2 Properties of Ophthalmic Lenses

curvature (R) =

1/r

The smaller circle has (greater/smaller) curvature (R) and a (longer/shorter) radius

greater/shorter

Power =

Function of lens clock

measure sagittal depth of lens surface

Sagitta

the distance between a point on the circle and the midpoint of a chord of the circle

Exact sag

What can be calculated with the sagittal depth?

Power of the surface

When is the approximate sag formula used?

when s is small compared to r (as with spectacle lenses)

How does a lens clock work?

Each lens clock is calibrated for a single index of refraction. The center pin moves to directly measure sag (s), and using the separation of outer pins (1/2 of that distance is semi-chord y), surface power is indirectly measured

Lens clock equation

combine surface power and sag depth formula

How do you find true surface power with a lens clock for a different n?

solve for r=(n’-n)/P where n’ is lens clock, then plug the true r into original surface power using true index of lens

If lens clock assumes a higher index of refraction than the material, it gives an (over/under)estimate of n and surface power

overestimate