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696 LEE December
decibels. The data were taken in a 16-mm projector using the Motion Picture Academy 400-Cycle Signal Level test film. The exciter-lamp color temperature was approximately 3100 degrees Kelvin. The curves illustrate the desirability of matching the load resistance to the resistance of the photoconductive cell. A total of about 20 lead-sulfide cells has been tested, from five different manufacturers. The sensitive areas ranged from l/& X 1/& inch to y4 X */4 inch. This alone can account theoretically for a range of output up to 12 decibels, since the size of the light spot was just about correct for the smallest surface. However, it is worthy of note that some of the cells which give the largest output are not those with the 1/g X Vs-mch area, but l/y X Y4-inch. The variations from cell to cell are undeniably large. This situation seems to be improving, however, and it is encouraging that five cells recently received from one manufacturer fell within a 10-decibel sensitivity range. All manufacturers report encouraging results in their attempts to reduce variations from cell to cell, and any single cell seems to be quite stable and long-lived.
EXCITER-LAMP COLOR TEMPERATURE
Fig. 4, curve A, shows the results of measurements made on leadsulfide cells to determine the effect of exciter-lamp color temperature on signal output. The data were taken with several different exciter lamps in a 16-mm projector, using the Motion Picture Academy 400Cycle Signal Level test film as a signal source. The exciter-lamp voltage and current were measured, and the brightness temperature of each lamp as a function of voltage was independently measured by a Leeds and Northrup optical pyrometer. Corrections to the measured brightness temperature were computed from data for tungsten given on page 1993 of the "Handbook of Chemistry and Physics," Chemical Rubber Publishing Co., Cleveland, Ohio, 24th edition, 1940. The absolute values of color temperature are probably not very precise. However, they are precise enough to show that, as one might expect from the spectral response of the cells, the lead-sulfide cell is much less sensitive to exciter-lamp color temperature than either the Si or 84 phototube.4
Curve B in the same figure is a plot of the total radiated power from a tungsten lamp as a function of color temperature, determined by thermopile measurements. Since curve A was taken simply by varying the voltage on an exciter lamp in a given optical system, the data represent the response obtained from a constant area of the light source.