dioptre
(daɪˈɒptə(r))
[The usual spelling in Great Britain of diopter in sense 5.]
A unit for expressing the power of a lens, equal to the reciprocal of its focal length in metres.
| [1876 Compte-rendu: Congrès internat. des Sciences médicales 1875 (Brussels) 610 L'unité du système métrique, c'est le mètre: l'unité dioptrique..sera nommée dioptre.] 1882 E. Nettleship Dis. Eye (ed. 2) i. 15 Some system of numbering is required which shall indicate the refractive power of the lenses used for spectacles... In the second system, which is fast displacing the old one, the metrical scale is used; the unit is a weak lens of 1 metre (100 cm.) focal length, and known as a dioptre (D). 1907 Practitioner June 820 Patient was myopic to 2·0 dioptres. 1964 S. Duke-Elder Parsons' Dis. Eye (ed. 14) v. 51 A lens with a focal length of half a metre will be twice as strong as one with a focal length of 1 metre: the refractive power of such a lens is therefore 2 dioptres. |