Lens Forms & Base Curves

finished lens

lens that is surfaced on the front & back to the correct power & thickness, not edged for a frame yet though (uncut)

1D, 0.6mm

a ____ increase in base curve increases the vertex distance by _____

stock lens

premade lens, does not have to be custom surfaced, ready to be edged

base curve

beginning curve on which the power is based

front

for SV lenses, the base curve is always on which surface?

toric base curve

weaker or flatter curve on the back surface of a SV spherocyl lens in minus cyl form

cross curve

stronger or steeper curve on the back surface of a SV spherocyl lens in minus cyl form

back base curve

on minus cyl form lenses, toric base curve is the same as…

base curve

the weaker or flatter curve on the front surface of a SV spherocyl lens in plus cyl form

cross curve

the stronger or steeper curve on the front surface of a SV spherocyl lens in plus cyl form

sphere curve

back surface of SV spherocyl lens in plus cyl form

toric base curve

on plus cyl form lens, base curve is the same as…

base curve / toric base curve/cross curve

full toric form of prescription writing for minus cyl form lenses

base curve/cross curve / sphere curve

full toric form of prescription writing for plus cyl form lenses

same

the base curve of a segmented multifocal lens is always on the _______ side of the lens as the segment

no, base curve is always a sphere curve

are toric surfaces ground on the same side as the multifocal seg?

same

the base curve of a PAL is always on the _____ side as the progressive optics

lens clock

measures a preexisting lens base curve for accurate duplication or verification, operates on the principle of the sagittal depth formula

1.530

the lens clock assumes an index of refraction of what?

surface power in diopters

what does the lens clock show?

sagittal depth/sag formula

used to convert sagittal depth to dioptric power using assumed index of 1.53

(y²/2s) + (s/2)

(sag formula) r =

nominal power

• sum of the front & back surface powers of a lens

• can be measured for lenses made of 1.530 index material

• ignores the effect of lens thickness on lens power

toric surface

• maximum & minimum surface values

• measure by finding the orientation of 3 contact points on lens at max & min readings that correspond to the major meridians of lens power

no

can a lens clock read segment power of a fused MF seg?

horizontal in center of lens & above seg line

to measure the curvature accurately for a one-piece MF seg, how must you position the lens clock?

when replacing one lens in a pair, when ordering an identically powered 2nd pair of lenses

when do you need to specify base curve?

± 0.75D

what is the ANSI tolerance for base curve when specified?

1. do not try to match new glasses to old

2. do not use to get a thinner, better looking lens

3. do not use to solve ghost-image internal lens reflection issues

4. do not use to steepen the base curve for people w/ long lashes

when should you not specify base curve?

((n_new -1)F_measured)/ 0.53

lens clock conversion factor equation

F_new =

true base curve

value of the front lens surface as measured by a lens measure

nominal base curve

reference # used for the convenience of optical labs

refractive power

controls what happens to light at the surface of the lens