optical density(redirected from Absorbance Unit)
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optical density:see refractionrefraction,
in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass.
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a measure of the opacity of a layer of a substance to light rays. The optical density D is equal to the common logarithm of the ratio of the radiation flux F0 incident on the layer to the flux F that has been attenuated by absorption and scattering in passing through the layer: D = log (F0/F). In other words, the optical density is the logarithm of the reciprocal of the transmission coefficient of a layer of a substance: D = log (1/τ). In the definition of natural optical density, a concept that is also sometimes used, the common logarithm is replaced by the natural logarithm.
The concept of optical density was introduced by R. Bunsen. It is used to characterize the attenuation of optical radiation, or light, in layers and films of such various substances as dyes, solutions, stained glass, and milk glass, as well as in light filters and other optical products. The concept of optical density is especially widely used in the quantitative evaluation of developed photographic emulsions in both black-and-white and color photography; the methods used to measure it constitute the subject of densitometry. There are several types of optical density, depending on the nature of the incident radiation and the method of measurement of the transmitted radiation flux (Figure 1).
Optical density depends on the set of frequencies ν or wavelengths λ characterizing the initial flux; its value for the limiting case of one unique v is called the monochromatic optical density for this frequency. The specular monochromatic optical density (Figure 1,a) of a layer of a nondispersive medium is, not taking into account corrections for reflection from the front and rear boundaries of the layer, equal to 0.4343 kv l, where kv is the natural absorption coefficient of the medium and l is the thickness of the layer. Actually, kv l = kcl, which is the exponent in the equation of the Bouguer-Lambert-Beer law. If the scattering in the medium cannot be disregarded, kv is replaced by the natural attenuation coefficient. For a mixture of nonreacting substances or a series of media placed one on top of another, this type of optical density is additive—that is, it is equal to the sum of the optical densities of the individual substances or media. The same is true for specular polychromatic optical density, where the radiation has a complex spectral composition, in the case of media with nonselective absorption, that is, absorption independent of v. The specular polychromatic optical density of a series of media with selective absorption is less than the sum of the optical densities of the media.
REFERENCESGorokhovskti, Iu. N., and T. M. Levenberg. Obshchaia sensitometriia: Teoriia i praktika. Moscow, 1963.
James, T., and G. Higgins. Osnovy teorii fotograficheskogo protsessa. Moscow, 1954. (Translated from English.)
L. N. KAPORSKII