Prisms are used in ophthalmology to deviate light and measure eye alignment and vision. They have several key properties: they deviate light without changing its vergence, displacing the image towards the base. Prism power is measured in prism diopters and can be used to calculate the amount of deviation induced by lenses. There are several types of prisms including dispersive, polarizing, and reflecting prisms that are used in instruments and to diagnose and treat various eye conditions like deviations, visual field defects, and degenerative diseases.

1. Ophthalmic Prism & its
Properties
Mohammad Arman Bin Aziz
Instructor Optometrist
Institute of Community Ophthalmology

2. Prism is a portion of a refracting medium
bordered by two plane surfaces which are
inclined at a finite angle.
AxisAxisββ

3. Components
• Apex
• Base
• Refracting angle
• Angle of deviation
• Axis of a prism

4. Properties of prism
Prisms deviates light without
changing its vergence.
It obeys Snellen’s law i.e. it displaces
the image towards its base .
It also causes dispersion.

5. Image formation of prisms
Erect, virtual and displaced towards the apex of the prism

6. Angle of Deviation
• The angle of deviation
can be found by the
following formula (in
air):
• If n’=1.5, then
)1'( −= nβε
ε=β(1.5 −1) =
β
2

7. Deviation without dispersion
• The prism which produces
this phenomena:
Achromatic prism.
• Consists of two prisms,
usually one of crown glass
and the other of flint of
equal angular dispersion.
• Apex should be in opposite
direction.
• Used in Motion Picture
industry.

8. Prism Aberrations
• Chromatic aberration
Producing color fringes at the edge of the objects
viewed through the prism
• Asymmetrical magnification
• Curvature of field

9. Prism Power Notations
• Refracting angle
• Centrad
• Angle of apparent
deviation
• Prism dioptre
99

10. Prentice Rule
P=dF
Where,
P=Prism effect,
d=distance in cm,
F=lens power

11. Example
A patient looks through a (-3.00) lens 10
mm nasally OD. What is the amount of
prism induced?
BIcmP ∆
== 3)00.3)(0.1(

12.  Dispersive prisms
 Polarizing prisms
 Reflecting prisms

13. Dispersive prism
 Dispersive prisms are used to break up light into its
constituent spectral colors because the refractive index
depends on frequency;
 Blue light is slowed down more than red light
and will therefore be bent more than red light.
 Triangular prism
 Abbe prism
 Pellin-Broca prism
 Amici prism

15. Polarizing prism
• Prisms which can split a
beam of light into
components of varying
polarization.
• These are typically made of a
birefringent crystalline
material.
 Nicol prism
 Wollaston prism
 Rochon prism
 Glan-Foucault prism
 Glan-Taylor prism
 Glan-Thompson prism

16. Wollaston prism
2 right-angled prisms of equal
angle made of a double refracting
surface such as quartz or calcite
cemented together by their
hypotenuse faces to form a
rectangular unit.
A beam of unpolarized light
incident on it will emerge as two
diverging beams which are
oppositely polarized and almost
free of dispersion.

18. Reflecting prisms
• A prism in which light is internally reflected at one or more
of the plane surfaces before emerging.
• This happens due to total internal reflection.
 Penta prism
 Porro prism
 Porro-Abbe prism
 Abbe-Koenig prism
 Schmidt-Pechan prism
 Dove prism
 Dichroic prism
 Amici roof prism
 Penta prism,
 Right Angle prism,
 Corner cube prism,
 Rhomboid prism etc.

19. Erecting prism
A prism system placed in an
optical system for the purpose of
forming an erect image .
Examples:
-Porro prism
-Dove prism

22. Penta Prism

23. Right Angle Prism

24. Dove Prism

25. Types of prism of clinical use
1. Relieving prisms
2. Corrective prisms
3. Over-corrective prisms
4. Inverse prisms
5. Yoked prisms
6. Rotating prisms
7. Regional prisms
25

26. Uses of Prism In
Ophthalmic Instruments

32. • To detect microtropia
4-dioptre Base out prism test
• To detect Intermittent Deviations
•The difference between blur point and recovery point
should be (2-4) prism dioptre.
•If recovery point is much less than 4 dioptre. it is
intermittent deviation.

33. Therapy for the deviations
Exophoria
Esophoria
Vertical Phorias
Exotropia
Esotropia
Hypertropia / Hypotropia
Convergence
To measure convergence
As a therapy to convergence
Divergence insufficiency

35. To measure Visual acuity in
infants
Base down prism
Four diopter base down prism test

36. Degenerative disease (i.e. multiple sclerosis)
• Only some conditions can be helped with
prisms.
• Those conditions are:
 Bilateral internuclear ophthalmoplegia
 Cerebellar eye signs
 Sixth nerve paresis
 Pendular nystagmus
3636

37. Visual field defects
 Hemianopia
 Constricted visual field
 Prism relocation

39. References
o System for ophthalmic Dispensing by T.
Grosvenor
o Low Vision Manual by Jonathan Jackson