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

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noise spectrum, and the noise varies with the light level in the same manner as shot noise, the curves would be correct. If the noise characteristics are known, the same method of computation can be applied to any type photocell. The character of the noise introduced by the lead sulfide tube may be different from that of shot noise and perhaps the absolute noise level would be greater. Mr. Ken: Except that our experience has been that the signal-to-noise ratio was much higher on the lead sulfide tube.* Mr. Horak: I'm not too familiar with lead sulfide tubes, but I believe the noise level is perhaps a little higher and the signal level is much higher. Mr. Ken: That's right. George Lewin (Signal Corps Photographic Center): Are the optimum values you have arrived at by this investigation radically different from practice in commercial projectors? * Lowell O. Orr and Philip M. Cowett, "Desirable characteristics of 16mm entertainment film for Naval use," Jour. SMPTE, 58: 245-258, Mar. 1952. See especially p. 249, Use of Sulfide Photoresistive Cell. Mr. Horak: We measured one Eastman Model 25 Projector and it has a slit height of approximately 0.5 mil, which corresponds to an h/\c of about 0.55. This measurement was made without checking the focus and azimuth adjustments. As I pointed out, the film noise is the dominant factor. If you have sufficient illumination on the phototube, it doesn't really matter, within wide limits, what slit height you have. The determining factors are how much do you want to equalize and how critical do you want your adjustments to be. Anon: Can you manufacture projectors with these optimum slit heights? Mr. Horak: These slit heights are within practical manufacturing ranges. The 16mm projectors need to have equivalent slit heights of between 0.64 and 0.73 mil. The 35mm projectors have, I believe, a slit height of about 1.2 mils. The h/\c of 0.71 would be equivalent to 1.64 mils — that's the optimum for phototube noise — that would be 1.64 mils for the 35mm reproducers, and the standard is 1.2 mils. The standard was apparently selected on the basis of practical equalization rather than on the basis of minimum electrical noise level. Grim wood and Horak: Reproducer Slit Height 387