2. Lens Power Identification

Question 1

Nominal (approximate) power - Def

Answer 1

(measured w/ lens clock) - Only considers surface Ps (not thickness) - Assumes thin lens, ref at front and back surface takes place at same pt, Ok for minus lens (thinner in center&raquo_space; less sep btwn pts), Less accurate for plus

Question 2

Nominal (approximate) power - Formula

Answer 2

Pa = P1 + P2, where Pa = nominal (approximate) power, P1 = Front surface P, P2 = Back surface P

Question 3

Front vertex power (FVP) aka Neutralizing power - Def

Answer 3

(w/ ref from front pole of lens) - Light enters from back surface towards primary focal pt = Neg reciprocal of reduced distance from lens front pole to primary focal pt&raquo_space; Secondary focal pt of known lens coincident w/ primary focal pt of unknown lens (Used in trial lens neutralization, Used in add P det of front surface seg, Used in atypical lensometry when lens convex surface against lens stop)

Question 4

Front vertex power (FVP) - Formula

Answer 4

Pn = Peq / (1 - dP2) = P1 + P2 / (1 - dP2) where d = t / n

Question 5

Back vertex power (BVP) - Def

Answer 5

(w/ ref from back pole of lens) - Light enters from front surface towards secondary focal pt) = Reciprocal of reduced distance from lens back pole to secondary focal pt, (Used in typical lensometry when lens concave surface against lens stop, Used for CLs), NOT affected by change in front surface of lens

Question 6

Back vertex power (BVP) - Formula

Answer 6

Pv = Peq / (1 - dP1) = P2 + P1 / (1 - dP1)

Question 7

Effective power - Def

Answer 7

Ability of lens to focus // rays at specified plane, Increased by moving lens towards eye (for minus lens) / moving lens away from eye (for plus lens)

Question 8

Effective power - Formula

Answer 8

Peff = P / (1 + dP) where d = change in lens position (+d) = lens moved towards eye, (-d) = lens moved away from eye

Question 9

Vertex distance compensation - Def

Answer 9

CL power - (Used w/ changing spectacle Rx to CL)

Question 10

Vertex distance compensation - Formula

Answer 10

Pvd = P / (1 - dP) where Pvd = vertex distance compensation = CL P, P = back vertex P (for CL), d = change in lens position = VD for CL, (+d) = lens moved towards eye, (-d) = lens moved away from eye

Question 11

Equivalent power - Def

Answer 11

For thick lens, multiple lens system&raquo_space; Equiv thin lens system (dep on locations of primary and secondary principle planes), (Used in low vision)

Question 12

Equivalent power - Formula

Answer 12

Peq = P1 + P2 - d(P1)(P2) where d = t / n, t = sep btwn lens 1 and lens 2 OR thickness of lens, n = index of medium btwn thin lenses OR index of thick lens

Question 13

Base curve (for contact lens) - Def

Answer 13

Curvature on concave side

Question 14

Distance ref pt (DRP), Near ref pt (NRP), Prism ref pt (PRP) - Def

Answer 14

Designated pt on spectacle lens where Rx must be measured - optical center, in most cases / where add P is measured / where prism of lens is measured

Question 15

Fitting pt - Def

Answer 15

Pt on lens that should be in front of patient’s pupillary center for distance and/or near

Question 16

Linear motion vs. rotational motion

Answer 16

WITH motion - Minus lenses/axis located vs. AGAINST motion - Plus lenses/axis located

Question 17

Trial lens - Hand neutralization - Def

Answer 17

Place known trial lens back pole coincident to unknown lens front pole&raquo_space; Secondary focal pt of known lens coincident to primary focal pt of unknown lens

Question 18

Lensometer - Focusing system - Parts

Answer 18

1) Light source, 2) Target (cross hairs and/or series of dots in circle, focused by P wheel - at primary focal pt of std lens, w/ no lens against lens stop, w/ P wheel set at zero&raquo_space; Sph P of Rx when first meridian comes into focus, Turn P wheel in minus direction&raquo_space; Cyl P of Rx when second meridian comes into focus), 3) Std lens (light leaving directed towards secondary focal pt of unknown), 4) Lens stop (at secondary focal pt of std lens - Badal system&raquo_space; Peq = Pstd, so P eq = same, regardless of unknown lens&raquo_space; Constant image size&raquo_space; Does NOT matter whether center of lens placed at lens stop (still two principal meridains, no matter where they’re measured)

Question 19

Lensometer - Observation system - Parts

Answer 19

[Keplerian TS] - 1) Objective lens (w/ // light entering from unknown lens), 2) Reticule (concentric circles and cross hair, focused by eyepiece - at secondary focal pt of objective = primary focal pt of eyepiece - position doesn’t change even w/ lens against lens stop), 3) Eyepiece lens

Question 20

Neutralization vs. Verification

Answer 20

Neutralization (to det Rx of a pair of lenses) vs. Verification (to confirm accuracy of fabricated Rx lenses against prescribed Rx lenses)

Question 21

Power of unknown lens, det by lensometry - Formula

Answer 21

Punk = x (Pstd)^2 where Punk = P of lens, in given meridian = multiple of D change in P wheel (increase greater target movement over greater range&raquo_space; increase accuracy), x = distance of target movement, (-) x = To the L, towards back - away from observer, towards primary focal pt of std lens, (+) x = To the R, towards front - towards observer, towards secondary focal pt of std lens, Pstd = P of std lens