Chapter 4: Prisms

Question 1

Define the axis of a prism

Answer 1

A straight line bisecting the refracting/ apical angle (alpha) and the base.

Question 2

In a prism, the angle of deviation is determined by which 3 factors?

Answer 2

1)angle alpha
2) angle of incidence of the light
3) refractive index of the material

Question 3

For a prism, what is the angle of minimum deviation

Answer 3

When the angle of incidence = the angle of emergency

D= (refractive index -1)*apical angle

For a prism of refractive index of 1.5

D= alpha/2

Question 4

Describe the image formed from a base down prism

Answer 4

Virtual, erect, displeased to the apex

Question 5

What is the prentice position

Answer 5

When incident light is parallel to one of the surfaces of a prism, there for, all the deviation occurs on the second surface

Question 6

When are lenses used in the angle of minimum deviation and when are they used in the prentice position?

Answer 6

Angle of minimum deviation= plastic prism bars ie Maddox

Prentice position in trial ophthalmic lenses

Question 7

Define a prism diopter

Answer 7

A prism of 1 diopter produces a linear elevation of 1cm when viewed from 1m away from the refractive angle.

Question 8

A prism of 1 prism diopter power has an angle of apparent deviation of

Answer 8

0.5 degrees

Question 9

How does the prism diopter differ from the centrad?

Answer 9

Prism diopter is 1cm upwards deviation in a straight line.
Centrad is 1cm arc deviation which is slightly bigger but is negligible in clinical practice.

Question 10

Assuming a prism has a refractive index of 1.5 and a refractive angle of 10 degrees.
What is the
1. angle of minimum deviation
2. Prism dioptric power

Answer 10

1. Alpha/2 = 5 degrees
2. 10 dioptres

Question 11

Describe a Risley prism

Answer 11

Two prisms of equal power are stacked one in front of the other. One is rotated relative to the second with a scale that tells you the resultant power. When used in conjunction with a Maddox rod it can be used to detect phorias (misalignment of the eye).

Question 12

What does an orthoptic report of synopt. Without gls +20 degrees mean?

Answer 12

Using a synoptophore, a patients eye is deviated inwards by 20 degrees.

Esotropia = +
Exotropia = -

Question 13

What does PCT = distance eso + 40 dioptres mean?

Answer 13

Using prisms of 40 dioptres, a patients esotropia is neutralised, ie is now straight.
Remember, 1 prism diopter causes incline of 1cm when viewed from 1m away from the axis of the prism and causes an apparent angle of deviation of 1/2 a degree.
Therefore, a 40 dioptre prism is used to correct an esotropia of 20 degrees.

Question 14

Name an objective measure of squint/ heterophobia

Answer 14

PCT

Question 15

Name a subject test for squint and heterophoria

Answer 15

Maddox rod

Question 16

Define heterophoria

Answer 16

Heterophoria is an eye condition in which the directions that the eyes are pointing at rest position, when not performing binocular fusion, are not the same as each other, or, “not straight”.

Question 17

How do you assess fusion reserve

Answer 17

Place prisms of increasing strength infront of one eye until binocular vision breaks down

Question 18

What is a fresnel prism?

Answer 18

A Fresnel prism consists of a plastic sheet of parallel tiny prisms of identical refracting angle (Fig. 4.9). 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.

Question 19

When do you use a base out prism?

Answer 19

To correct esotropia.

IMPORTANT CONCEPT: Always orient prisms with the apex in the direction of deviation.

Question 20

When do you use a base in prism?

Answer 20

To correct exotropia

IMPORTANT CONCEPT: Always orient prisms with the apex in the direction of deviation.
The base is therefore in the opposite direction.

More info: https://www.eophtha.com/posts/fun-with-prisms-in-ophthalmology

Question 21

What is the effect of a Porro prism

Answer 21

Image is deviated by 180 degrees (reflected) and flipped upside down. No lateral transposition

Question 22

What is the effect of a dove prism

Answer 22

No deviation. Image is flipped upside down
No lateral transposition.