Journal of the Society of Motion Picture Engineers (1930-1949)

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700 LEE December For the great majority of the cells, the measured frequency defect ranged from 2 to 7 decibels at 7000 cycles per second. One cell showed a high-frequency drop of 8.4 decibels at 7000 cycles per second. Some additional comments are pertinent. There is no apparent correlation between sensitivity and frequency response. Such a correlation has been suggested by one of the groups working in this field,5 but our results do not confirm it. As a matter of fact, the cells which show the highest sensitivity lie at both extremes of the frequency response curve. Another theory has been advanced6 to the effect that a direct-current light bias tends to improve the frequency response of the photoconductive cells. Again, our results fail to confirm this supposition. In one of the projectors used, a directcurrent bias of the order of 3 to 5 foot-candles was present, but there is no appreciable difference between results obtained in the two projectors. [AUTHOR'S NOTE : Considerable clarification of .the data on frequency response has been possible since this paper was presented at the SMPE Convention. For this, I am indebted to Mr. John A. Maurer. At the conclusion of the paper, he asked if the sound optics had been refocused for infrared when lead-sulfide cells were substituted for the S4 vacuum phototube used as a reference. I answered that they had been refocused. Subsequently, it was found that in the case of one set of measurements this had been done, but in a second set this was not the case. When measurements were retaken with sound optics properly focused for all cells, the gross inconsistencies with which I was plagued at the tune of the Convention were removed. ] USE WITH COLOR FILM One question which has been raised very frequently relates to the use of the lead-sulfide cell with dye-image sound tracks. The modulation of the light beam in a sound optical system depends upon absorption of light by the track material. As pointed out by Gorisch and Gorlich,6 if the track material does not absorb completely in the wavelength region to which the phototube is sensitive, the signal output is decreased, and the noise level (caused by dirt, scratches, and so forth) increased. The net signal output from an optical sound system employing a photosensitive receptor is actually in the general case a very complicated story, depending upon the characteristics of the light source, film base, track material, and the detector. A