L6 Thick Lenses Flashcards Preview

Opthalmic Lenses and Dispensing > L6 Thick Lenses > Flashcards

Flashcards in L6 Thick Lenses Deck (9):
1

Back Vertex Power

- BS states that lenses are numbered in terms of their back surface power, F'v
- isnt necessarily the same as the back surface power
- takes into account the thickness along the axial part of the lens and also takes into account the refractive index
F'v = power of the lens at a2

2

F'v

Sum of the effective power of the front surface at the back surface and the back surface power

3

Thickness

When light travels through a medium other than air, the real thickness of the material (t) can be replaced by an equivalent thickness of air (d)
d=t/n

4

Accurate Transposition

- We can compensate a surface for the change in rx due to thickness of lens
- if the thickness and one surface power are known, then to make a lens of given back vertex power, the other surface power needs to be compensated for the thickness and worked with a diff tool

5

Nominal/uncompensated surface power

surface power that would be worked by the lab if they assumed the surface to be thin
F1N = F'V - F2

6

Reduced/ compensated surface power

actual surface power after taking into account the thickness
F1 or F2 - what we want to calculate

7

Vertex Power Allowance

difference between the compensated and uncompensated powers
F1-F1N or F2-F2N

8

Front surface compensation

when the back surface of the lens has already been worked to get the correct rx we need to take into account the lens thickness and calculate what power is required on the front surface to give the correct rx

9

Back Surface Compensation

When the front surface of the lens has already been worked to get the correct rx we need to take into account the lens thickness and calculate what power is required on the back surface to give the correct rx