A lens clock has three points of contact, ball tips that will not mar the surface of plastic lenses. The outer two points are stationary, the inner point moves in or out to measure the curvature of the lens. A large 1-3/4 dial displays the curve in diopters, with plus (+) curves shown in one direction and minus (-) curves in the opposite.
Measures to + / - 20 diopters in quarter diopters
These lens measures are calibrated to read the powers of lens materials with of 1.53 refractive index. Lenses of higher index materials will have a power that is greater than the indicated measurement.
Lens clocks may also be used to determine whether a lens surface is spherical or toric by placing the lens measure upon the optical center of a lens and then rotating it. If the needle does not move, the surface of the lens is spherical. If the needle moves the lens surface is toric.
A toric lens has two different powers, in two orientations that are perpendicular to each other. One of the lens surfaces is shaped like a cap, while the other is usually spherical. Toric lenses are primarily found in eyeglasses, contact lenses and intraocular lenses (IOLs) and are used to correct astigmatism.
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