Ophthalmic Optics Formulas Flashcards
Minimum blank size
Total decentration
(A + DBL) - pt PD
Frame PD
A + DBL
ED
Double the distance of the edge of the lens farthest away from geometric center
Frame difference
A size - B size
Exact sag formula
H is half the chord length
Thickness equation for biconvex lens
CT = Sf + ET + Sb
Crown glass
n = 1.523
V = 58
plastic CR-39
n = 1.498
V = 58
Polycarbonate
n = 1.586
V = 30
Trivex
n = 1.53, V=44
Lens clock correction formula
Nominal power
Front surface power + back surface power
F1+F2
Spherical equivalent
SE = sph + cyl/2
Distance target moved on lenso
f is focal length of the standard lens in the lensometer, Fv is Fbvp
Back vertex power
Front vertex power
OC locations of flat-top segs
For 22, 25, 28mm -> OC is 5mm below top of seg
For 35mm -> OC is 4.5mm below top of seg
For 40, 45mm -> OC is on the line (0mm)
OC locations for round segs and executives
Round segs -> OC half way through diameter of round seg
Executive seg -> OC is on the line (0mm)
Inset
frame pd - pt pd (divide by 2 if monocular situation)
Seg inset
distance pd - near pd (divide by 2 if monocular situation)
total inset
frame pd - near pd (divide by 2 if monocular situation)
also is seg inset + inset
Image jump
Prism = cF
(distance bw seg OC to dist OC)(add power)
TCA
amount of prism looking through / V
LCA
Power of lens / V
power error
(tangential error + sagittal error) /2
Effective power
left to right direction is positive
prism diopters
= cm/m (image jump over distance)
Angle of deviation (approx)
D = A (n-1)
Prism based on thickness of base and apex
Prism = 100g(n-1) / d
d is distance bw apex and base
g is thickness difference bw apex and base
(match units)
One degree of deviation angle for a prism is how many prism diopters?
1.75 pd
Prentice rule
Amount of BD necessary in progressive lens
Add power x 2/3
Spec mag
Reflection formula
I is intensity of the light incident on the surface
Index relationship with reflection/transmittance
Increased n = increased reflections = decreased transmittance
lenses seem dimmer to look through
Ultimate transmission
T(ultimate) = (T1)(T2)(T3) …
Ideal AR coat index formula
Lens maker equation
P = (n2 - n1) / r
Correcting vertical imbalance
Clinical methods - Slab off (BUMM, base up more minus), multiple specs, CLs, Fresnel prisms
Academic methods - dissimilar segs, compensated R segs
Prentice rule for image jump in ADDs
Prism = dP , where d is the distance from the top of seg to near OC, and P is the power of the ADD only
