Prism and its prescription in various conditions

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.