Prism

1. PRISM
Dr.Shazia Gul
KMU-IHS Kohat

2. Definition
“A prism is defined as a portion of refracting medium
bordered by two plane surfaces that are inclined at a finite
angle”
apex
apical angle
axis
base

3. The angle α between the two surfaces is called the
refracting angle or apical angle of the prism
A line bisecting the angle is called the axis of the prism
The opposite surface is called the base of the prism
apex
apical angle
axis
Base

4. Refraction by the prism
When light passes through a prism, it obeys Snell’s law at
each surface. So, the ray of light is deviated towards the
base of the prism.
The ray coming is called as ‘incident ray’ and the ray
deviated is called as ‘emergent ray’.

5. Passage of light via prism
Apical angle Angle of
deviation
Angle of Incident Angle of emergence
Incident ray
Emergent ray

6. Angle of deviation
When a light ray passes via prism, it deviates and
changes its direction. Thus the net change in direction
of the ray is called as “angle of deviation (D)”.
(OR)
The sum of the deviations produced at each of the
surfaces of the prism.

7. Determination of “angle of deviation”
The angle of deviation for a prism in the air, is
determined by the following three factors:
The refractive index of the material of which the
prism is made
The refracting or apical angle of prism
The angle of incidence of the ray considered

8. For any particular prism, the angle of deviation D is least
when the angle of incidence equals the angle of
emergence. Refraction is then said to be symmetrical and
the angle is called the angle of minimum deviation.
Under these conditions the angle of deviation is given by
the formula:
D = (n-1) α
For a glass prism of refractive index 1.5
D = (1.5 – 1) α
D = (0.5) α or (1/2) α
D = α/2
The angle of deviation equals half the refracting angle for a
glass prism.

9. Image formation
The image formed by prism is
 Erect
 Virtual
 Displaced towards the apex of the prism.

10. Image formation by prism
Image
object

11. Prism on the basis of position
There are two primary positions in which the power of a
prism may be specified:
1) The position of minimum deviation
2) Prentice position of prism

12. Position of minimum deviation
When the angle of incidence equals the angle of emergence, refraction is
then said to be symmetrical and it is called the position of minimum
deviation.
Apical angle Angle
of deviation
Angle of Incident Angle
of emergence
Incident ray
Emergent ray

13. The power in the position of minimum deviation which is
specified for plastic ophthalmic prisms:
e.g. prism bars

14. Prentice position
In the Prentice position one surface of the prism is normal to the ray of
light so that all the deviation takes place at the other surface of the
prism.
Angle of incident
(90)
Angle of
deviation
Incident ray Emergent ray

15. The deviation of light in the prentice position is greater than
that in the position of minimum deviation, because in the
prentice position the angle of incidence does not equal the
angle of emergence. Therefore the prentice position power
of any prism is greater than its power in the position of
minimum deviation.
It is the prentice position power which is normally specified
for glass ophthalmic prisms:
e.g. trial lens prisms

16. Notation of prism
The power of any prism can be expressed in various units
e.g
Prism dioptre
Angle of apparent deviation
Centrad
Refracting angle

17. Prism dioptre ( )
A prism of one prism dioptre power (1 ) produces a linear
apparent displacement of 1cm, of an object O, situated at
1m.
Angle of Apparent Deviation
The apparent displacement of the object O can also be
measured in terms of the angle θ, the angle of apparent
deviation. Under conditions of ophthalmic usage a prism of
1 prism dioptre power produces an angle of apparent
deviation of 1/2°. Thus 1 prism dioptre = 1/2°.

18. THE PRISM DIOPTRE AND ANGLE OF APPARENT DEVIATION
1
1cm ½ θ
1m
Displacement : 1cm in form of line

19. THE CENTRAD ( )
The centrad differs from the prism dioptre only in that the image
displacement is measured along an arc 1m from the prism.
1
1cm
1m
Displacement : 1cm in form of ‘arc’

20. Refracting Angle
A prism may also be described by its refracting angle
However, unless the refractive index of the prism material
is also known, the prism power cannot be deduced.
Summary of Prism Units
Thus a glass prism of refracting angle 10° (a ten-degree
prism) deviates light through 5° and has a power of 10
prism dioptres (10∆), assuming its refractive index is 1.5.

21. USES OF PRISMS
The main uses of prisms are :
Prisms for diagnostic purposes
Prisms for therapeutic purposes

22. Diagnostic uses:
( 1 )Assessment of squint and Heterophoria :
Prism cover test ( PCT )
For measurement of angle objectively
Maddox rod test
For measurement of angle subjectively

23.  Assessment of diplopia
To assess likelihood of diplopia after proposed squint
surgery in adults
 Fusional reserve test
For 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 binocular
single vision in children under 2 years of age.

24. Four-dioptre prism test
This is a delicate test for small degrees of esotropia
( microtropia )
In this test a four dioptre prism placed base-out before
the deviating eye causes no movement as the image
remains within the suppression scotoma. When placed
before the normal ( fixing ) eye, movement occurs.

25. ( 2 ) Assessment of simulated blindness
For assessment of simulated blindness, a prism is placed
in front of a seeing eye, the eye will move to regain
fixation

26. Forms of Prism Used in Diagnosis
Forms of prism used in assessment include:
 Single unmounted prisms
 The prisms from the trial lens set
 Prism bars
These are bars composed of adjacent prisms of increasing
power.

27. Single unmounted prisms
The prisms from the trial lens set
Prism bar

28. Therapeutic uses of Prism :
Therapeutically prisms are used for:
Convergence insufficiency
To relieve diplopia in squint

29. ( 1 )…Convergence insufficiency
The commonest therapeutic use of prisms in orthoptics is in
building up the fusional reserve of patients with
convergence insufficiency.
The prisms are used base-out during the patients' exercise
periods.

30. ( 2 )…To relieve diplopia in squint
In certain cases of squint, to relieve diplopia , include:
 Decompensated heterophorias
 Small vertical squints
 Some paralytic squints with diplopia in the primary
position
Prisms are reserved for those patients for whom surgery is
not indicated.

31. Forms of Therapeutic Prism
(1) Temporary wear
Prisms used in treatment include clip-on spectacle prisms for
trial wear. An improvement on these are Fresnel prisms,
which are available in all powers employed clinically.
A Fresnel prism consists of a plastic sheet of parallel tiny
prisms of identical refracting angle. The overall prismatic
effect is the same as that of a single large prism. The
sheets are lighter than a glass prism and can be stuck on
to the patient's glasses.

32. Fresnel prism

33. Fresnel prism

34. (2) Permanent wear
Permanent incorporation of a prism into a patient's
spectacles can be achieved:
1. By decentration the spherical lens already present
2. By grounding prism in the spectacles

35. Decentration
Decentration of the spectacle lenses
When the optical center of the lens and the visual axis are
not align, is termed as decentration.

36. Why we decentered the lens?
When the lens is decentered prismatic effect is produced
The prismatic effect is proportional to the decentration and the
power of the lens.
Prismatic effect is:
P = F * D
P= is the prismatic power in prism dioptres
F= is the lens power in dioptres
D= is the decentration in centimetres

37. Example:
OD = +10DS
OS = +8 DS
The lens is temporally displaced 5mm, the prismatic effect?
Sol:
OD OS
P = F * D P = F * D
P = 10 * 5mm P = 8 * 0.5
(1cm = 10mm) P = 4 D
(5/10 = 0.5 cm) Base out
P = 10 * 0.5
P = 5 D
Base out

38. Prescription of prism :
When prescribing prisms the orientation is indicated by
the position of the base
e.g. “base-in, base-out, base-down, and base-up etc.
Generally, when prescribing prisms, the correction
is split between the two eyes.
To correct convergence the prisms must be base-out
To correct divergence the prisms must be base -in

39. Base-in Prism
To achieve Base-in prism (Rt Eye)
 the lens is decentered temporally
( in case of minus lenses)
and
 the lens is decentered nasally
( in case of plus lenses)

40. Base-out prism
• To achieve base-out prism (Rt Eye)
lens is decentered temporally
( in case of plus lenses)
and
lens is decentered nasally
( in case of minus lenses)

41. Cont :
To correct vertical deviation the orientation
of the prisms is opposite for the two eyes.
e.g
Base-down
( right eye )
Base-up
( left eye for right hypertropia )

42. Determination of prism
• How we can determine either the prism is due to
decentration of lens?
Or
the prism is grounded in the glass?
 it can be determine by the lensmeter

43. It can achieved simply by locating the optical center by
moving the lens about until the lensmeter target is
centered.
If the greatest distance from the optical center to the
edge of the lens is more than half the diameter of the
usual kens blank size ( 60 mm). Prism must have been
ground into the lens.

44. Prisms used in optical instruments
Instruments in which prisms are used are:
Slit lamp microscope
Applanation tonometer
and
Keratometer.

45. Prisms are used in instrument due to some
characteristics:
Prisms are commonly used in ophthalmic
instruments as “reflectors of light”.
The prism is designed and orientated so that
total internal reflection occurs within it.
It can also be seen that prisms give greater
flexibility in dealing with an image than do
mirrors.

46. Types of Prisms
Dispersive prisms
 Reflective prisms
 Polarizing prisms

47. Dispersive prisms
Dispersive prisms are used to break up light into its
constituent spectral colors because the refractive index
depends on frequency, the white light entering the prism is
a mixture of different frequencies, each of which gets bent
slightly differently. Blue light is slowed down more than red
light and will therefore be bent more than red light.

48. Reflective prisms
Reflective prisms are used to reflect light, It works on total
internal reflection.

49. Types of reflective prism
• Pentaprism
• Right angle prism
• Porro prism
• Porro-Abbe prism
• Abbe-Koenig prism
• Schmidt-Pechan prism
• Dove prism
• Dichroic prism
• Amici roof prism

50. Polarizing prisms
There are also polarizing prisms which can split a beam of
light into components of varying polarization.

51. Types of polarizing prisms
• Nicol prism
• Wollaston prism
• Rochon prism
• Glan-Foucault prism
• Glan-Taylor prism
• Glan-Thompson prism