Diopter

DIOPTER. A lens with a focal length of 5 inches (No. 5) refracted 5 times weaker, i.e., = 1/5 of lens No. 1, etc. But the magnitude of the focal length, especially when designated in inches, was an inconvenient measure for performing various calculations and for expressing the refractive power (refraction) of a lens or any optical system. Therefore, it was proposed to use metric measures for calculating refractive power, but without limiting this to a simple replacement of inches with centimeters. It was deemed more expedient to designate the refraction of a lens not by its focal length, but by its optical power. The initiator of this innovation was Professor Nagel in the late 1860s. At his suggestion, a glass with a focal length of 1 m was taken as the unit of measure, which he called "Meterlinse." The name Diopter was proposed by the French oculist Monoyer in 1872 and quickly came into use. Abbreviated, Diopter is designated by D or dptr. The refractive power of a lens is the reciprocal of its focal length. For a lens with a focal length of 1 m, it is equal to 1 D. A lens with a focal length of 0.5 m has a refractive power of 2 D, and so on. But since in practical ophthalmology it is necessary to use lenses weaker than one D, and furthermore the interval between whole diopters is too large, especially in weaker lenses, it became necessary to introduce fractional numbers into practice, designated in decimal fractions, such as: 0.25 D, 0.5 D, 0.75 D, 1.25 D, 1.5 D, etc. All linear quantities refer to air. If there is a linear quantity equal to a in a medium with a refractive index n, then for air this quantity corresponds to n * a. The latter quantity is called the length reduced to air. The main advantage of the dioptric calculation lies in the clarity of the image, the simplification of arithmetic operations when combining two or more lenses, and the uniformity of the calculation value for all countries. At the present time, dioptric calculation is used exclusively in ophthalmological optics, which has completely supplanted the old designation of the refractive power of lenses. If it is required to convert from one measure to another, the following procedure is used: assuming that 1 m is approximately equal to 40 English inches, and knowing that a lens of 1 D corresponds approximately to a lens with a focal length of 40 inches, when converting from one system to another, one should divide the number 40 by the number D to obtain the glass number according to the old system, or divide 40 by the lens number to obtain the number D; e.g.: a lens of 4 D = 40/4 = lens No. 10 with a focal length of 10 inches; lens No. 5 = 40/5 = a lens of 8 D, etc.