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Flashcards in 4 - Prisms Deck (10)
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Movements thru prisms

Light -> base

Image + eyes -> apex


Deviation power (Z or Δ)

Δ =y/x

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


Prism power and apex angle

d = A(n-1)

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

Every 1 degree deviation = 1.75Δ prism


Prism and power thickness

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


Prentice’s rule

Δ = dF

PD = distance from optical center in CENTIMETERS * lens power


Prentice’s rule
-4 uses

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


Combining prisms problems

Draw vectors and use pythagorean theory


Correcting vertical imbalance

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


Image jump
-what 2 things you need to know to solve

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)


Total prismatic effect problems

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