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