# dioptre

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## dioptre

A measure of lens power. The reciprocal of the focal length in metres. A lens of 1 m focal length has a power of 1 dioptre. A lens of 50 cm focal length has a power of 2 dioptres. One of 33.3 cm focal length has a power of 3 dioptres. Dioptres are used by opticians in preference to focal lengths because they can immediately be added together or subtracted during vision testing.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005

## dioptre

1. A unit proposed by Monoyer to evaluate the refractive power of a lens or of an optical system. It is equal to the product of the refractive index in the image space and the reciprocal of the focal length in metres. (Symbol: D.) Thus a lens with a focal length (in air) of 1 m has a power of 1 D, one with a focal length of 1/2 m, has a power of 2 D, etc. 2. It is also incorrectly used to represent a unit of curvature, being equal to the reciprocal of the radius of curvature expressed in metres. See curvature of a surface; myodioptre; fundamental paraxial equation; refractive error; vergence.
Fig. D3 Effect of an 8 Χ prism

 Table D3 Relationship between dioptres and focal length (in air) focal length dioptre value (cm) (in) 0.25 400 157 0.50 200 79 1.00 100 39 1.50 67 26 2.00 50 20 2.50 40 16 3.00 33.3 13 4.00 25 10 5.00 20 7.9 6.00 16.7 6.6 7.00 14.3 5.6 8.00 12.5 4.9 9.00 11.1 4.4 10.00 10 3.9 12.00 8.3 3.3 14.00 7.1 2.8 16.00 6.2 2.4 20.00 5.0 2.0

 Table D4 Relationship between prism dioptres and degrees prism dioptres (Χ) degrees (º) minutes (′) 1 0.573º 0º34′ 2 1.14º 1º8′ 3 1.71º 1º43′ 4 2.28º 2º17′ 5 2.85º 2º51′ 6 3.43º 3º26′ 7 4.0º 4º0′ 8 4.57º 4º34′ 9 5.14º 5º8′ 10 5.71º 5º43′ 15 8.57º 8º34
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
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