Lens clock

A lens clock measures the curvature of a surface, but gives the result as an optical power in diopters, assuming the lens is made of a material with a particular refractive index.

The outer two probes are fixed while the center one moves, retracting as the instrument is pressed down on the lens's surface.

As the probe retracts, the hand on the face of the dial turns by an amount proportional to the distance.

A typical lens clock is calibrated to display the power of a crown glass surface, with a refractive index of 1.523.

If the lens is made of some other material, the reading must be adjusted to correct for the difference in refractive index.

of the surface can be obtained from the optical power given by the lens clock using the formula where

, the true optical power of the surface can be obtained using A biconcave lens made of flint glass with an index of 1.7 is measured with a lens clock calibrated for crown glass with an index of 1.523.

The actual power, as read by a vertometer or lensometer, might differ by as much as 0.1 diopters.

To do this, the contact lens is placed concave side up on a table or other hard surface.

in the formula above, and can be calculated from the optical power reading, if the distance between the outer prongs is known.