Equidensitometry is the technique of measuring equidensities in a photographic deposit or photographic layer, such as photographic films and photographic plates.
Four techniques of obtaining equidensities are:
Interpretation or measurement of an image is sometimes aided by simplification of the density pattern. This results in data reduction and enables keying (areas or lines with iso-densities to unique patterns, grey values or colours). This keying to singular grey- or colour values is known in modern image processing as using L.U.T.s.
The fact that a certain density of a photographic layer has been caused by a specific amount of actinic radiation (visual light, UV-light, X-Rays, autoradiography, etc.) can be used to qualify and perform spatial measurements of a physical phenomenon. A calibrated photographic process is used to obtain quantitative measurements as well.
Equidensitometry with Agfacontour made it possible to obtain reproducible equidensities similar to pseudosolarized photographic films and prints. The Agfacontour film contained two special emulsions with each different spectral sensitivity. Using color filters (yellow or magenta) during the exposure on Agfacontour one could control the width of the equidensity. By varying the exposure one could change the density in the image that showed on Agfacontour as equidensity. Although the material exhibited very low sensitivity (long exposures were necessary) the results were very reproducible and second order equidensities were sharp and clear, something that with pseudosolarization was almost impossible to achieve without special procedures.
In the following fields equidensitometry is used extensively: