Equidensitometry

a method of photometric measurement without instruments, widely used in astronomical photometry, spectrochemical analysis, X-ray diffraction analysis, and interferometry.

In equidensitometry, a halftone photographic image of an object is converted into an image consisting of lines made up of geometric points at which the value of the optical density does not exceed some narrow range. In the first applications of the technique, a negative of the object and a positive obtained from the negative were superimposed, and a combined image was then printed in transmitted light. The combined printed image was a set of curved lines of equal density (called equidensities, which gave the technique its name). Later, the method was replaced by direct printing from the negative and partial reversal of the positive by means of the Sabattier effect (seePHOTOGRAPHIC EFFECTS). In the 1960’s and 1970’s a third variant was used, in which the printing was done on special double-layer photographic materials.