Densitometry

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Densitometry

 

a branch of photographic sensitometry devoted to measurements of the absorption and scattering of light by developed photographic layers.

The methods of densitometry make possible the quantitative evaluation of the final photographic effect based on the optical density of blackening in the photosensitive layer. There is a nearly linear relationship between the surface concentration of silver in the blackened area and the optical density; their ratio is called the photometric equivalent of blackening. The optical density increases with the degree of dispersion of the silver in the blackened area. Correspondingly, the extent of the optical usefulness of the silver in the blackened area increases with the disperseness of the silver halide in the original light-sensitive layer and with the relative magnitude of the exposure of this layer to light. Because of the nonhomogeneous nature of the blackened areas, the light absorption in these areas is accompanied by strong diffusion. Therefore, the magnitude of optical density depends on the geometric structure (aperture) of the light beams that are incident on the blackened area and are directed to a receiver after passing through the blackened area.

A distinction is made between regular density (D ║) and integral density (DE), which are measured upon illumination of the blackened area by a beam of parallel light rays and upon reception, in the first case, of only that part of the light beam that has not changed direction during passage through the blackened area and, in the second case, of the entire light beam. In addition, diffuse density (D#), which is measured using an ideally diffuse beam of light to illuminate the blackened area, and effective density (D ψ), which is measured under intermediate conditions encountered in practice, are also distinguished. The difference D ║ -DE is a measure of the light diffusion in blackened areas. The diffuse density is commonly used in sensitometry.

The optical density of blackening is measured using densitometers and microphotometers.

The measurement of color fields in developed materials for color photography constitutes a special branch of densitometry.