An Etymological Dictionary of Astronomy and Astrophysics

A unit of optical measurement that expresses the refractive power of a lens or prism. In a lens or lens system, it is the reciprocal of the focal length in meters.

Etymology (EN): L. dioptra, from Gk. di-, variant of dia- “passing through, thoroughly, completely” + op- (for opsesthai “to see”) + -tra noun suffix of means.

Etymology (PE): Dioptr loanword from Fr.