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[dahy-op-ter] /daɪˈɒp tər/
Optics. a unit of measure of the refractive power of a lens, having the dimension of the reciprocal of length and a unit equal to the reciprocal of one meter.
Abbreviation: D.
an instrument, invented by Hipparchus, to measure the apparent diameter of the sun or moon or to estimate the size or elevation of distant objects.
Also, especially British, dioptre.
1585-95; < Latin dioptra < Greek: instrument for measuring height or levels, equivalent to di- di-3 + op- (for ópsesthai to see) + -tra noun suffix of means
Related forms
dioptral, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for diopter
  • The higher the diopter value, the higher the magnification.
  • The mirror was rough ground about a diopter out of spec.
  • From each plot center a three-diopter prism plot was established.
  • The diopter is often part of the right-hand eyepiece on a binocular.
British Dictionary definitions for diopter


a unit for measuring the refractive power of a lens: the reciprocal of the focal length of the lens expressed in metres
Derived Forms
dioptral, adjective
Word Origin
C16: from Latin dioptra optical instrument, from Greek, from dia- through + opsesthai to see
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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diopter in Medicine

diopter di·op·ter (dī-ŏp'tər)
Abbr. D.
A unit of measurement of the refractive power of lenses equal to the reciprocal of the focal length measured in meters.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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