4 - Prisms

Question 1

Movements thru prisms

• light
• image
• eyes

Answer 1

Light -> base

Image + eyes -> apex

Question 2

Deviation power (Z or Δ)
-equation

Answer 2

Δ =y/x

y: linear distance the beam is shifted in CENTIMETERS
x: distance between the prism and the wall in METERS

Question 3

Prism power and apex angle

-equation

Answer 3

d = A(n-1)

Deviation angle in degrees = apex angle*(n-1)

Every 1 degree deviation = 1.75Δ prism

Question 4

Prism and power thickness

-equation

Answer 4

Δ = 100*(g(n-1)) ÷ ℓ

PD = 100 * (diff in thickness bw apex and base(n-1)) ÷ apex to base length

g and ℓ must be in the same units

Question 5

Prentice’s rule

-equation

Answer 5

Δ = dF

PD = distance from optical center in CENTIMETERS * lens power

Question 6

Prentice’s rule

-4 uses

Answer 6

Decentration
Vertical imbalance (binocular)
Image jump
Total prismatic effect (monocular)

Question 7

Combining prisms problems

Answer 7

Draw vectors and use pythagorean theory

Question 8

Correcting vertical imbalance

Answer 8

Slab off - use Base Up to the more Minus lens (BUM)
Dissimilar segs
Contact lenses
Multiple specs instead of multifocals
Fresnel prisms

Question 9

Image jump

-what 2 things you need to know to solve

Answer 9

Add type
Add power

Then use Prentice’s rule where d is the sag drop for that type of BF (e.g. 5mm for FT-28, so use .5cm)

Question 10

Total prismatic effect problems

Answer 10

Separate into D and N problems:

• draw power cross for distance
• draw picture and find distance their looking at from OC

Insert each into a Prentice’s equation and add