This investigation was carried out in 1943 and 1944 and was undertaken primarily to determine the relation between the sphere and pot-opal types of densitometers, and the factors which affect their reproducibility from specifications, preliminary to the formulation of an American Standard specification for the measurement of photographic transmission density.
The two sources of error which have been given the greatest consideration are the directional variation in the luminance of the sphere or opal glass used to illuminate the test sample, and the “build-up” of the luminance of the illuminator by light reflected back from the test sample.
The net difference between the pot-opal and sphere densitometers was found to be about 0.04 at low densities, decreasing to about 0.01 to 0.02 at high densities.
The differences in density as measured by the sphere and the pot-opal densitometers were determined by computations based on the optical properties of the instruments and the test materials, with final checks by direct measurement. The differences between the pot-opal and sphere densities determined by these two methods agree within about 0.005 between 0 and 2.0, and within about 0.01 from 2.0 to 3.0.
It is concluded that the inherent difference between the sphere and pot-opal densitometers is small and that either can be readily reproduced from specifications.
K. S. WEAVER, "Measurement of Photographic Transmission Density," J. Opt. Soc. Am. 40, 524-533 (1950)