prisms
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prismsTRANSCRIPT
PRISMS & ITS OPHTHALMIC USES
HappyNew Year2014 PRISMS & ITS OPHTHALMIC USES Dr. Rutvi R. SadatiaPrismDefinition It is a piece of glass bounded by plane surfaces inclined towards each other.
REFRACTING/APICAL ANGLE OF THE PRISMThe angle between the two surfaces
AXIS OF THE PRISM A line bisecting the angle
APEXThe thin edge where the intersecting surfaces meet
BASEThe opposite surface Refraction by a prismLight passing through a prism obeys Snells law at each surface.
Rays of light passing through prism are bent towards the base.
Angle of deviation (D)The net change in direction of the ray is called angle of deviation
Determined by 3 factors
1.the refractive index of the material of which the prism is made2.the refractive angle of the prism3.the angle of incidence of the raysAngle of minimum deviationAngle of deviation is least when the angle of incidence equals the angle of emergence
The angle of deviation equals half the refracting angle of the prism
Image formationIf we turn the light around, making the image the object, and view the object through the prism, we will see a virtual image of the object.
The object being viewed through the prism appears displaced toward the apex of the prism.
Although the light rays themselves bent toward the base
The image formed by a prism is erect,virtual & displaced towards the apex of the prism
If a prism is introduced into the path of convergent light, all the light rays are bent towards the base of the prism, and the image is also displaced toward the base of the prism
In this case, the image is real, and real images are displaced toward the base of a prism
Positions 2 primary positions
The frontal position---plastic prism that is parallel to the infraorbital margin
The prentice position---glass prism the posterior face of the prism is perpendicular to the line of sight
NotationPrism Diopter
Prism power defines the amount of light-ray deviation produced as the light ray traverses a prism.
One prism dioptre power produces a linear apparent displacement of 1 cm,of an object,situated at 1 m.Angle of apparent deviation
The apparent displacement of the object measured in terms of the angle of apparent deviation
2=1
3. The centrad
This unit differs from the prism dioptre only in that the image displacement is measured along an arc 1 m from the prism
The centrad produces a very slightly greater angle of deviation than the prism dioptre
But the difference in practice is negligibleUses 1.Diagnostic
2.therapeutic
3.Instruments
4.MiscellaneousDiagnostic 1.Assessment of squint & heterophoria
Measurement of angle objectively by prism cover test,prism reflex test of Krimsky
Measurement of angle subjectively by maddox rod
To assess likelihood of diplopia after proposed squint surgery in adults
Measurement of fusional reserve
4 D base out test
Forms of diagnostic prismsSingle unmounted prismsTrial lens set prismsPrism bars
Prism bar cover test
Krimskys test AC, Prisms, baseout, of increasing power are placed beforethe fixating eye until the light reflex is centered on the cornea of the deviating eye. D, Optical principles of the prism reflex test
Maddox rod testA, Maddox rod in testing position for horizontal heterophoria. B, Patient sees the line going through the light: no horizontal phoria is present. C, The line is seen to the left of the light (crossed diplopia): exophoria. Add prisms, base-in, to OD until the line is centered on the light. The power of the prism is read and equals the amount of phoria. D, The line is seen to the right of the light (uncrossed diplopia): esophoria. Add prisms, base-out, to OD until the line is centered.
A, Maddox rod in testing position for vertical phoria. B, No vertical phoria is present. C, Right hypophoria (usually left hyperphoria also). Add prisms, base-down, to OS until the line is centered. D, Right hyperphoria. Add prisms, base-up, to OS until the line is centered.
Measurement of fusional reserve
Increasingly powerful prisms are placed before one eye until fusion breaks down.This is very useful in assessing the presence of BSV in children under 2 years old
Vergence with prismsDistance (6m) in D Near(33 cm) in D Convergence 14-2035-40Divergence 5-815-20Vertical vergence2-42-44 D base out test
Test for microtropia4 D placed base out before the deviating eye causes no movement as the image remains within the suppression scotomaWhen placed before the normal eye,movement occurs
Therapeutic1.Convergence insufficiency
2.To relieve diplopiaConvergence insufficiencyThe commonest use of prisms in the orthoptic department
Done on synaptophore
Pt.s eyes are exposed to fusion slide pictures and pt is made to fuse them by gradually moving the arms of synaptophore by which power of base out prisms is gradually increasedExercise may be continued at home
Pt is given 4 D with which to practice for a week & augmenting by 2 prisms at weekly intervals
Exercise may be done for distance & nearTo relieve diplopiaIn cases of decompanseted heterophorias,small vertical squints & some paralytic squint in the primary position
Prisms are reserved for those pts for whom surgery is not indicatedForms of therapeutic prisms1.Temporary wear
Clip on spectacle prisms for trial wear
Fresnel prisms:
It is a series of small side-by-side prisms that act as a single large prism. It is typically used to avoid the weight and some of the aberrations of conventional prisms.
The Origin of Fresnel Lenses
The French physicist, Augusti-Jean Fresnel is credited with the invention of the concept of this form of lens. Their initial use was primarily to replace the very thick lenses used in lighthouses. To obtainlenses powerful enough for lighthouses, massive heavy lenses were required. By designing the lens as a series of small segments, rather than one large lens, the great weight could be removed.
The first Fresnel lens was placed in the Cordouan Lighthouse at the mouth of the Gironde estuary in 1823. Augusti-Jean Fresnel lived only to age 39, but did important work in diffraction opticsand polarization.
The most popular form of Fresnel prism is a membrane molded from clear polyvinyl chloride. Known as a Press-On prism
it is applied with water to the back surface of an ordinary spectacle lens. Press-On prisms are available in a variety of powers. Visual acuity is reduced because of light scattering at the groove edges, but the chromatic aberration of the prism themselves produces most of the visual decrement.
The advantages of these prisms far outweigh the disadvantages, and they are widely used in the fields of strabismus and orthoptics
Because of their ease of application and lower expense
Press-On prisms are especially useful for patients whose strabismus is changing (eg, patients with thyroid eye disease)
Fresnel lenses are also available with concentric groove construction to approximate spherical lenses
Prism AberrationsChromatic aberration produces colored fringes at the edges of objects
Asymmetrical magnification and curvature of field
2. permanent wear
Permanent incorporation of a prism into a pts spectacles can be achieved by decentring the spherical lens.
Rays of light incident upon a lens outside its axial zone are deviated towards (convex lens) or away from(concave lens) the axis.Thus the peripheral portion of the lens acts as a prism.
The refracting angle between the lens surface grows larger as the edge of the lens is approached.Thus the prismatic effect increases towards the periphery of the lens.Use of non axial portion of a lens to gain a prismatic effect is called decentration of the lens
P=FDP= the prismatic power in prism diaoptresF=the lens power in dioptresD=the decentration in cm Priscription of prismsThe correction is split between the 2 eyes
To correct
convergence-base outDivergence-base inHypertropia-base downHypotropia-base upInstruments1. Applanation tonometer2. Indirect ophthalmoscope3. Slit lamp microscope4. Keratometer5. Operating microscope6. Synaptophore7. Haidingers brush8. Koppes goniolens
Forms of intrumental prisms1. Right angle prismDeviation 90
2. Porro prismDeviation 180Image inverted but not transposed left to right
3. Dove prismNo deviationImage inverted but not laterally transposedMiscellaneous1. recumbent spectaclesTo allow a pt lying in bed to keep the reading matter on his chest
2.hemianopic spectaclesA prism of 8 dioptre is used with base towards the blind side
3.low visual aidsBase in prisms are incorporated in binocular magnifierReferences Clinical optics-Andrew R Elkington & Helena J FrankAmerican academic of ophthalmology 2012-2013Strabismus simplified-pradeep sharmaRefraction & optics- N C Singhal