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(US), diopter
a unit for measuring the refractive power of a lens: the reciprocal of the focal length of the lens expressed in metres


A measure of the power of a lens or a prism. The diopter (also called dioptrie) is usually abbreviated D. Its dimension is a reciprocal length, and its unit is the reciprocal of 1 m (3.28 ft). See Focal length, Lens (optics)

The dioptric power of a prism is defined as the measure of the deviation of a ray going through a prism measured at the distance of 1 m. A prism that deviates a ray by 1 cm in a distance of 1 m is said to have a power of one prism diopter. See Optical prism

Spectacle lenses in general consist of thin lenses, which are either spherical, to correct the focus of the eye for near and far distances, or cylindrical or toric, to correct the astigmatism of the eye. An added prism corrects a deviation of the visual axis. The diopter thus gives a simple method for prescribing the necessary spectacle for the human eye.



the unit of focal power of a lens and other axially symmetrical optical systems. It is indicated by the symbol D; 1 D is equal to the power of a lens or spherical mirror with a main focal length of 1 m. Focal power, expressed in diopters, is the reciprocal of the principal focal length in meters. The power of a converging lens is taken as positive, and that of a diverging lens, negative. The focal power of eyeglasses is given in diopters. Eyeglasses for nearsighted persons have negative focal power (negative number of diopters); those for farsighted persons have positive focal power.



a simple device for fixing the direction to an object—that is, sighting. It consists of two metal plates (the ocular and object diopters) fastened to the ends of an alidade or to a divided circle. The ocular diopter has a small circular hole or narrow slit, and the object diopter has a sight or small fine hair stretched at a certain distance from the ocular diopter. The diopter is used by turning it until, upon looking through the ocular diopter, the object diopter is projected on the object to be sighted. With a sufficient distance between the ocular and object diopters, the accuracy of sighting can be 2-5 minutes of arc. The diopter was described by Heron of Alexandria (c. first century A.D.) and was extensively used earlier in geodetic instruments. At present it is found only in some compasses.


A measure of the power of a lens or a prism, equal to the reciprocal of its focal length in meters. Abbreviated D.