9 - Oblique Prisms Flashcards
What is Oblique prism?
When there are prisms in two different directions horizontal and vertical.
Practice finding the compound prism from a V+H Prism
Practice resolving prism from compounded to H+V Prism
What are the steps to obliquely Crossed Prism
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
Practice examples of resolving obliquely crossed prism
What is a rotary (Risley’s Prism) prism?
Combination of two prims
What happens when the bases are next to each other in a rotary prism?
Add the Base prism together.
What happens when the bases are opposite to each other in a rotary prism?
The prismatic effect will cancel each other out.
What is the distance from the front surface of the cornea to the center of rotation?
13.50mm
see figure 15.27 page 303 SOD
What is the effective power of the prism?
What is Prentice’s Rule? 2
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.
What do the letters represent in the Effective Power Formula? Pe = P/ 1-(s/l)
Pe= Effective Prism
P=
s=
l=
What do the letters reporesent in the Prentice’s Formula?
c= decentration in cm
F= focal length in D (Power)
Decentering = lens moving
Looking away from OC = Eye Moving
How much prism is present when looking through the optical centre?
No prism is induced and the object is not displaced from its actual location.
What is the rules for decentering the lens of a Minus Prescription?
Up - Base Down
Down - Base Up
In - Base Out
Out - Base In
What is the rules for decentering the lens of a Plus Prescription?
Up - Base Up
Down - Base Down
In - Base In
Out - Base Out
When resolving what formulas do we use?
V/H = Psin0
V/H = Pcos0
P = Prism Power
0 = Base orientation
What do we need to be careful of with these formualas? Psin0 Pcos0
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.
