# optical density

(redirected from optical densities)
Also found in: Dictionary, Thesaurus, Medical.
Related to optical densities: optical absorbance

## 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.
.

## Optical Density

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.

Figure 1. Types of optical density of a layer of a medium classed according to the incident radiation’s geometry and the method used to measure the transmitted radiation flux, (a) The specular optical density Dǀǀ is determined by directing a parallel flux perpendicular to the layer and measuring only the part of the transmitted flux that retains the original direction, (b) To determine the totally diffuse optical density D, a parallel flux is directed perpendicular to the layer and the entire transmitted flux is measured, (c) and (d) Methods of measuring two types of doubly diffuse optical density D; the incident flux is uniformly diffuse. The difference DǀǀD is a measure of the light scattered in the layer under examination. In the sensitometric system used in the USSR the terms “regular,” “integral,” and “diffuse” are found instead of “specular,” “totally diffuse,” and “doubly diffuse,” respectively.

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.

### REFERENCES

Gorokhovskti, 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

## optical density

[′äp·tə·kəl ′den·səd·ē]
(optics)
The degree of opacity of a translucent medium expressed by log I0/ I, where I0 is the intensity of the incident ray, and I is the intensity of the transmitted ray. Abbreviated OD.
References in periodicals archive ?
where the rate coefficients [k.sub.1] and [k.sub.2], and the maximum optical densities [E.sub.1max] and [E.sub.2max] are four unknown constants to be estimated.
Figures 1 and 2 illustrate the optical densities of the steps on the reference films of the small and large films, respectively.
This result was validated by evaluating bacterial presence using optical densities. After incubation for 48 hours, 31 out of the 36 were contaminated (86%).
We then normalized resultant optical densities to actin to control for differences in loading.
If the correlation is significant, then the wind tunnel drag for multirow barriers, barriers with rows with different optical densities, and barriers with variable spacing between the rows can be correlated with an expression of the equivalent optical density (EOD).
Speed index was taken to be 1 + base + fog (11) and contrast index was taken to be the difference in optical densities of the steps with minimum and maximum OD values that were not part of the toe or the shoulder, respectively.
To facilitate the comparison, the optical densities of phospho-ERK shown in Figure 1A and C are plotted with the SRE reporter activation shown in Figure 1B and D.
The distance between the tower and the upwind-most row was kept great enough so that tower readings could not possibly be influenced by the windbarrier when the wind was from a southerly direction.) By changing the number of rows and/or optical density per row, data was obtained from the following combinations of row numbers and per-row optical densities: one row with 12.5% [OD.sub.PR]; two rows at 12.5% and 25% [OD.sub.PR]; four rows and eight rows at 12.5%, 25%, 50% and 75% [OD.sub.PR].
Isolates were suspended in buffered saline, and solutions were adjusted in volume until optical densities (OD) were identical (1.6 OD at 610 nm).
Another example of the excellent correlation between measured optical densities and AEC readings at both 80 kVp and 100 kVp is shown in Table 1.
Adjusted optical densities (OD) for each dilution were calculated by subtracting the [OD.sub.410] of the control antigen from the [OD.sub.410] of the SNV antigen.

Site: Follow: Share:
Open / Close