Conventional methods of microphotometry measure the portion of an incident beam of light that is transmitted through a given spectral line on a photographic film or plate. This percentage transmittance is, in effect, a measure of the density of the line; for density is equal to the
logarithm of the reciprocal of the transmittance. These methods are well founded and are not likely to be replaced for those densities on or near the linear portion of the H and D curve. However, as the density approaches the photographic saturation level, the methods become less reliable
and eventually fail. The chief causes for this failure are (1) the diminishing efficiency of the photographic emulsion when most of the silver halide has been exposed, and (2) the lack of reliability of the microphotometer when the transmittance is below about 3%.
U. S. Geological Survey, Denver, Colorado 80225
Publication date: March 1, 1967
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