dioptre -
a unit of measurement of the refractive power of a lens which is equal to the reciprocal of the focal length measured in meters; used by oculists
A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres). It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at ⁄3 metre.
