Optical prism for new ophthalmic residents

1. Prisms
Done by: Othman Al-Abbadi, MD

2. Reflection
• Occurs, at some degree, at all interfaces even
when most of the light is transmitted or
absorbed.
• Laws:
– The Incident ray, the Reflected ray & the Normal..
All lie in the same plane.
– Angle of Incidence = Angle of Reflection

3. Reflection at spherical mirrors
C F
P
F C
P

4. Clinical application
• The anterior surface of the cornea acts as a
convex mirror.

5. Light Refraction
• It’s the change in direction of light when it
passes from one transparent medium into
another of different optical density.
• The higher the density, the slower light and
the higher the refractive index.

6. • Absolute Refractive index: a comparison of
light velocity in a vacuum to another medium.
• Refractive indices to some media:
– Air= 1
– Water= 1.33
– Cornea= 1.37
– Crystalline lens= 1.386-1.406
– Crown glass= 1.52
– Flint glass= 1.6

7. Snell’s law
• To measure the refracting angle, we use the
Snell’s law of refraction

8. • On entering an optically dense medium from a
less dense medium, light is deviated towards
the normal.
• Medium 1nMedium 2 = n2 / n1

9. • When passing obliquely from air through a
glass plate then to air, light is deviated
laterally.. Thus, the direction of the light is
unchanged but is laterally displaced.

10. Refraction on curved interfaces
• If n2 > n1  convergence
• If n1 > n2  divergence
• Refracting power= (n2 – n1 )/r
r  radius of curvature in meters

12. Critical angle
• As the rays meet the interface more obliquely,
a stage is reached where the refracted ray
runs parallel with the interface.
• More obliquely
 total internal reflection

13. • Critical angle for the cornea??
 48.5
• How to overcome this reflection??
 applying higher refractive index
material

14. Light dispersion
• The dispersive power is not related to
refractive index
• Shorter wavelengths  more deviation
• Rainbow  total internal reflection with
dispersion
– Primary
– Secondary

15. Prisms
• A portion of a refracting medium bordered by
two plane surfaces which are inclined at a
finite angle.
• The ray is deviated towards the base of the
prism obeying the Snell’s law… forming erect,
virtual & apically displaced image.
• The net change in
direction is the angle
of deviation.

16. • Factors determining the angle of deviation in
air:
1. The refractive index of the prism material,
2. The refracting angle of the prism,
3. The angle of incidence of the ray.

17. Primary positions
1. Angle of minimum deviation,
2. Prentice position.

18. Angle of minimum deviation
• When the angle of incidence equals the angle
of emergence.
• Symmetrical refraction.
• Under these strict condition,
D= (n-1) * A
• For glass prisms
D= A/2

19. Prentice position
• When the incidence angle is zero, so that all
the deviation takes place at the other surface
of the prism.
• The “prentice position” power of the prism is
greater than the “angle
of minimum deviation”
power.

20. In ophthalmic use
• The prentice position is specified for glass
ophthalmic prisms… & the angle of minimum
deviation power is specified for the plastic prism.
• Stacking prisms one on top of another doesn’t
bring cumulative power of both prisms, so it’s not
advised… except if used on different planes (one
H & one V) because of perpendicular planes
which result in independence refraction.

21. Notations of prisms
1. Prism dioptre ( Δ )
Linear of apparent displacement of 1 cm of an object
that’s situated at 1 m.
2. Angle of apparent deviation ( θ )
1 prism dioptre = ½ angle of apparent deviation
3. The centad ( Ϫ )
The image is measured along an arc of 1 m from the
prism.
Slightly greater than the prism dioptre.
4. Refracting angle of the prism

23. Vector addition
• When the patient needs prismatic correction
both H & V, we can give one stronger prism
mounted at an oblique angle.
• Calculation:
1. Graphically; by
drawing a rectangle,
2. Mathematically;
by the Pythagoras’
theorem.

24. Risley prism

25. Orthoptic reports
1. The synoptophore
Measuring the angle between the
visual axes of the eyes in degrees…
using (+) for XT & (-) for ET
e.g: Synopt without gls +20
2. Prism cover test
Measuring the angle of squint by the alternating cover
test while placing prisms of increasing power before one
eye until movement is eliminated… measured in dipotres
e.g: PCT= distance eso +40Δ

26. Prescribing prisms
• The correction is split between the two eyes
upon prescribing.
• E.g:
– Convergence  both base-out
– Divergence  both base-in
(the apex of the prism is directed toward the
deviation)
– Hypper-/hypotropia  one base-up & the other
base-down

27. 1. The following are true about prism:
a. its orientation is defined by its apex
b. light is deviated towards the apex
c. light with shorter wavelength is deviated
more than light with longer
wavelength
d. the angle of the prism apex is called the
refracting angle
e. all the ophthalmic prisms are calibrated
according to the Prentice's position

28. 1. The following are true about prism:
a. its orientation is defined by its apex
b. light is deviated towards the apex
c. light with shorter wavelength is deviated
more than light with longer
wavelength
d. the angle of the prism apex is called the
refracting angle
e. all the ophthalmic prisms are calibrated
according to the Prentice's position

29. 2. The image formed by a prism is:
a. erect
b. magnified
c. laterally inverted
d. virtual
e. deviated towards the apex

30. 2. The image formed by a prism is:
a. erect
b. magnified
c. laterally inverted
d. virtual
e. deviated towards the apex

31. 3. The angle of deviation of a prism is
determined by:
a. the refracting angle
b. the angle of incidence of the ray
c. the refractive index of the prism material
d. the width of the base
e. the thickness of the prism

32. 3. The angle of deviation of a prism is
determined by:
a. the refracting angle
b. the angle of incidence of the ray
c. the refractive index of the prism material
d. the width of the base
e. the thickness of the prism

33. 4. The following are true about prisms:
a. they can control torsional diplopia
b. they can control diplopia caused by an eye
which is deviated out and up
c. the prism power can be calculated form the
refracting angle alone
d. a prism with 2 doptres will produce a linear
displacement of 2 cm of an object situated at 1 m
e. a prism with 1 dioptre produce a stronger
deviation than one with an angle of
apparent deviation of 1

34. 4. The following are true about prisms:
a. they can control torsional diplopia
b. they can control diplopia caused by an eye
which is deviated out and up
c. the prism power can be calculated form the
refracting angle alone
d. a prism with 2 doptres will produce a linear
displacement of 2 cm of an object situated at 1 m
e. a prism with 1 dioptre produce a stronger
deviation than one with an angle of
apparent deviation of 1