9 - Oblique Prisms

Question 1

What is Oblique prism?

Answer 1

When there are prisms in two different directions horizontal and vertical.

Question 2

Practice finding the compound prism from a V+H Prism

Question 3

Practice resolving prism from compounded to H+V Prism

Question 4

What are the steps to obliquely Crossed Prism

Answer 4

1 - Take each obliq prism - resolve into H + V Prism
2 - Add H components from the 2 prisms into 1
3 - Add V components from the 2 prisms into 1
4 - Combine resultant V+H components into a new, single prism

Question 5

Practice examples of resolving obliquely crossed prism

Question 6

What is a rotary (Risley’s Prism) prism?

Answer 6

Combination of two prims

Question 7

What happens when the bases are next to each other in a rotary prism?

Answer 7

Add the Base prism together.

Question 8

What happens when the bases are opposite to each other in a rotary prism?

Answer 8

The prismatic effect will cancel each other out.

Question 9

What is the distance from the front surface of the cornea to the center of rotation?

Answer 9

13.50mm
see figure 15.27 page 303 SOD

Question 10

What is the effective power of the prism?

Question 11

What is Prentice’s Rule? 2

Answer 11

Prism Power is the amount light is displaced in CM at a distance 1m away from the lens/prism.
The displacement in cm over the distance in m.

Question 12

What do the letters represent in the Effective Power Formula? Pe = P/ 1-(s/l)

Answer 12

Pe= Effective Prism
P=
s=
l=

Question 13

What do the letters reporesent in the Prentice’s Formula?

Answer 13

c= decentration in cm
F= focal length in D (Power)

Question 14

Decentering = lens moving
Looking away from OC = Eye Moving

Question 15

How much prism is present when looking through the optical centre?

Answer 15

No prism is induced and the object is not displaced from its actual location.

Question 16

What is the rules for decentering the lens of a Minus Prescription?

Answer 16

Up - Base Down
Down - Base Up
In - Base Out
Out - Base In

Question 17

What is the rules for decentering the lens of a Plus Prescription?

Answer 17

Up - Base Up
Down - Base Down
In - Base In
Out - Base Out

Question 18

When resolving what formulas do we use?

Answer 18

V/H = Psin0
V/H = Pcos0
P = Prism Power
0 = Base orientation

Question 19

What do we need to be careful of with these formualas? Psin0 Pcos0

Answer 19

They can each be used for horizontal and vertical components so take into account the angle given to see what is likely to be the horizontal and vertical.