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

Table I. Slit Image Heights and Relative illuminance Data Relative illuminance in db Slit height at film plane Measured Calculated Equivalent Theoretical (avg.) Correction factors in db Signal level Shot thermal noise noise 0.127 0.127 0 0 0 0 0.290 0.300 7.47 7.82 -0.35 -0.18 0.447 0.432 10.63 12.77 -2.14 -1.07 0.605 0.490 11.73 15.18 -3.45 -1.73 0.861 0.600 13.48 16.10 -2.62 -1.31 1.047 0.735 15.26 18.25 -2.99 -1.50 1.180 0.870 16.72 18.80 -2.08 -1.04 length curve with the theoretical amplitude-wavelength curve of a perfect slit. The values of slit heights which are used here in plotting measured noise levels have been determined by this technique. The precision of determining slit height from slit-loss data becomes rather poor for very small slit heights, since it was not practical to make measurements at wavelengths of much less than 1 mil. However, measurements of the light distribution along the height of the image of the primary slit show that the uniformity is very good for the smaller values of secondary slit height when the secondary slits are properly aligned with respect to the primary slit image. At the smaller slit sizes, the equivalent slit height approaches the value determined by dividing the measured height of the physical secondary slit by the magnification. For the smallest value of secondary slit, this calculated value is used since it is corroborated by the equivalent slit-height data. The theoretical curves are also computed on the assumption that the illuminance upon the phototube is proportional to the slit height. Measurements show that this assumption is not, in this instance, justified. Although the data are consistent within each slit height, there are inconsistencies between the series of data for the several slits due to factors such as the necessary realignment of optics when changing slits, adjustments of lamp position for minimum microphonics, etc. From the relative illuminance data for the several slits, appropriate correction factors are determined which are applied to all signal and noise level data. Table I gives the slit image heights, relative illuminance data, and the correction factors. For each of the seven slits, a series of measurements were taken of signal levels for both chopped light and calibrated film, film noise level, phototube noise level, and amplifier noise level. Nondiffusing neutral densities placed in the light beam ahead of the phototube were used to control the ratio of phototube noise to amplifier noise. The illuminance at the phototube, with the smallest slit in place and no density or film in the light path, was such that phototube noise was 7 db above thermal noise. Since the total noise level and the thermal noise level were known, the phototube noise level could be calculated. The correction factors of Table I were applied to these data, and all measurements were adjusted to a common signal reference level. The slit sizes are expressed as ratios of slit height to the wavelength corresponding to a cutoff frequency of 8000 cycles/sec. The results are shown by the curves of Fig. 7. Curve A is the theoretical relation between shot noise and slit height taken from Fig. 3. The crosses are experimental points. Similarly, Curve B is the theoretical thermal noise versus slit-height relation of Fig. 4, the squares representing experimental data. 384 November 1952 Journal of the SMPTE Vol. 59