- A prism refracts light, causing the object to be displaced towards the apex of the prism while maintaining the light's vergence. It produces an erect virtual image that is displaced towards the apex. - The power of a prism is measured in prism diopters, which is the angle of deviation in centimeters of light passing through the prism at a distance of 100 cm. Prisms are used to treat binocular vision disorders by compensating for phorias and tropias. - Prismatic effects can be produced by decentration of lenses. The amount of prismatic effect depends on the lens power and distance from the lens pole, as described by the Prentice rule. Prismatic effects are

1. PRISMS IN
OPHTHALMOLOGY

2.  A prism is a refracting medium having two plane surfaces
,inclined at a finite angle .
 A Prism causes light to change direction without changes its
vergence but the lens will change the vergence of light rays .

3.  Parts of prism
Apex
Base
 Apex : Intersection of refracting surfaces forms the apex of
prism. Thinnest portion of prism .
 Base of prism (opp to apex): The thick side of prism which
subtends the apical angle . Orientation of prism is described
relative to the base

4. Terminologies Used In Prism
Refracting Angle
Angle Of Deviation

5.  Refracting angle :
angle included by the two faces of the prism (β)(α)
greater the angle , more the prism will deviate light .
 Angle of deviation :
amount of light deviate from its original path , in degrees(ξ)

6. ACTION OF PRISM
 A Prism refracts the light rays towards base so the
object is displaced towards apex .
 Eye must make a movement toward apex to maintain
fixation on an object .
 Eye moves by an amount equal to angle of deviation .
 Image formed by prism is erect virtual and displaced
towards apex .

7.  Image of prism and object displaced towards apex

8.  Power of prism
denoted as prism diopter
defined as deviation of 1 unit at a distance of 100 units.
for convenience amount of deviation is considered in
centimeter (cm)

9.  1 prism diopter
Deviation of 1 cm at a distance of 100 cm .

10.  For small angled prisms
1 degree = 1.75 diopter &
1 diopter = 0.57 degree
 Ophthalmic prisms are never prescribed in power greater than
5 diopter for each eye
 Measurement of phorias and tropias involve high power
prisms.

11. Orientation of prism
 Orthoptic prism made of glass
generally caliberated according to prentice position
should be placed with rear surface perpendicular to the
deviated eye’s visual axis
 40 diopter glass prism erroneously held in frontal plane –32
diopter of effect achieved

12.  American made plastic prism & prism bars
caliberated according to the angle of minimum deviation
should be placed with rear surface in the frontal plane to
achieve the caliberated effect

13. Uses of prisms
 Binocular vision disorders
 Phorias and tropias can be compensated
 In low vision
Field enhancing channel lens
Fresnel prisms
 In various ophthalmic instruments like keratometer ,Goldman
Applanation Tonometer etc .

14. Effect of prism on movement of eyes
 Mono ocular prismatic effects
image of object is displaced towards the apex
eye moves towards the apex through an angle equal to the
angle of deviation of the prism
examples : BO moves inwards
BI moves outwards

15.  Binocular effects
when bases of prisms are placed in same direction – both eyes
moves in same direction .
Examples –BI in OD
BO in OS

16.  When the base of prism is in opposite direction – eyes moves
in opposite direction ( vergence or disconjugate movement )
 Example – Base In – Eyes moves outwards (divergence )
Base Out –Eyes moves inwards (convergence)

17. Resultant horizontal prismatic effects
 When prism make eye move in same direction –net effect is
subtractive
e.g., basein in one eye &base out in other eye
 When prism make eye move in opposite direction – net effect
is additive
e.g., base in in both eyes

18. Resultant vertical prismatic effects
 When bases of prism are in same direction (both base up or
base down )-net effect is subtractive .
 When bases of prism are in opposite direction (one base up
and other base down ) – net effect is additive .

19. Prismatic effects of lens
 Spherical lenses – considered to be
made of infinite number of prisms.
plus lens – base to base
minus lens – apex to apex
 For both lenses power of prism
increase from the pole of lens
towards periphery .

20.  Prismatic effect of lens is function of
distance from pole of lens
power of lens

21. Prismatic effect of lens :prentice rule
 Prentice rule
the prismatic effect at any point of spherical lens is
equal to the distance of point from the pole of the lens , in
centimeter, multiplied by power of the lens
 P=Df
Where p= prismatic effect at a given point
d=distance from pole of lens
F = power of lens

22.  Base in prismatic effect – induced by convex lens decenterd
nasally and concave lens decentered temporally
 Base out prismatic effect –induced by convex lens decentered
temporally and concave lens decenterd nasally

23.  When a spectacle wearer converses to read while wearing
lenses that are centered for inter pupillary distance
 plus lens wearer experiences a base out prismatic effect
 minus lens wearer experiences a base in prismatic effect
 Clinical significance : incorporation of base in prism in
spectacle magnifier

24. Prismatic effects of cylindrical lens
 Prentice rule can be applied to cylindrical lenses whose axis is
either horizontal or vertical
 Power of cylinder is 90 degrees to axis
 Prismatic effect will only be in the power meridian

25. DECENTERATION
 An ophthalmic lens is said to be centered when its optic axis
is aligned with the visual axis
 When not aligned to the visual , then the lens is said to be
decentered

26.  Prism can be induced when looking through a lens away from
the optical center .
 Prism can be induced intentionally or unintentionally

27. Prism reference point (PRP)
 The point on the lens ,as specified by manufacturer at which
the prism value of the finished lens is to be measured .
 It is also known as Major Refraction Point (MRP)
 When no prism is prescribed, PRP is the optical center .

28. Rule for decenteration
 For a lens of plus power
decenteration is in the same direction as that of the base
of the prism
 For a lens of minus power
direction is in the direction opposite to the base of the
prism

29. Prism and prosthetic eyes
 Prism prescription in front of prosthetic eye - to give more
natural and cosmetic appearance .
Example – when the angle of gaze of the right prosthetic eye is
down , base down would be oriented in OS lens ,so that
apparent image will be displaced up to match the other eye.

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