basic optics refractive errors

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7/28/2019 BASIC OPTICS Refractive Errors

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Brian Lorimer COMT,RO

BASIC OPTICSRefractive Errors



T HE H UMAN

E Y E



BASIC OPTICS

• Optics is the properties of light as it isacted on by optical systems

• Geometric optics: uses line diagrams todepict the behavior of light

• Wavefront optics: uses principles of

waves to describe the behavior of light



LIGHT AS WAVES



VERGENCE

Parallel Beam Convergence Divergence



VERGENCE

• All naturally occurring sources of light are divergent• Light rays traveling parallel have zero vergence

• Light rays that focus on a point are convergent

• The unit of measurement of vergence is the diopter

1

D= Vergence (Diopters)= ________________________

Distance from the source in meters



BASIC OPTICS

• Therefore:

• The closer the light is to its source, the

greater the vergence

• The farther the light is from its source,

the lower the vergence, approachingzero as d goes toward infinity



BASIC OPTICS

• Diverging light has negative power (-)

• Converging light has positive power (+)



REFRACTION• Refraction of light occurs when light

passes from one medium to another of different refractive index (ie. density)



Refractive Componentsof the Eye

• Cornea: responsible for the majority of therefractive power of the eye (40 D)

• Lens: 20 D of refractive power, changeswith accommodation

• Axial length



NORMAL EYE(EMMETROPIA)



REFRACTIVE

PHYSIOLOGY

• Light rays are focused on the retinabecause they are refracted by passing

through the cornea and lens (Snell’s Law)

• Corneal refractive power is constant

• Lens refractive power is modifiable with

accommodation

• Axial length of the eye is constant except

under certain conditions



REFRACTIVE ERRORS

• Ametropia: a refractive error is present

• Myopia: Near sightedness• Hyperopia(Hypermetropia): Far sightedness

• Presbyopia: Loss of accommodativeability of the lens resulting in difficultieswith near tasks

• Astigmatism: the curvature of the corneaand/or lens is not spherical and therefore

causes image blur on the retina



REFRACTIVE ERRORS

• Anisometropia: a refractive power differencebetween the 2 eyes (> 2D)

• Aniseikonia: a difference of image sizebetween the 2 eyes as perceived by the

patient

• Aphakia: (Phakos=lens), aphakia is no lens

• Pseudophakia: artificial lens in the eye



RETINOSCOPE



Refraction

• Refraction is about identifing the far point in the eye and moving it around,

usually to the fovea in the macula, but

could be moved, just in front of the

macular leaving a little myopia left for

reading



What is the Far Point

• The farthest an eye can see• An emmetrope it is infinity or 20 feet

• 4.00D myope without glasses it is 25cmin front of the eye

• 1.00D myope without glasses it is

1meter in front of the eye• 3.00D hyperope without glasses 33.3

cm behind the eye



What is the Near Point

• The closest the eye can see• The near point is the sum of the power

of the eye and accommodation• An emmetrope with 10.00D of

accommodation and a -4.00D error with

out glasses has a near point of 14.00Dor 7.15cm in front of the eye




• A 50 year old with 2.00D of accommodation and a -1.00D error

without glasses has a far point of 1

meter and a near point of 33.3 cm

• With glasses on the far point is infinity

or 20 feet and the near point is 50cm infront of the eye




• A +3.00D without glasses hyperope with6.00D of accommodation has a far point

of 33.3 cm behind the eye and a far

point of 33.3 cm in front of the eye

• With glasses on the far point is infinity

and the near point 16.6 cm



Remember

• A myope has too much plus in their eyes

• Whereas a hyperope has too muchminus



AGAINST MOTION

(retinoscopic reflex)



WITH MOTION

(retinoscopic reflex)



MYOPIA (Near Sightedness)



MYOPIA – WHY?

• The eye’s axial length may be too long:

axial myopia (most common form)

• Increase in the curvature of the cornea or the surfaces of the crystalline lens

• The eye’s refractive power may be toostrong (cornea and lens): refractive

myopia



MYOPIA - SYMPTOMS

• Typically do not have “eye-strain”,“watering” of the eyes or headaches as

often as hypermetropes do

• Usually detected by the young when theydiscover they cannot see things at a

distance as well as their friends do

• The teacher complains that the child

makes too many mistakes copying things

from the black-board



MYOPIA

• Increases with age roughly until the

person stops growing in height



MYOPIA• A myopic person can still see some objects

clearly, provided the object is closer than thefar point

• For a -2 D myope, the far point is 0.5 meters(D=1/distance in meters), so any objectsinside 0.5 m are clear as long as they are nottoo close at which point clarity may belimited by accommodation

• Pseudomyopia: accomodative spasm. Thepatient cannot relax accomodation whenlooking in the distance. For example, an over anxious student



MYOPIA

NORMAL VISION

UNCORRECTED

MYOPIA

CORRECTED

MYOPIA



PATHOLOGICAL CAUSESOF MYOPIA

KERATOCONUS



PATHOLOGICAL CAUSES OF

MYOPIA

CATARACT DIABETIES



PATHOLOGICAL CAUSESOF MYOPIA

MARFAN’S STAPHYLOMA



CORRECTION OF

MYOPIA

• Glasses (minus lenses)

• Contact lenses

• Corneal surgery (PRK/ EPI-LASIK

LASIK/ LASEK)

• Lens surgery (IOL / clear lens

extraction)



HYPEROPIA (farsighted)



HYPEROPIA – WHY?

• The length of the eyeball is shorter thanit should be

• Hyperopia forms a stage in normal

development of the eyes—at birth eyesare hypermetropic (2.5 to 3.0 Diopters)

• When persists in adulthood it representsan imperfectly developed eye

• Lens changes (cataract)



HYPEROPIA - SYMPTOMS

• “Eye-strain”(ciliary muscle is straining tomaintain accommodation) / “watering” /“redness”

• Headaches in later part of the day• Complain of blurring of text specially after

some time

• Young children with significanthypermetropia can also develop aconvergent squint



HYPEROPIA• Hyperopia may be partially

compensated for by using the eyes’accommodative ability

• When accommodative ability cannotkeep up with demand hyperopia is

manifest and images are blurred in the

distance and for near



NORMAL VISION

UNCORRECTED

HYPEROPIA

LENS CORRECTED

HYPEROPIA



PATHOLOGICAL CAUSES OF

HYPEROPIA

DISLOCATED LENS

PATHOLOGICAL CAUSES



PATHOLOGICAL CAUSES

OF HYPEROPIA

RETINAL DETACHMENT

RETINAL FLUID

CHOROIDAL TUMOR

1 mm = 3D



CORRECTION OF HYPEROPIA

•Glasses (plus lenses)•Contact lenses

•Corneal surgery (EPI-LASIK/LASIK /PRK/LASEK)

•Lens surgery: intra ocular contact lensimplants or clear lens extraction



ACCOMMODATION



ACCOMMODATION

• Accommodation can be suspended by

the use of cycloplegic drugs

• It takes more accommodation to see at

near than in the distance



ASTIGMATISM

• The curvature of the cornea or lens mayvary in different meridians causing and

image blur on the retina

• Corneal astigmatism is most common

with the classic example of the cornea

being shaped like a football instead of abasketball



ASTIGMATISM SYSTEM



PATHOLOGICAL CAUSESOF ASTIGMATISM

• Corneal: post surgical, traumatic,infectious

• External pressure on cornea: lid masses• Lens: pressure on lens from tumors



Compound Hyperopia Compound Myopia

Simple Hyperopia Simple Myopia

TYPES OF ASTIGMATISM



Mixed Astigmatism

TYPES OF ASTIGMATISM

CORRECTION OF



CORRECTION OF

ASTIGMATISM

• Glasses (cylindrical lenses)

• Contacts ( toric soft / hard )• Corneal surgery (PRK/ Lasik

Lasek / Epi-Lasik)



PRESBYOPIA

Muscles pull and relax to change the shape of the lensinside the eye.

PRESBYOPIA



PRESBYOPIA



PRESBYOPIA

• Age 10—14D

• Age 20—10D

• Age 44__ 4D

• Age 60__ 1D

• Age 75__ 0D



EMMETROPE USING NOACCOMMODATION

Light from 40 cm

PRESBYOPIC PATIENT



PRESBYOPIC PATIENT

USING MAXIMAL

ACCOMMODATION

Light from 40 cm

2.5 D of accommodation



ACCOMMODATION NOTSUSTAINABLE

Light from 40 cm



CORRECTED PRESBYOPE

Light from 40 cm

Reading lens

( +1.25D)



PRESBYOPIA

• By the time the patient is 65 years old,essentially no accommodative ability

remains

• A reading add of 2.5 D allows the

patient to read at 40 cm ( the standard

reading distance)



PRESBYOPIA

• All patients eventually becomepresbyopic if they are corrected for

distance vision: emmetropes, myopes

and hyperopes

• A myope, however, will never need

reading glasses if they are in the -2.5range because they can simply remove

their glasses and their far point is 40cm



CORRECTION OF

PRESBYOPIA

• Glasses ( reading glasses /

progressive)• Contacts (mono vision)

• corneal (refractive—mono vision)

• Lens surgery (IOL - Multifocal )



Excimer Laser System

WHAT IS EXCIMER



WHAT IS EXCIMER

LASER SURGERY ?

• First conceived in 1983.

• Treatment to correct nearsightedness,farsightedness or astigmatism.

• May reduce or eliminate the need for contact

lenses or glasses.

HOW DOES EXCIMER



HOW DOES EXCIMER

LASER SURGERYCORRECT VISION ?

• Short pulses of ultraviolet (UV)

light remove a small amount of

tissue from the cornea to correct

the curvature.

HOW DOES EXCIMER



HOW DOES EXCIMER

LASER SURGERYCORRECT VISION ?

• The amount removed is typically lessthan the thickness of a human hair.



EFFECTS OF EXCIMER LASER

~ 0.1mm

Excimer laser ablation of human hair

Courtesy IBMCourtesy IBM

EXCIMER



EXCIMER

LASER SURGERY

Surface

•PRK - photorefractive keratectomy

•LASEK - laser epithelial keratomileusis

•EPI-LASIK - epithelial LASIK

•LASIK - laser in-situ keratomileusis



The Excimer Laser

precisely removes corneal

tissue altering the cornealcontour—now light rays

can focus on the retina

LASER SURGERY - Surface



SURGERY - LASIK



SURGERY - LASIK

HYPEROPIC LASIK



HYPEROPIC -LASIK



SURGERY – LASEK / EPI- LASIK



Thank you

basic optics refractive errors

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