These lectures has prepared for postgraduate student (Ophthalmology) according to the curriculum of Bangladesh College of Physician and Surgeons (BCPS) and Bangabondhu Sheikh Mujib Medical University (BSMMU) Bangladesh

2. • A prism is defined
as a portion of a
refracting medium
bordered by two
plane surfaces
which are inclined
at a finite angle.

3. Axis of the prism: A line bisecting the angle
Apex: The thin edge where the intersecting
surfaces meet
Base: The opposite surface.
Refracting/ Apical angle
of the prism: The angle
between the two surfaces

5. Prism Terminology
 Apex: Tip of the prism where the two refracting
surfaces meet. Thinnest portion of the prism
 Base
 The bottom of the prism or the side opposite the
apex or apical angle
 Thickest portion of the prism
 Orientation of an ophthalmic prism is described
relative to the base

6. Prism Terminology
 Refracting angle:
 Angle included by the two faces of the prism
(β)(α)
 Greater the angle, the more prism will deviate
light
 Angle of Deviation:
 Amount light deviates from its original path, in
degrees (ε)

7. Prism Action
a) Prism deviates light toward the base of the
prism
b) Objects appear to move toward apex of the
prism
c) Eye must make a movement toward apex to
maintain fixation on an object
d) Eye moves by an amount equal to the angle
of deviation

8. The deviation angle in a prism
depends upon:
 Refractive index of the prism: Larger the refractive
index, the larger the deviation angle.
 Angle of the prism: The larger the prism angle, the
larger the deviation angle.
 Angle of incidence: The deviation angle depends on
the angle that the beam enters the object, called angle
of incidence. The deviation angle first decreases with
increasing incidence angle, and then it increases.

10. Characteristic of prism
 A prism does not change the vergence of
the rays.
 A prism does not magnify or minify the
image.
 A prism also disperses incident pencil
rays into its component colours.

11. Image formation
 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.

12.  There are two primary positions in which
the power of a prism may be specified
a) The position of minimum deviation
b) The prentice position.

13. The deviation angle & Angle of
minimum deviation
A beam passing through an object like a prism or
water drop is deflected twice: once entering, and
again when exiting. The sum of these two
deflections is called the deviation angle.

14. A ray of light is
deflected twice in a
prism. The sum of
these deflections is
the deviation angle.
When the entrance
and exit angles are
equal, the deviation
angle of a ray
passing through a
prism will be a
minimum.

15.  Angle 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

16. The Prentice position

17. The Prentice position
The Prentice position is an orientation of a
prism, used in optics and ophthalmology. In
this position, light enters it at an angle of
90° to the first surface, so that the beam
does not refract at that surface. All the
deviation caused by the prism takes place at
the exit surface.

18. The Prentice position
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.

19. • In ophthalmology, glass prisms, e, g, trial
lens prism, were classically calibrated for
use in the Prentice position,
• while plastic prisms, i, e, prism bars were
calibrated for use in the frontal position.

20. For example, a 40 dioptre plastic prism held
in the frontal plane will have an effective
power of 41 diopters,
but if it is held in the prentice position its
effective power becomes 72 diopters.

21. Notation of prism
The power of any prism can be express in
various units.
The Prism Dioptre (∆)
A prism of one dioptre power (1∆) produces
a liner apparent displacement of 1 cm, of an
object O, situated at 1 m.

22. Notation of prism
Angle of apparent deviation:
Under condition of ophthalmic usage a
prism of 1 prism dioptre power produces an
angle of apparent deviation of ½ 0. Thus 1
prism dioptre= ½ 0
(Prism diopters in the US and degrees in Europe)

23. Notation of prism
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 negligible.

25. Use of prism
1) Diagnostic
2) Therapeutic
3) Instruments
4) Miscellaneous

26. Diagnostic use of PRISM
 Assessment of squint & heterophoria
a) Measurement of angle objectively by prism cover test
b) Measurement of angle subjectively by maddox rod
c) To assess likelihood of diplopia after proposed squint
surgery in adults.
d) Measurement of fusional reserve
e) 4 ∆D base out test

27. Measurement of angle objectively by
prism cover test

28. prism cover test: Procedure
The prism cover test measures the angle of deviation
on near or distance fixation and in any gaze position.
It combines the alternate cover test with prisms and
is performed as follows:
 The alternate cover test is first performed to
establish the direction and approximate extent of
deviation.

29. Prisms of increasing strength are placed in front of one
eye with the base opposite the direction of the
deviation
For example, in a convergent strabismus the prism is
held base-out, and in a right hypertropia, base down
before the right eye.

30. The alternate cover test is performed continuously as
stronger prisms are introduced, typically using a prism
bar consisting of a column of prisms of progressive
strength. The amplitude of the re-fixation movement
should gradually decrease as the strength of prism
approaches the extent of deviation.

31. The end-point is approached when no movement is
seen. To ensure the maximum angle is found, the prism
strength can be increased further until a movement is
observed in the opposite direction (the point of
reversal) and then reduced again to find the neutral
value; the angle of deviation is then taken from the
strength of the prism.

32. Measurement of angle subjectively by
Maddox rod
• The Maddox rod consists of a series of fused
cylindrical red glass rods that convert the appearance
of a white spot of light into a red streak. The optical
properties of the rods cause the streak of light to be
at an angle of 90° with the long axis of the rods;
when the glass rods are held horizontally, the streak
will be vertical and vice versa.

33.  The rod is placed in front of the right eye (Fig: A). This
dissociates the two eyes: the red streak seen by the right eye
cannot be fused with the unaltered white spot of light seen by
the left eye (Fig: B).
 The amount of dissociation (Fig: C) is measured by the
superimposition of the two images using prisms. The base of
the prism is placed in the position opposite to the direction of
the deviation.
 Both vertical and horizontal deviations can be measured in
this way but the test cannot differentiate a phoria from a
tropia.

34. (A) Maddox rod test;

35. (B) appearance of a point of light through the Maddox rod;

36. (C) possible results

37. • Maddox wing
• The Maddox wing dissociates the eyes for
near fixation (1/3 m)
• and measures heterophoria.

38. Maddox wing
The Maddox wing
dissociates the
eyes for near
fixation (1/3 m) and
measures heterophoria.

39. To assess likelihood of diplopia after
proposed squint surgery in adults.
Squint surgery in adult sometimes may cause
intractable diplopia, but before surgery if we
assess the squint with prism we can be aware of it
to the patient.

40. 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 vision in
children below two years of age.

41. 4 ∆D base out test
This is a delicate test for small degrees of esotropia
(microtropia). A four-diopter 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.

42. Forms of diagnostic prisms
i. Single un mounted prisms
ii. Trial lens set prisms
iii. Prism bars: These are bars composed of
adjacent prisms of increasing power.

43. Therapeutic uses of prism

44. Therapeutic prism: Convergence
insufficiency
The commonest therapeutic use of prisms in
the orthoptic department is in building up
the fusional reserve of patients with
convergence insufficiency.

45. Convergence insufficiency: Base out
prism exercises
Base out prisms can also be used to
stimulate the converge reflex. The base out
prism induces crossed diplopia and the
patient must converge to overcome the
prism strength and obtain BSV.

46. Convergence insufficiency: Base out
prism exercises
• Some practitioners give a patient a single
prism and have them do gradual exercises
or near tasks, while other practitioners
have the patient use a prism bar and to
overcome increasing prism strengths
while focusing on a near target.

47. To relieve diplopia
To relieve diplopia in certain cases of
squint, these include decompanseted
heterophorias, small vertical squints and
some paralytic squints with diplopia in the
primary position. Prisms are reserved for
those patients for whom surgery is not
indicated

48. Forms of therapeutic prism
Temporary wear prisms: Used in treatment include clip-
on spectacle prisms for trial wear. Eg:-Fresnel prism
(pronounced fre-nell') prisms,)
• To understand how a Fresnel prism works, imagine
cutting off the tops of a large number of equally
powered prisms and gluing them, one above the other,
onto a thin piece of plastic. A Fresnel prism is only 1
mm thick. It consists of 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.

49. • Advantages of a Fresnel Prism: It is very thin and
extremely lightweight. It is flexible and can be
applied to an existing spectacle lens. It can be cut
to any shape with scissors or a razor blade.
Reduce magnification differences considerably.
• Disadvantages: They are harder to clean than
conventional lenses. Fresnel prisms also cause a
slight loss of visual acuity caused by reflections
for prisms greater than 10Δ.

51. Prisms in optical instruments:
1) Slit lamp bio microscope.
2) Applanation tonometer
3) keratometer

52. Different types of prism used in
ophthalmology
Porro-prism: , is a type of reflection prism used in
optical instruments to alter the orientation of an image.
• Porro prisms are most often used in pairs, forming
a double Porro prism. A second prism, rotated 90°
with respect to the first, is placed such that light will
traverse both prisms. The net effect of the prism
system is a beam parallel to but displaced from its
original direction, with the image rotated 180°. As
before, the handedness of the image is unchanged.

53. • Double Porro prism systems are used in small optical
telescopes to re-orient an inverted image (an
arrangement is known as an image erection system),
and especially in many binoculars where they both
erect the image and provide a longer, folded distance
between the objective lenses and the eyepieces.
• Commonly, the two components of the double
Porro system are cemented together, and the prisms
may be truncated to save weight and size.

54. Single Porro prism. Double Porro prism

55. • Dove prism:
• A Dove prism is a type of reflective prism which is
used to invert an image. Dove prisms are shaped
from a truncated right-angle prism. A beam of
light entering one of the sloped faces of the prism
undergoes total internal reflection from the
inside of the longest (bottom) face and emerges
from the opposite sloped face. Images passing
through the prism are flipped, and because only
one reflection takes place, the image is inverted
but not laterally transposed.

56. Dove prism:
A Dove prism is a type of reflective prism which is
used to invert an image. Dove prisms are shaped
from a truncated right-angle prism. A beam of light
entering one of the sloped faces of the prism
undergoes total internal reflection from the inside of
the longest (bottom) face and emerges from the
opposite sloped face. Images passing through the
prism are flipped, and because only one reflection
takes place, the image is inverted but not laterally
transposed.

57. The Wollaston prism
• A Wollaston prism is an optical device,
invented by William Hyde Wollaston, that
manipulates polarized light. It separates light
into two separate linearly polarized outgoing
beams with orthogonal polarization. The two
beams will be polarized according to the
optical axis of the two right angle prisms.

58. The Wollaston prism
• The Wollaston prism consists of two
orthogonal prisms of birefringent material—
typically a uniaxial material such as calcite.
These prisms are cemented together on their
base (traditionally with Canada balsam) to form
two right triangle prisms with perpendicular
optic axes.

59. The Wollaston prism
• Outgoing light beams diverge from the prism as
ordinary and extraordinary rays due to the
differences in the indexes of refraction, with the
angle of divergence determined by the prisms'
wedge angle and the wavelength of the light.
Commercial prisms are available with divergence
angles from less than 1° to about 45°.
• Use in: Keratometer