Diopter (dpt or D)
Diopter (dpt or D) :
A metric unit used in optics to measure the refractive power of a lens. Each converging lens has a focal length, defined to be the distance from the center of the lens to the point at which the lens focuses light. (Diverging lenses, which can be used to convert focused light to parallel rays, have a negative focal length.) The shorter the focal length, the greater the refractive power of the lens. The refractive power of the lens, in diopters, equals 1 divided by the focal length of the lens, in meters, so 1 diopter = 1 m-1. The unit is often spelled dioptre outside the United States. The lower case Greek letter delta is also used as a symbol for the diopter. See also: Prism Diopter
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