A Photometer for the Measurement of Low Illuminations

In the manipulation of photographic materials the provision of satisfactory darkroom illumination is of vital importance. This illumination must be of such intensity and quality that a negligible amount of fog is produced when these materials are exposed to its action for the time required to perform the necessary operations on the material. At the same time it is desirable that the illumination be of sufficient intensity and quality so that the operator may work with comfort and certainty. In the earlier days of relatively low-speed materials sensitive only to the blue and blue-green, the problem of illuminating the darkroom was a relatively simple one. These materials could be manipulated under relatively high levels of red illumination without danger of producing objectionable fog. Satisfactory seeing was obtainable with illumination levels adjusted so that the time to produce a perceptible fog was several times as long as that required for the manipulation of the materials. Under these conditions the precise measurement of darkroom illumination was not of great importance since the allowable latitude in intensity was relatively great. With the advent of the high-speed materials sensitive to all wavelengths of the visible spectrum, this problem of providing satisfactory illumination in the darkrooms has assumed greater importance and more and more difficulty has been met in obtaining a darkroom illumination which is satisfactory from the standpoint of both photographic safety and visual requirements. It is necessary to use the maximum allowable illumination which can be tolerated without producing perceptible fog so that the operator may see as much detail as possible. This means that the variation in illumination which can be tolerated is very small and hence a method of measuring these illumination levels with relatively high precision is very desirable. Practically all of the portable photometers and illuminometers on the market are designed specifically for measuring illuminations of relatively high value, such as exist in offices and factories illuminated by natural and artificial means. While these instruments are very satisfactory for this purpose, they do not, in general, function satisfactorily when an attempt is made to use them in darkrooms illuminated for the manipulation of modern high-speed panchromatic materials. The instrument described in this paper was constructed specifically to meet these special requirements of dark room photometry and in its design careful consideration was given to those fundamental visual characteristics which determine the precision with which photometric readings can be made.

Print ISSN

0096-6460

Published

1928-09

Content type

Original Research

DOI

10.5594/J00766