Definition of dioptre in English:


(US diopter)


  • A unit of refractive power, which is equal to the reciprocal of the focal length (in metres) of a given lens.

    ‘vision corrected to within one dioptre of perfect sight’
    • ‘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 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 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.