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 [syn: diopter, dioptre]
Diopter \Di*op"ter\, Dioptra \Di*op"tra\, n. [L. dioptra, fr.
Gr. ?. See 2d Dioptric.]
An optical instrument, invented by Hipparchus, for taking
altitudes, leveling, etc.
[1913 Webster]
