L7 Opthalmic Prisms and prismatic effects

Question 1

what is the main property of prisms

Answer 1

they deviate the direction of light toward the base, inducing compensatory eye movements

Question 2

horizontal prism summation

Answer 2

compounding effect when prism in same direction (bir & bil or bor & bol)

cancelling effect when prism in opposite direction (bir & bol or bor & bil)`

Question 3

vertical prism summation

Answer 3

compounding effect when prisms in opposite direction (bur & bdl)

cancelling effect when prisms in same direction (bur & bul)

Question 4

formula for calculation of prismatic effects

Answer 4

P = decentration (in cm) * power of lens

Question 5

prismatic effect of an astigmatic lens

Answer 5

• power cross
• there will be no prism along axis meridian
• there will be prism along power meridian

Question 6

3 methods to incorporate prism into the spectacles

Answer 6

1. fresnel prism
   - pvc
   - can be trimmed to required sizes/shapes
   - up to 30 prism
   - stick onto back of lens with water
   - lightweight and flexible/removed when not needed
   - not cosmetic friendly
2. grindin prism
   - prism can be created by thickness between apex and base
   - lab will charge as custom made
3. decentration prism
   - want px to experience the prismatic effect