One of the mysteries of the English language finally explained.
A unit of refractive power that is equal to the reciprocal of the focal length (in meters) of a given lens.
- ‘The difference between the focusing powers of these two axes, measured in dioptres, is the astigmatism.’
- ‘For a typical 3-mm-diameter lens, the dioptric power can be controlled between - 100 and + 50 diopters.’
- ‘The focusing power of a lens is measured in dioptres.’
- ‘Refractive error is measured in dioptres, and myopia is designated with a minus sign.’
- ‘The presence of 1 dioptre of refractive error reduces Snellen visual acuity to less than 6/12 that is, below the level needed to drive.’
Late 16th century (originally as diopter, denoting an alidade): from French, from Latin dioptra, from Greek, from di- ‘through’ + optos ‘visible’. The term was used in the early 17th century to denote an ancient form of theodolite; the current sense dates from the late 19th century.
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