesand two erect prism boxes and one objective.

(2)The illumi.nation system:1)Various illumination systems are available: The most important for

ophthalmic surgery is coaxial illumination especially for visualization of theposterior capsule and for vitreous surgery (but illumination is not exactlycoaxial but almost coaxial with the observation system).

2)Halogen lamps or fibreoptic delivery systems: are sed as the light source toreduce heat near the microscope and to' allow easier change of bulbs duringsurgery.

3)Jllumination of the operating microscope may lead to retinal lesions due to:Immobility of the patient's eye during surgery, focussing of the coaxialmicroscope on the retina, maximum mydrias, prolonged duration of the

. procedure and the use of high illumination levels.(3)Operating microscope equipped with slit lamp biomicroscopy:

1)Use in posterior segment surgery:Adjustment of t le operating microscope with its slit lamp: The proper

angle between the pathway of the slit illumination beam and the axis ofobservation beam must be between 5-.110

, so that both beams focus on thesame fundus spot even under moderate mydriasis.

Advantages of operating microscope wit slit lamp over indirectophthalmoscope in ) surgery:I-Higher magnification during special stages which provide .

(a) Less traumatization.(b) More satisfactory anatomical, visual and diagnostic results.

2- iomicrosapic observation ofthejundus un er high magnification(a)A better localization and treatment of fine retinal tears.(b)Observation of the extreme retinal periphery.(c)Control treatment of retinaJ tears to avoid under or over treatment.(d)Asepsis as the room lights are not turned off.(e)Adjustment ofsclerochoroidal indentation in optical cross section of

biomicroscopy ..

Disadvantages of the operating microscope equipped with the slit lamp:l-Reducedfield of observation than indirect ophthalmoscope-70vercome by:

{a)Use low M for entire fundus and. high M for retinal tear itself.(b)Goldmann 3-mirror contact lens for excellent view of the fundus

(chapter 23) but its bulk interferes with cryopencil application orlocalization of retinal tears by sclerallocalizers.

(c)Volk's double aspheric 60 D, 78 D or 90 D non-contact lens (cannotinterfere with cryopencil application or localization of retinal tears).

2-It takes longer time for the surgeon to control and manage it.2) Use in anterior segment surgery:

Aojustment of the operating microscope with its slit lamp: By changingthe angle between the slit beam and axis of observation to an angle of 30°.

Tlie operating microscopes designed for anterior segment surgery aretotally inadequate for biomicroscopic observation of the fundus due to:I-The slit lamp is already placed and fixed at an angle of 30°.2-Coaxial illumination with no angle between the illumination and the

observation system will result in a binocular examination with alocalized beam and not a true optical section.

+90lilens

Fig. ~8.2: Operating microscope with:(a) Stereoscopic diagonallnverlet;::,'(b) Non-eontact binocular indirect ophthalmomicroscope (with a frontiens of +90D usually). (c) Assistant binocular stereoscopic microscope.

(4)Operating microscope equipped with non-eontact erect binocular indirectophthalmomicroscope ( 810M, Fig.28.2):

Principle:Of indirect ophthalmoscopy and enables up to 120° of non-contactobservation of the fundus with a selection of front lenses withdifferent viewing angles (+90D lens usually).

Advantages:I)Wide observation angle (up to 120°).

2)Non-contact to the cornea.3)Perfect mobility of the eye to see the far periphery of the fundus easily.4)Used with small pupil, corneal scars, lens opacities and vitreous

.substitutes including gas.(5)5terioscopic diagonal invertor (5DI, Fig.28.2) :

I)It consists of 2 right angled prisms2 Porro prisms,ChapLer 5 and Fig.5.1 Oc)willen are placea oerween me L eye-pieces ana oOJecuve 01 me UjJeli::tllllg

microscope.2)It erects the inverted image of the wide angle observation system such as the

binocular indirect ophthalmomicroscope or a wide field contact lens whilemaintaining correct stereopsis-7vitreous surgery with panoramic viewing.

(2) SURGICAL LOUPES(1) LOW POWER SURGICAL LOUPE (BASIC SURGICAL LOUPE):

1)A low power Galilean telescopE.; (2x to 4x) combined with an add on thefront to establish the desired working distance:

I-The add is usually combined with objective lens of telescope as one lens.2-The nonnal working distance is equal to the focal length of the add.

2)The total magnification: Is derived by multiplying the power of the Galileantelescope by the magnifying power of the add (M = Ml x M2).

3)Optical principles (Fig. 28.3): .I-The working front add collimates the light from the tip of the object O.2-This bundle of collimated light enters the Galilean telescope as a particular

angle 8 to the optic axis.3-The same bundle emerges from the telescope still collimated, but at an

increased angle 8', where the magnifying power of the telescope is equalto the ratio of 8' to 8.

4-Tip of object is seen by eye as at infinity and all object 0 subtends angle W.(2) HIGH POWER SURGICAL LOUPE: Some of the high power (6x-8x) surgical

loupes are astronomical telescopes \lith image inverting prisms which iscalled expanded field telescopes.

--- ----- ---~--- -'- --r----e::--__----·--=.---.--------·-<::":....::- - ~

L1 -:-::-::::=---e:::- - ---- ------- ---~--o

L.-.-J

Working"add"

Galileantelescope

(3)SURGICAL LENSES(l)SURGICAL VITRECTOMY LENSES:

1)Conventional vitrectomy lenses:I-Landers biconcave lens. It has a refractive power of -83 D, diameter of 10

mm, field of view of 24 ° and is designed to view the fundus in an air filledphakic eye.·

2-!l1achemer magnifying len : It has a 9.8 mm diameter, a 30° field of viewand is used to visualize the minute deep structures in the vitreous and tomanipulate retinal membranes.

3-Machemer flat lens: It is a lens with a plano anterior surface which providesa 36° tleld of view of central posterior pole and central vitreous and is idealfor photography.

4-Peyman widefield lens. it has a concave -60 D anterior surface for 48° wideangle viewing of a phakic filled fluid eye, of the posterior and peripheralfundus in phakic and aphakic eyes and [or endolaser applications in the fluidor air filled vitreol s cavity.

5-l'olentino pris17llenses: rolentino 20°, 30" or 60° prism lens is used forvisualization of the posterior peripheral fundus and vitreous beyond theequator with minimal distortion.

6-Woldoff prismatic biconcave lens: It allows a clear view of the retinalperiphery in the gas filled phakic or pseudophakic eye and in laserendophotocoagu lati on.

2)Wide field Jolk v:trectomy lenses: Are used with the operating microscopeequipped with the stereoscopic diagonal inverter:I-Supa Macula .JuD lens: It has a refractive power of 58D, diameter of 21.4

mm, highest image magnific~ttion of 1.03 X, a central field of view arounJthe macula and of the vascular arcades and so is ideal for submacular surgcIYand treatment of macular holes .

.2-Centra! Retina 85D lens: It has a reii'aGtive po\\'er of85D, diameter of 19.5mm, higher image magnification of O.71X, a field of view till the equatorand so is ideal for vitrectomy in diabetics and membrane peeling to equator.

3-A1im ltad 156D lens: .has a refractive power of 156D, diameter of 18.4mm (in the standard Mini Quad lens) or 21.4 mm (in the Mini Quad XLlens), high magnification of 0.38 X,widest field of fiew of the entire retinaincluding ora serrata and so is ideal for retinal detachment surgery, treatinggiant retinal tears and anterior prolife ative vitreoretinopathy, air/fluidexchange, overall fundus evaluation ofvitreoretinal relationships,examination of peripheral retina in a vitreous cavity filled with air,tv1anagement of dislocated lenses and is excellent for small pupil viewing.

(2)SURGICAL GONIO-LENSES FOR DIRECf CO IOSCOPY: Chapter 24.(3)BYRNE EXPULSrVE HAEMOHRHAGE LENS: Is used to block the flow

from the eye in expulsive haemorrhage .

(4)TEMPORARY KERATOPROSTHESIS:l)Cobo keratoprosthesis: With a clear planr

intraoperati ve visualization of the pC'-during corneal transplant surger'

2)Landers-Foulks keratoprost1surfaces, provides a -8f"corneas with.lacerr

3)Landers wide .r32 D to fr

OPHTP'(l)DIAr'

6) WoldoH prismatic biconcave lens: It allows a clear view of the retinal periphery in the gas fillec

phakic or pseudophakic eye and in laser endophotocoagulation.(2) Wide field Yolk vitrectomy lenses (Fig. 27.4): Are used with the operating microscope

equipped with the stereoscopic diagonal inverter:-1) Supa Macula 580 lens: It has a refractive power of 58D,

diameter of 21.4 mm, highest image magnification of 1.03x, a

central field of view around the macula and of the vasu",,;'

arcades and so is ideal for submacular surgery and 'Teatmentof macular holes.

2) Central Retina 850 lens: It has a refractive power of 85D,diameter of 19.5 mm, higher image magnification of O.71x, afield of view till the equator and so is ideal for vitrectomy indiabetics and membrane peeling to the equator.

3) Mini Quad 1560 lens: It has a refractive power of 156D, Fig. 27.4: Wide field Volkdiameter of 18.4 mm (in the standard Mini Quad lens) or 21.4 vitreclomy lenses:

(a) Silpra Macilla 58V lells;mm (in the Mini Quad XL lens), high magnification of 0.38x, (b) Cenlral Relilla 85 V lens;widest field of view of the entire retina including ora serrata (c) Mini Qilad 156V lells.

and so is ideal for retinal detachment surgery, treating giant retinal tears and anteriorproliferative vitreoretinopathy, air/fluid exchange, overall fundus evaluation of vitreoretinalrelationships, examination of peripheral retina in a vitreous cavity filled with air, managementof dislocated lenses and is excellent for small pupil viewing.

(8) Gonio lenses for direct gonioscopy: Chapter 23.(e) Byrne expulsive haemorrhage lens: Is used to block the flow from the eye in expulsive

haemorrhage .

(0) Temporary keratoprosthesis:(1) Cabo keratoprostltesis: With a clear plano anterior surface which allows intraoperative

visualization of the posterior pole in expulsive haemorrhage during corneal transplant surgery.(2) Landers-Foulks keratoprostl1esis: With anterior and posterior concave surfaces, provides a -86

D tens in the aphakic iluid filled eye and is used in corneas with lacerations.(3) Landers wide field keratoprostl1esis: It has a convex anterior surface with 32 D to facilitate

viewing of the peripheral retina and the posterior pole .

(a)

¥~

(b~1

• OPHTHALMICLENSESUSEDWITHOPHTHALMICINSTRUMENTS:

(1) Diagnostic lenses:1) Non-con tact (movable) lenses:

1- Condensing lenses in indirect ophthalmoscopy: Chapter 16.

2· Movable lenses used in conjunction with the slit-lamp (Chapter 22):(a) Hruby lens. (b) EI-Oayadi lens. (c) Yolk double aspheric 60 D, 78 D or 90 D lens.

2) COlltnet lenses:1- Contact lenses used in conj unction with the slit lamp (Chapters 22 & 23):

(a) Goldmann posterior fundus contact lens. (b) Goldmann 3-mirror contact lens.(c) Schlegel contact lens. (d) Panfundoscope lens.

(e) Goniolenses for indirect gonioscopy.2- Contact goniolenses: Used for direct gonioscopy (Chapter 23).

3- Other contact lenses: (a) For localization of intraocular foreign body.(b) With electroretinography.(c) With orbitonometry.

(2) Therapeutic contact lenses:

1) Laser lellses (Cl:apter 25): 1- Argon laser lenses. 2- YAG laser lenses. 3- Diode laser lenses.2) Sllrgicallenses (Chapter 27): 1- Yitrectomy lenses.

2- Goniolenses for direct gonioscopy.3- Expulsive haemorrhage lens.~- Temporary keratoprosthesis.

(2) SURGICAL LOUPES(1) LOWPOWERSURGICALLOUPE(BASICSURGICALLOUPE):

1) A low power Galilean telescope (2x • 4x) combined with an add on the front to establish the desiredworking distance:

1- The add is usually combined with the objective lens of the telescope as one lens.2- The normal working distance is equal to the focal length of the add.

2) The total magnification: [s derived by multiplying the power of the Galilean telescope by themagnifying power of the add (M = Ml x M2).

3) Optical principles (Fig; 27.5):

1- The working front add collimates the light from the tip of the object O.2- This bundle of collimated light enters the Galilean telescope as a particular angle B to the optic axis.3- The same bundle emerges from the telescope still collimated, but at an increased angle B', where the

magnifying power of the telescope is equal to the ratio of 9' to B.

~- The tip of the object is seen by the eye as if it were at infinity, with the entire object 0 subtending theangle B'.

L-JWorking Galilean"odd" telescope

Fig. 27.5: Optical principles of the basic surgicalloupe.

(2)HIGHPOWERSURGICALLOUPE:Some of the high power (6x-Hx) surgical Iou pes are astronomical telescopeswith image inverting prisms which is called expanded field telescopes.

29KERATOMETER (OPHTHALMOMETER)

DEFINITION AND INDICATIONS: It is an instrument which is used to measure:(l)The radius of curvature (r) of the anterior corneal surface (by using the first

Purkinje image) for: 1)Contact lens fitting.2)Progress of keratoconus.3)Calculation of distances between ocular media.

(2)Dioptric power of the cornea (D) and corneal stigmatism.(3)Radius of curvature (r) of contact lenses.

DISADVANTAGES:(1)Disadvantages of standard (in the office) keratometry:

])Jt neglects:] -The refraction of the posterior surface or the cornea(about 0.50DC axis 1800)~.

2-Lenticular astigmatism (about 0.50 D C or more).2)lt cannot estimate the refraction of the central part of the cornea but

that of two points about 1.25 mm on either side of this point.3)lt gives the value of cylinder at the corneal plane and not at the

spectacle plane which leads to high error (up to 3 to 5 D) in highcylinders.

(2)Disadvantages of operative (during surgery) keratometry:Less accuratethan in the office due to:I)Patient's visual axis is not coincident with the optical axis of the device

as the anesthesia prevents patient's eye movement.2)Reflections from the front of lhe cornea and from the front of air bubble

in opened AC.3)Distorted corneal retlections from corneal drying during surgery.4)Distorted corneal shape due to eye speculum and eye decompression.5)Most surgical keratometers are only calibrated for one working distance

and so the astigmatic difference between the meridians may be differentfor different objective lenses of the operating microscope to which thekeratometer is attached.NE: The keratometer is combined 11t'ithmodern autorefractometers

and makes use of 3 infrared beams and measures the cornealcurvature on each of 3points affixation.

THE OPTICAL PRINCIPLES OF KERATOMETERS:(A)IMAGE FORMATION ON CONVEX CORNEAL SURFACE (FIG.29.1a):

(1)The anterior cornea surface acts as a convex surface) reflects a sma Jpart of incident light: So the keratometer measures r of the central 3mmof the cornea using the first Purkinje image (the centra14mm of the corneais a spherical refracting surface for vision).

(2)The fonowing formula is used (Chapter 7): I I 0 =V I U

I)If 0 is at infinity, I will be at F (of convex corneal surface )-780 V = 1'/2.

2)ln practice, I iS,very close to F (Fig.29.1a)-7S0 V equals r /2 :So, I / 0 =' r / 2U So, r = 2U I 0

3)fn all keratometers, U is constant = focal distance of viewing telescope.4)0 is fixed and I is adjusted to measure r (in Helmholtz and in Bausch &

Lomb keratometers) or 0 is variable with a standard I (in Javal Schiotzophthalmometer).

(B)DOUBLING OF THE IMAGE SEEN BY THE EXAMINER:(1)ln Helmholtz ophthalmometer:

Observer'~view

r ) , : Y

(a)/mage formatio/l Oil COIlVexcorneal surface. (b)Optical principle .••..Fig. 29.1: Helmholtz ophthalmometer.

1)Refraction through a glass plate:A ray of light falling obliquely on aglass plate, is deviated through the glass and the emergent ray is parallelto the incident ray, but shifted laterally and this displacement depends onthe angle of incidence (Chapter 4 and fig.29.1 a).

2)Doubling of the image by two rotating glass plates in the optical 'ystem (Fig.29.1b):

I-Doub inn ohe Image b two g ass pate:(a)Ofknown thickness, index of refraction and angle of inclination

(varied by the observer).(b)To overcome the natural movements of the patient.

2-A beam 0 ig It IS passed t u·ou h a raticule cOr' c: 0 be seen onpatient's cornea where an image I of graticule is formed by reflection.

3- he re ected ig It asses bac { into the instr e Through twoparallel-sided glass plates X and Y which are inclined to each other todisplace light laterally as it passes through them, giving rise to twovirtual images l' and I"which are viewed through a telescope.

4-The ang e 0 me tnatlon of the lass pates is varied by the observer'Until the edges of l' and I" touch and the diplacement E produced bythe 2 inclined grass plates is measured.

5 Calcu aho . The displacemnt E can be calculated and it equals the sizeof image I produced by the surface of unknown r as the distancebetween the centres of I' and I" equals the diameter of 1, from whichthe radius of corneal curvature can be calculated using the fom1Ula:

r= 2UI I 0Where,U=Distance of object of known size from curved surface(known).

I=Size of image = Displacement E.

o =Size of object (Known).6- loptric power of the cornea D is determined from the ormula of

simp e spherical refractive sur ace ( hapter.): D =n2 - nl/ r:Where, n2 = Refractive index of the comea(1.3375).

n1 = Refractive index of air (1).So, D = 1.3375-1/ r = 0.3375-1/ rein m.) = 337.5 I rein mm)So, Resultant keratometric formula D = 337.5 / rein mm)

(2)ln Javal-Schiotz ophthaimometer:

/-'----""" "

/ "/ A B \

(~ OJ\ -(---) I\ a b I" /'- ./..... _--_ ....

(a) Mires A and B. (b)Space ab(object size).Fig. 29.2: Javal-Schiotz ophthalmometer.

1)The object consists of a pair of mires A and B, mounted on curved sidearms which project from each side 0 viewing telescope (Fig. 29.2a):I-Each mire consists of a small lantern with a coloured window.2-0ne mire is step shaped while other is rectangular (Fig. 29.2b) and space

ab between the two mires is the object size used in the measurement.3-The arms on which the mires are mounted can be rotated about the axis

of the telescope so that readings can be made in any direction.2)Doubling of image by Wollaston prism (double refracting prism,Fig.29.3):

i-It consists 0 nvo rectangular quartz ri ms c nented together in theviewing telescope (between the objective and eyepiece lenses) with theoptical grain of the crystal at right angles(Fig.29.3a):

(a)Optical system. (b)Wollaston prism.Fig. 29.3:Javal-Schoitz ophthalmometer.

(a)Quartz is a double refracting substance and thus it splits a single beamof incident light to form two polarized emergent beams.

(b)The optical grain of crystal separates the two emergent beams by afixed angle, while dispersion produced by first prism is neutralizedby that of second prism.-7sharp images are formed(Fig.29.3b).

~ D~ 0<t------io 4----)-a b . 8 b

(b) Objec! size carteCI

~~~ ~ D• •

[&.~ ~~a b( •a b

(C) Object size 100 la'ge

O(al 1 '11dioptru corneal (b AdjuslInents lor reading lhe axis of

~. ~D astigm(llisrn astigmatism and corneal radius

8 ( 'I>B~ ) :,

Fig.29.4:Images of mires. Fig.29.5:Adjust of axis and radius of corneal astigmatism.2-The mires are reflected upon the cornea and the 0 server sees four

image :The two peripheral images are ignored while the two centralones are app~6ximated until their inner edges touch (Fig. 29.4b) to beindicated on a dial to give the .meridian of least refraction.

3-Next the arc ·s urned al righ angles to this meridian'(a tIe im gcs 0 IC mlrcs arc sh III apposi 10.1.The curvature of

the cornea is uniform and there is no cornea astigmatism.(b III {; C um me H. ~ln e' v· p s·· 0 Ie .mag 'S 0

lallg", , cae s cp v He 1 S ovcrla } lcd b 'therc 'mgar .",.In' I" lea es 0 ash·' ~tism.a)Thus if the inner images of a and b (Fig. 29.4b) are aligned

correctly in one corneal meridian but overlap by one and half stepsin the meridian at 90° to the first, 1.5 D of corneal astigmatism ispresent (Fig. 29.5a).

b)\Vhen an astigmatic cornea is examineds the two images aredisplaced vertically and so adjustments for reading the axis ofastigmatism are done and the degree of astigmatism can bemeasured (Fig. 29.5b).

NB:Thc mires of tllr Haal!-Strcit Javal Schiotz KcraromCt~-(J)lllcorporate a "orizontalline to facilitate vertical alignment.(2)Step-slwped mire is green and rectangular mire is red, and both are

internal illuminated, so t"at the area of overlap appears white.4-Calibratiol1S of the instrument: he instrument is calibrated in terms of

corneal curvature r and in terms at dioptric power of the cornea 0and It can also be used to measure r of contact lenses.

OBJECT/VElENS tOOUBIED

IM"'CE

I '\I \I t"'I\-~,I • I

lA.l..lp

Fig.29.6:0ptical system of Bausch and Lomb keratometer,(3)ln Bausch and Lomb keratometer (Fig. 29.6):

1)Ob'ect and image sizes:Object size is constant and image size is variable,2)lmage doubling: By two doubling prisms in the optical system.3)Double images: Are at 90° from each other to allow the measurement of

both powers of an astigmatic cornea without rotating portions of theinstrument between measurements,

(a.) (h).Jc) (d) (e)Fig. 29.7: Mire used in Bausch and Lomh kcratomctcr:(a)Mire cOlljigllra!ioll;(bjEnIl11iller'sview after aligllment;(c)Adjilstmenf of horizontal meridian;(d) View ill oblique astigmatism;(ej Adjustment of obliq.ue meridian.

4 Mire used in the kera ometer, ig.29.7(a),(b),(c),ld),(e):1 i .tion Fi r.29,7a: .mire with 2 plus and 2 minus signs.2·· ~ cenira rin is dOH led a e a igning the. instrument( ig.2 , I : With

the patient's eye indicating that the instrument is not correctly focussedon the corneal image.

3- <l'ustment of the horizontal meridian( 'ig.29.7c)· Occurs when the 2plus signs of the centml and Jeft images are superimposed.

4-W there is oblIque astigmatism( <ig.29.7d . The 2 plus signs are notin aligmnent with each other.

5- tment ot he ob it ue me idian rg.29.7e· Would bring the 2 pI ssigns into coincidence.

5) nterposition of +1.25 or -1"00 0 lens: For a range of 36-52 D (i.e. 6.5-.9.38 mm radii of curvature)to be extended to one 0[30-61 D(5.6-l 0.9mm) for the extremes of corneal curvature.

HA: TER30FOCIMETER (LENSMETERY

DEFINITON:It measures the vertex power of a lens accurately and direCtly(Trade names: Focimeter, lensmeter, ultimeter, vertometer and refractionometer).

USES:(I)Measurement of the vertex power ofa lens accurately and directly.(2)Comparison of vertex power of lenses in trial frame with manufactured lenses.(3)Measurement of the axis of a cylinder.(4)Location of optical centre of lens to detect and calculate the power of a prism.(5)Measurement of the back vertex power of a soft contact lens.(6)Measurement of the posterior central curve (base curve) of a hard contact lens.

OPTICAL PRINCIPLES:(A)OPTICAL SYSTEM:

(1)Standard focimeter: Consists of two main parts (Fig. 30.1):1) Thefocu~'sing ~ystenz:

1- The standan lens A collimating lens of high plus power which.-"renders light parallel. ~2~ Ie target: s usuaut a ring of small dots, formed by a light source

placed behind a punched disc with a circle of small holes.3- The un 'nown ens: The spectacle lens or the contact lens being tested

is placed in a special rack against the lensmeter aperture stop.2) The observation system:

I-A telesco e: With an adjustable eyepiece (the light entering from thefocussing system is viewed through the observation system).

2 'e e C Ieee contams a graticu c and rotractor scale: To measurethe direction of the axis ofa cylinder. ,,~

(2)Automa e focimeter: It replaces the viewing telescope by a projectionscreen and can give either a visual and/or printed readout of spherical andcylindrical lens powers and prism power (Badel type optical system can beapplied to automated focimeters BUT most automated focimeters on themarket employ the non-Badel type with a different optical system(astronomic optics) utilizing mirrors, prisms and lenses.

(B)LENSMETER DESIG S(FIG.30.1):i-Badel (optometer) lensmeter design: The standard lens is located at a

distance equal to its focal length from the lensmeter aperture stop againstwhich the unknown lens is placed.

2-Non-Badel lensmeter design: The standard lens is located near thelensmeter apertur~ 'stop against which the unknown lens is placed.

NB:Badel principle came to dominate lensmeter designs (lue to its advallta!!es willcn ar:(l)Ullifor11lI1leclla/lical motio/l perdioptre change ofpower.(2)Morejlexibifity of the lensmeter as the condition of a sharply focllssed

target is governed by thick lensformula with:1) Target magnification is independent of botlt unknown lens ami sltarpnesj'

of foctls, tit tiS eliminating errors in high plus lens measurement.2) Target position is a simplefuJ1ction of unknown lens power, eliminating

longer target distances required to measure high minus lenses.(C}OPERATING PRINCIPLES:(1)Measurement of the power of a spherical lens:

l)Before use the instrument should be set to zero and the eyepiece adjusteduntil the dots and the graticule scale are sharply focussed.

2) The distance through which the target is moved is directly related to thedioptric power of the unknown lens under test.

3) The power of the lens can be read off in dioptres from the calibrations onthe focimeter.NB:Focimeter measures the vertex power of tlte fellS surface ill COlltact with lite lens

rest and so 'he back surface of the jpectacle lens must be against the lens rest.

Axis.scale

Wi,,"'""'""";:::..,.,,"'\A~~/®--=-f}-- (~ ~:.:./• ~"""'E-).e-Pie-«-r<-licl-e ~/'---'- , Targcl mC;;

Viewing lclcscorc .

I PO\lol'r~lak_SpcClodc kns 10 S 0 -S -10

Lcnsmctcr aperture Staudilrd lens Tarbe( mireI'~ SPW.c!e k~

--J)~~ LJl~J _=====t=Ft\ . :rr-f-+-t----r-VieWing lelescope 302010 0 -S -·10 -IS

PowcJ scale

Fig. 30.1: Optical prindplcs oftllc focimeter. '(2)Measurement of the power of a cylindrical lens:

1)The target must befocussed separately: For the two principal meridians.2) The dots of the target are then seen as drawn out lines: he length of the

lines being proportional to the difference between the two principalpowers i.e. to the cylindrical power of the unknown lens under test.

3) Examination of a cylindrical lens on the focimeter:1-The instrument is adjusted until one set of line foci is in focus (Fig.

30.2£1) and the reading (+ 1.00 D) is recorded.2- Instrument is further adjusted until second set of line foci come into

focus(Fig.30.2b )and reading( -L3D)and axis (180°) oflines are recorded.- The first reading gives .the spherical power of the lens.

4- The cylindrical power is calculated by algebraic subtraction of the firstreading from the second i.e. (+3)-(+1) = +2D.

5- The axis of the cylinder corresponds to the axis of the second reading,+1.0 OS

i.e. 180°. So, The lens power = +2.0 DC axi~ 180-;;

120 90 60

150~30

180t' "III l1,1 0

.• 1.0 0

120 90 60

150~30

180r <::1- 10.• 3.0-0

(a) (b)Fig.30.2:Identification of a cylindrical lens. Fig.30.3:J\.ttatchment to measure BC of hard CL.

(3)Detection of the optical centre of the lens: By marking it by a markerwhich is incorporated in most focimeters.

(4 )Detection and calculation of the power of a prism:.1) The spectacles with marked optical centre are put onto the patient and the

degree of decentration of the lens is measured: By the relationshipbetween the centre of the patient's pupil and the optical centre of the lens.

3) Then the prism power is given by the equation: P = D hWhere, P = The prismatic power in prism dioptres.

D = The lens power in dioptres.h = The decentration in centimetres.

(5)Measurement of the back vertex power of a soft contact lens:The contactlens surface is dried before being placed on the focimet~r lens rest:I)Rapid measurement is done to avoid image distortion from lens driness.2)The lens can be measured while it is immersed in a saline filled cell but

the focimeter reading must be corrected by a specific conversion factor.(6)Measurement of posterior central curve(base curve)of hard contact lens:

l)An attachment (Fig.30.3) afthe same material as the hard contact lens,fitsover the lens stop of the focimeter with:1- Its convex surface lies in the plane of the lens stop.2-1ts concave surface supports the lens being measured.

2)The hard contact lens is placed, convex surface down, on the concavesurface of the attachment or holder: With a small amount of fluid (ofsame Rl of the lens) is placed between them.

3JPCC(BC) o.fhard contact lens is calculatedfrom the fallowing equation:tl + t2

(I-n) 1+ n (Dy-Dl)

pec = (Dy - DI)

Where, PCC=Posterior central curve (base curve,BC).Dy =F ocimeter reading.01 = Power of the holder's front surface.t I =Holder thickness.t2 = Thickness of the contact lens.n = Refractive index (1.49).

AORTHOPTIC INSTRUMENTS

DEFINITION: Are instruments used for assessment and binocular training of muscleimbalance.

TYPES:( 1) Stereoscopes.(2) Amblyoscopes: 1)Minor amblyoscopes.

2) Major amblyoscopes (synoptophores).

thehips.

PRINCIPLES:(1)Two photographs of an object taken from slightly different aspects (such as

would be seen if the observer viewed the object first with one eye only and thenwith the other eye) are used as objects.

(2)When these two photographs are viewed, the two superimposed images willgive the impression of a simple stereoscopic photograph(i.e. 'Nith a lengt.h, breadth and a depth, Fig. 31.1).

(1) STEREOSCOPESTY PES(FIG.31.1):

ct d 'j. 0 X,.~ d

1\

S.I \ 0

I \I \

tI \

\I ,, \ I ,

r.I \ \I I

A\ . , ,

I ,s: I \

I \, \ I \

I I I \

ItsI , , ,

P. 8 Ai.- B

(a) ReJ1ecting mirror. (b) Prism. (c) Lells.Fig.31.l :Stereoscopes.

(1)Reflecting mirror stereoscope: Two tilted plane mirrors A andB with t.he image X, of the objects 0 and 0', formed by eithermirror is at the central line of the instrument to facilitate theirfusion.

(2)Prism stereoscope:Component~ :Two prisms base-out A and B with the image X,

of the objects 0 and 0', formed by either prism is displaced at thecentral line of the instrument:

1)Distance between the two objects is t.wice the distance throughwhich the image is displaced.

2)Distance of object from eyepiece equals that of the image asthe prism only displaces the image.

Orthophoric lines:ffwo lines from a point midway between eyepiecesto infinity and the angle between each line and the axis of thest.ereoscope is the same as the deviation produced by the prism.

274

(3)Lens stereoscope:Components 'fwo lenses A and B, with the objects 0 and 0' placed

or inside f of lens A and lens B respectively, tQ ge~an erect imageX at the central line of the instrument.

Two orthophoric lineSJ:Pass from the midpoint between the opticalcentres of the two lenses and principal foci of lenses.

Formula/or lens stereoscope.

1) I ~~= d; I Where, dl=Dislance separating the two lenses.

d2= Distance separating the two object.d3=Distance between object and the lens.f=Focallength of each lens:

2)When the stereoscope is constructed with d 1 = d2, then itfollows that d3 = f (thus, the objects are at principal foci ofthe lenses and so their images \Yill form at infinity and willbe focussed on the retina of an emmetropic observer withoutany effort of accommodation).

3)The objects are situated on the two orthophoric lin~s and sothey will be fused into one image at the central line of theinstrument normally:I-If the two objects are moved away from the two

orthophoric lines:The images will be fusedonly in exophoria with excessive convergenceinsufficiency.

2-lfthe two objects are moved away from the twoof esophoria with excessive convergence.orthophoric lines:The images can be fused only in cases

(2) AMBLYOSCOPESTYPES OF AMBLYOSCOPES:(1) MINOR AMBLYOSCOPE: 2 halves joined by a hinge and each half consists pf:

1)Two cy indrical tubeS:A short tube joined to a long tube ( of4cm diameter) a:t 1200 angle.

2)Oval mirro . At the junction of the two tubes with Clnangle of3)SUd"ecarrier At the distal end of each long tube.

450 to reflect the picture of the slide.4)Convex lens (eyepiece):

I-Situated at the proximal end of the short cylindrical tube.2-lts power is +7.000 to produce relaxation of accommodation.3-lts F is 14.3 cms which is the total of:

(a) The distance between the lens and the mirror ( 1.6 cms).(b) The distance between the mirror and the slide( 12.7cm).

4-So, the slide is placed at f of eyepiece.(2)MAJOR AMBLYOSCOPE (SYNOPTOPHORE):

Components:l)Metal Columns: The major amblyoscope consists of two metal

columns attached to the base of the instrument and each metalcolumn carries a tube which cOFltains:I-Sma I mirror:Near the proximal end of the tube at 45° to

reflect the picture of the slide.2-Slide carrier: At the distal end of the tube, to hold slides

containing targets:(a)For different grades of binocular vision.(b)For Maddox rod test.

3-Collvex lCIls(eyepiece :At the proximal end of the tube( theconvex lens power, F and position are the san1e as in theminor amblyoscope).

))Thp illumination svstem:Lamosibehind a Qlass filter to illuminate the slides.. ... '. -Procedur.e:

1)The angulation and position of the 2 tubes can be changed to detect anyangle of deviation.

2)The position of each eyepiece is adjusted for each eye to see a picture from theslide carrier.

3)The observer is situated at the distal end of the synoptophore to see theret1ections oflight( fTomthe illuminated tube )in the patienf s cornea untilcorneal reflections are central and symmetrical.

4)The angulation of the tube as measured on the scale indicates any angle ofdeviation in true strabismus(horizontal,vertical or torsional angle ofdeviation )in deQrees or in orism diootres.. - ... .

Accessories:J)After images:f3y halogen light sources and condensing systems2)Haidinger's bruches·Pair of removable Haidinger's bruches units.

Uses:I)Measurement of the angle of strabismus(heterophoria and heterotropia).2)Assessment of the grade of binocular vision and the fusional reserve.3)Orthoptic training for development of 3 grades of binocular vision.4)Measurement of angle kappa with assessment of angle alpha.5)After images for: I-Diagnosis of abnormal retinal correspondence.

2- Pleoptic treatment.6)Haidinger's brushes for: I-Testing of macular function.

2-Assessn1ent ot mnbJyopia.3-Pleoptic treatlnent.

H 3REFRACTIVE SURGERY

(1) CORNEA: I)Refractive. keratoplasty: 1- Lamellar.2- Non-lamellar.

2) Photorefractive keratoplasty (surgery):1-Eximer lase.r:

(a)Photorefractive keratectomy (PRK).(b)Photorefractive intrastroma keratomileusiS' (LASIKand).

2-Diode laser: Photorefractive astigmatic keratectomy.(2) LENS: 1) Intraocular lens implantation:

. I-Aphakic IOL (Chapter 19): High plus aphakic lens(single focus ormultifocal IOL in PC): (a) In refractive cataract surgery.

(b) For aphakia.(c) For presbyopia.

2-Phakic IOL.Minus or plus phakic lenses(single focus or multi focalIOL in AC): (a) Iligh myopia.

(b) High hypermetropia.(c) presbyopia.

2) Lens extraction: For high myopia of more than -20 D.(3) SCLERA: 1)Scleral expansion surge (using silicone bands).

2)Anterior ciliary sclerostomy(AC ).(l)REFRACTIVE KERATOPLASTY

1) LAl\tlELLAR REFRACTIVE KERATOPLASTYTTPES OF LAMELLAR REFRACTIVE K~~RATOPLASTY:

Fig.32.1: Kcratophakia. Fig.32.2: Kcrutomiicusis. Fig.32.3: Epikc."atophakja.(t.t.)For aphakiaJh) For myopia. (a)For llphakia.(h)For myopia.

(1) Keratophakia(Fig.32.:1.):Definition: Insertion of a cryolathe-cut (i.e. frozen and cut) doner comeal tissue

lens in-between the lamellac ofthc patient's corncal stroma.Principle: t increases the anterior corneal curvaturc.Indication: High hypermetropia in aphakia.

(2) Intrastromal lens implantation:1) Intrastromal lens:

Definition: A synthetic lens of minus or plus power is inserted in-between thelamellae of the patient's cornea (instead of donor corneal tissuelens in keratophakia).

Principle: I-Hydrogel lens: Its refractive index is similar to the cornea (1.38)and it changes the posterior corneal curvature more than theanterior corneal curvature.

2-Polysulphone lens: Its refractive index is higher (1.63) and itchanges the refractive power of the cornea by altering the path oflight as it passes through the cornea-lens interface within thecornea (unl ike tissue and hydrogel lenses).

Indication : High myopia or high hypermetropia (up to 9 D).2) Intrastromal/ens ring: It steepens the corneal periphery and flattens the

corneal centre in low degrees of myopia.(3) Keratomileusis and keratomileusis in situ:

Definition:l)Keratomiieusis(Fig.32.2): A segment is removed from patient'scornea (autoplastic keratomileusis) or [rom a donor cornea or apreserved donor tissue is used (homoplastic keratomileusis) andthen shaped on a cryolathe before being reattached.

2)Keratomileusis in situ: The patient's own cornea is shaped by themicrokeratome.

Principle: It changes the anterior corneal curvature.Indications I-High M:(a)Up to-15D in autoplastic keratomileusis.

(b) Up to-25D in homoplastic keratomileusis).2-High H as in aphakia.

(4) Epikeratophakia(Fig.32.3):Definition. A cryopreserved lathed donor corneal tissue lens is sutured onto the

patient's cornea after removal of its epithelium.Principle: It changes the anterior corneal curvature.Indications: l)High hypermetropia as in aphakia (up to 20D).

2)To correct high myopia.(5) Lamellar keratoplasty. ,

Definition: A cryopreserved lathed donor corneal tissue lens is sutured onto thepatient's cornea after removal of its epithelium.

Principle: It changes the anterior corneal curvature.Indications: )High hypermetropia as in aphakia (up to 20D).

2)To correct high myopia.(5) Lamellar keratoplasty.

TISSUE LENS PREPARATION:(1)Shaping of tissue lenses is done with a cryolathe: Which freezes the tissue as

it is being lathed (cut).

(2)Freezing of the tissue is necessary for:1)It holds the tissue in the lathe.2) It increases the solidity of the tissue for lathing.

(a)A/lterior surface oftlIe corneal tissue (b) Tissue is removedfrom posterioris placed ()fl a concave plastic base. surface of tile corneal tissue

Fig.32.4: Tissue lens preparation.(3)The anterior surface of the corneal tissue is placed on a concave plastic

base(Fig.32.4a):To- match the desired postoperative anterior curvature of thepatient's cornea.

(4)A cut is made in the posterior surface of the corneal tissue which is of thedesired depth, radius- of curvature and diameter to mach the curvature ofthe patients stromal bed(Fig.32.4b):I) lv/ore tissue is removed from the centre of the posterior surface of the tissue

lens: To decrease (flatten) the anterior corneal curvature for correl;liull Ul'myopIa.

2) More tissue is removed from the periphery of the posterior surface of thetissue lens: To increase (steepen) the anterior corneal curvature Corcorrectionof hypermetropia.

3) Additional tissue is removed from the periphery: to create a peripheral wingfor attachment of the tissue lens to the cornea in epikeratophakin and inkeratomileusis for aphakia.

DETERMINATION OF TISSUE LENS PARAMETERS IN LAMELLARREFRACTIVE KERATOPLASTY:

(1) Determination of the desired power correction at the corneal plane:1) For refractive keratoplasty alone:From the formula using:

l-The refraction at the corneal plane in dioptres.2-The refraction at the spectacle plane in dioptres.3-The vertex distance in metres.

2) For combined epikeratophakia and 'cataract extraction (especially inchildren): From the formula using:

1-The reti'action at the corneal plane in dioptres.2-The axial length of the eye in millimetres.3-The keratometric reading in dioptres.

(2) Determination of the desired postoperative anterior corneal curvature:1)Form ula for a thin lens: From the formula using:

I-The postoperative radius of anterior curvature in metres.2-The preoperative radius of anterior curvature in metres.3-The refraction at the corneal plane in dioptres.

2) Formula for a thick lens: The central corneal thickness is incr~ased inkeratophakia and in epikeratophakia and so formula of the back vertex powerof thick lens is applied for thick cornea using:

I-The back vertex power of the cornea in dioptres.2-The power of the anterior surface in dioptres.3-The refractive index of the c.ornp,::l

NB:The central corneal thickncss is dccrcascd inkcralomilcusis donc for corr·cctionof myopia:So r2 is reduced by 0.004095 nUll/or each tlioptre of myopic correctioll.2) NON LAMELLAR REFRACTIVE KERATOPLASTY

(A)REFRACTIVE SURGERY FOR MYOPIA:Radial keratotomy::ic Definition: Multiple radial deep incisions (initially 16, now 8 or 4) are made in1e the cornea between the periphery and its optical zone, almost down

to the level of Descemet's membrane (Fig. 32.5).Ie Principle: Multiple deep radial peripheral incisions lead to flattening of the)f central corneal curvature.

Indication: Stable myopia of -20 to -5D only.

Fig.32.5:Radial keratotomy. Fig.32.6:Corneal wcdge rcsection. Fig.32.7:Two arcuate incisions.(B)REFRACTIVE SURGERY FOR ASTIGMATISM:

Indication: This is usually done for correction of postoperative astigmatism (aftercataract extraction or keratoolastv).- - .

Procedures:(l)To steepen the flatter corneal meridian (astigmatic keratectomy):

J) Corneal wedge resection (Fig.32.6):DefinitiOl :A crescentic wedge shaped area is resected from the cornea (at

the healed keratoplasty incision centred across axis of flatcorneal meridian to become steeper.

Principle: It steepens the flatter corneal meridian (each 0.1 mm resectedresults In ID of steepening).

Indication: igh degree of postoperative astigmatism after healed·keratoplasty incision.

2) Wound revision by block resection and resuture:Definition: Block resection of thinned misaligned tissue of the cataract

wound and then the new wound is resutured in its full thicknesswith monofilament nylon suture.

Principle: It steepens the flatter corneal meridian.fndicatio : High degree of postoperative astigmatism against the rule atter

ectatic superior limbal cataract wound.(2)Tojlatten the steeper corneal meridian:

1) Selective suture cutting or removal:Dejllllfion Suture cutting or removal after 8 weeks for cataract wounds

and 12 weeks for corneal incision (not earlier to avoid an opposite error).Principle: It flattens the steeper corneal meridian (the rule is to remove

the sutures in the axis of the plus cylinder usually at 120 'clockmeridian in postoperative astigmatism with the rule).

fndication:Postop, rative astigmatism with rule resulted from tight sutures.NB:ln astigmatism against the rule, suture cutting or removal will almost always

increase the error: So it is best to leave all sutures in place to support thewoundfor long time.

2) Astigmatic keratotomy:,1-Arcuate -ircu m eren ia re axing inCISIOns

: One or two deep arcuate corneal incisions (Fig.32.7): ••(a)Single arcuate incision: Across the axis of steeper

corneal meridian.(b)Two arcuate incisions: At opposite poles of steeper

corneal meridian.1 n lp e: To flatten the steeper corneal meridian.

n ication: High degree of postoperative astigmatism:(a)A fier healed cataract incision (single arcuate incision).(b)After healed keratoplasty incision (2 arcuate incisions).

NB:Two compression sutures arc phlccd across the keratoplasty scar at 90" tothe stccper meridian (Fig. 32.8) : To induce a temporary overcorrection bygaping the two arcllate relaxillg illcisiol1s to prevent early apposition.

2-TraRczOId(sfe adder' incisions:e l17lfLOn: Two groups of transverse and radial incisions which are

separated with a Clear central zone (Fig.32.9):(a) ! Wr) scnes ( r. e . re made. perpendicular to and centred on opposing poles of the

steep corneal meridian.(b) Wv ra ia inciswns: Are made to each side of each set

of transverse incisions, but not intersecting them ..(c) (;,'ar cpn{rat ZOJ!L': Of more than 3 myn separates the

2 groups of incisions (to avoid induced glare andirregular astigmatism).

(C)R1(1

rmci I : To flatten the steeper corneal meridian.n ication: Postoperative astigmatism after healed cataract or

keratoplasty incision (as in arcuate relaxing incisions).

Fig.32.8:Compression sutures. Fig.32.9:Trapczoid incisions. Fig.32.10:Radial-T incisions.3- Sector radial incisions with or without perpendicular T-cuts:

e mition Sector radial incisions are made with or withoutperpendicular T-cuts (a slight off-set of the T-cuts is made toavoid poor healing, Fig.32.1 0).

rmci e. To flatten the steeper corneal meridian.n lca/wn Small degree of astigmatism (with surgery of myopia).

(C) REFRACTIVE SURGERY FOR PRESBYOPIA:(I )Scleral surgery:

1)Scleral expansion surgery (using silicone bands):Separate injection-moulded polymethylmethacrylate (PMMA ) segments were placed in partial-thickness scleral pockets, in each of the four oblique quadrants of the eye.

2)Anterior ciliary sclerostomy(ACS):Principle: Radial cuts in the sclera overlying the ciliary bodY-'7allowsclera

to extend and increase the space between the crystalline lens andthe ciliary bbdY-7s0pull the ciliary muscle and restore theaccommodation tone.

Procedure: I-It involves 8 equally placed radial incisions of the conjunctivaand sclera owerlying the ciliary body in each of the 4 obliquequadrants (full-thickness sclerotomies enhance surgical effect).

2-Limbal peritomies owerlying the 4 oblique quadrants:(a)To measure the length and depthofthe incisions.(b)To lessen bleeding.

3-Ultrasonic biomicroscopy(UBM):(a)To measure the depth of the incisions,

(b)To set the blade depth prior to incision of the sclera.4-Silicone scleral expansion plugs:May be added for:

(a)Keep the incision opened.(b)Ciliochoroidal detachment with increased uveoscleral outflow.

(2)Conductive keratoplasty(CK):ls a non-ablative, laserlss technique:Indication I:1) Emmctropic prcsbyopcs(who need only reading glasses).

2) Hypermetropic presbyopes(needs reading and distance glasses).Aim: or one eye only to improve near vision and preserves binocular vision.

Procedur : 1)Low frequency energy(0.6 watts for 0.6 sec. for each application)is applied to corneal stroma by a probe tip inserted into peripheralcornea at 8-32 treatment points->heating of corneal stroma to 65° C(optimal temperature of collagen shrinkage).

2)A full circle of CK spots are applied to the midperiphery cornealperipheral flattening and central steepening, preserving thecorneal tissue of the visual axis-7increase the focal power of theeye, bringing near objects into better focus.

(3)Accommodative or multifocallOL:Chapter 19.(D)THERMOKERATOPLASTY FOR HYPERMETROPIA:

Definition: Appropriately sited corneal coagulations:(l)Refractive thermokeratoplasty: By a deep hot needle.(2) Photorefractive thermokeratoplasty: By holmium- YAG or diode la,ier.

Principle: Steepening of the central corneal curvature.Indication: Low hyp _rrnetropia.

(E) PENETRATING KERA'[OPLASTY.(2) PHOTOR[~FRACTiVg SURG ERY (CHAPTER 26)

(A)EXIMER LASER:Chapter 26:(1)Photorefrac .ive ~erate . omy:

Definition Ablation of concentric areas of the superficial stroma of the centralcornea by excimer laser.

Principle flattening ofthe central corneal curvature (as in radial keratotomy).Indication: Stable myopia of -2D to - 5D only (as in radial keratotomy).

(2)Photorefractive kerato ni eusis(excimer laser keratomileusis In situ,LASIK):Definition' Ablation of the corneal tissue of the corneal bed by ex imer laser.

Principle. (1 )Flattening of the central corneal curvature in myopia.(2)Ablation of steeper axis of cornea to in astig natic keratectomy.

Indications: (l)High myooia up to -15 D.{2)Astigmatism.

(B)DIODE LASER: Paracentral coagulation spots to steepen the flatter axis at thecentral comca(Chapter 26).