Prisms are used in optometry to deviate light and relieve diplopia. A prism consists of two angled refracting surfaces that obey Snell's law to deviate light towards the base. Prism power is measured in prism diopters based on the amount of deviation produced at 100cm. Prisms can be prescribed to relieve diplopia in cases of strabismus or paralytic squint. The type and amount of prism prescribed depends on whether it is being used for correction, relief of demand on vergence, or strengthening weak eye muscles.

2. GHAZANFAR IKRAM
Optometry Student

3.  Prism is a portion of a refractive medium,
bordered by 2 plane surfaces which are
inclined at a finite angle.
 A prism consists of two angled refracting
surfaces
 The simplest form of prism is two flat
surfaces coming together at an angle at the
top.

5.  Obeying Snell's Law of refraction light
passing through the prism is deviated to the
base
 Angle of deviation - D = (n-1)α ;
 where n- Refractive index, α – refractive angle
 Thus, D = α/2
 RI of glass –> 1.5

6.  Image formed through prism is virtual, erect
and displaced towards the apex.

7.  Prism power is defined by the amount of
deviation produced as the light ray traverses
the prism.
 The deviation is measured as the numbers of
centimeters of deflection measured at a
distance of 100cm and expressed in prism
diopters.

8.  Angular Units:
 The deviation produced by the prism is
 expressed in degrees or radians.
 Prism Dioptre:
 One prism dioptre produces a deviation of
one unit at a plane 100 units from the prism.
 Centrad:
 One centrad produces a deviation of one unit
of arc at a distance of 100 units from the
prism.

9.  1)Ground prism’s:
 Permanently incorporating the prism into
glasses by decentring the present spherical lens
or by mounting on them.
 Limit 8-10 prism D in each eye or 12
 2) Fresnel prism (or) Wafer prisms:
 Temporary
 Series of plastic Prisms of 1mm.
 Small prisms stacked to give an effect of a large
prism to over come wt.,aberration, and achieve
higher power.
 1-30 prism D in each eye

10.  Thin Prism:
 Thin prism have no dioptric power but can be
combined with dioptric lenses in a refractive
correction.
 They have an apical angle of less than 10º to
15.
 Ophthalmic prisms are, generally, thin
prisms.

11.  Thick Prism:
 A prism whose apical angle is greater than
15º to 20º.
 White light incident on a thick prism will
appear to be dispersed into the colour
spectrum when emergent from the second
surface.
 A prism displaying this phenomenon is often
known as a Newton prism.
 Dispersion is usually seen in thick prism

12.  The orientation of the prism in front of the
eye will affect the position at which the eye
perceives any object to be viewed through
the prism.
 It is, therefore, important to specify
accurately the orientation of the prism so
that its effect on the eyes is known when
incorporated into a refractive correction.

13.  When prescribing prism it is, of
course,necessary to indicate the direction of
the prism base.
 While most cases will involve prism in one of
the four main directions, up,down, in and out,
oblique prism may also be ordered.

14.  There are two accepted methods for
indicating the direction of an oblique prism.
 a)standard notation
 b)360 degree notation

15.  Standard Notation:
 This is the same axis notation as used for the
 axis of astigmatic lenses.
 This notation requires further indication of the
direction of the base.
 For example, it is not sufficient to order RE: 4
diopter at 135.This could be either up and out at
135 or down and in.
 So the prescription needs the direction as well as
the angle.

16.  B)360° notation:
 This system of notation is the same as standard
notation in the top two quadrants but continues
to 360 °in the bottom quadrants.
 This system requires no other notation that the
angle.
 So, RE: 4 diopter prism at 135 would mean up
and out, there is no other possibility since down
and in would be RE: 4 diopter prism at 315.

18.  2)To relieve diplopia in certain cases of
squint:
 Including decompensated heterophoria,small
vertical squint and some paralytic squint with
diplopia in primary position.

20.  Due to the difference in thickness between
the base and the apex of a prism, refractive
corrections incorporating prism power for
one eye only, the spectacles may be
dispensed with the prism power split
between the two eyes.
 This is usually due to a noticeable and
cosmetically unacceptable difference in
thickness between the spectacle lenses if
they were made up as prescribed.

21.  It is important that the effect on the eyes as a
pair is maintained when the prism power is
split between the spectacle lenses.
 This can be achieved using the following
rules.
1) If the prismatic power is prescribed
monocularly in a refractive correction that is
similar between the two eyes, the prismatic
power should be split evenly.

22.  2) Prism power with horizontal base direction
should have the same base direction in both
eyes.
 3) Prism power with a vertical direction
should have opposite base directions in each
eye, with the base direction for the eye in
which the prism was originally prescribed
remaining the same.

23.  The following correction is prescribed:
 R Plano 3 PD UP 4 PD IN
 L Plano
 The two prisms could be compounded
i.e.replaced by a single oblique prism.
 The resultant prism would be positioned with its
base between the base directions of the two
prescribed prisms.
 The exact orientation of the single resultant
prism is determined by the power of the two
prescribed prisms.

24. V
O
H
R
The exact position of the
resultant prism can be
determined using Pythagoras’
Theorem:
=> (OR) 2 = (OV) 2 + (OH) 2
=>(OR)2 =( 3)2 + (4)2 = 25
=>OR = 5
tan (ROH)= 3/4
Angle ROH = tan-1(3/4) = 36.87º
The resultant prism power is
5PD orientated at 37 degree.
So R Plano 5 prism dioptr UP @
37 L Plano

26.  Relieving
 Corrective
 Inverse
 Yolked
 Regional

27.  Optically reduces the demand to the
controlling fusional vergence system for
bifixation of the target.
 Prescription is less than the angle of deviation
 It moves the image to the range of fusion
 Rxed for intermittent strabismus and
symptomatic heterophorias.
 The most commonly Rxed prism

28.  Optically eliminates the oculomotor
deviation’
 Rx prism is equal to the magnitude of
deviation.
 Apex toward deviation.

29.  You put the base in the same direction as the
deviation and optically increases the demand
to the controlling vergence system.
 Strengthning weak muscle and prevent
contracture of antagonist muscle
 Eso---BI
 Exo---BO

30.  Move both eyes in the same direction
 For example Base right prism
(BO RE and BI LE)
 More used in Hemianopias
 Also used in paretic muscle and in nystagmus

32.  Different amount of prisms are needed in
different field of gaze or for different
distances.
 Prism is placed on a particular part of lens
where you prescribe.