Cinematographic annual : 1931 (1931)

STRAIGHT LINE AND TOE RECORDS 179 (3) The intensity scale represented by the flashing lamps, where the time of exposure is about 1/20.000 of a second and the light intensity varies up and down from 10.000 or 15,000 meter-candles. The specifications of light quality cannot be stated generally. This scale is not involved in the present discussion. (4) The intensity scale of the printer, where the light source is usually a 60watt tungsten lamp and the positive film is exposed through the negative. The time of exposure is about 1 /40 second and the intensity of light transmitted by the negative varies from a few score to a few hundred meter-candles. The relations between the gammas derived from exposures on the above scales involve the failure of the reciprocity law and the dependence of gamma on the color of the exposing light. It is known, for example, that blue light gradations yield a lower gamma than gradations of yellow light and we may expect that, apart from failure of the reciprocity law, the daylight time scale gamma will be lower than the tungsten time scale gamma for the same development. The relation of printer intensity scale gamma to daylight time scale gamma will involve the color difference between tungsten and daylight as well as the difference between time scale and intensity scale exposures and the difference in source intensities. Experiment has shown that for positive film in a given developer, the H & D curve for a definite gamma has the same shape at the toe, straight line, and shoulder, whether obtained by time scale or intensity scale exposure.* It has also been shown experimentally that for a given development time in the same developer the following ratios exist between the daylight time scale of the Eastman sensitometer and the light-valve time scale of the Western Electric recording machine, and between the Eastman scale and the tungsten intensity scale of the printer: If the Eastman daylight time scale gamma = 1.00, then (1) the light-valve tungsten time scale gamma = 1.05 (2) the printer tungsten intensity scale gamma = 0.95 (3) the Cinex (tungsten time scale) gamma = 1.05 These ratios are derived from data collected for the purpose from most of the Hollywood laboratories that process variable density "sound film. The values in the table apply to a wide range of positive and negative gammas. The intensity gradations of the printer scale are understood to be gradations of effective printing densities of the negative printed. The significance of this specification will be made clear in what follows. H&D curves are customarily plotted with visual ** densities as ordinates against logarithms of exposure as abscissas. Exposures are given in meter-candle-seconds, and densities are measured visually by the polarization photometer head (or equivalent) with diffuse illumination of the film area being measured. In the Bell & Howell backshutter printer, the light source sends light to the negative through two apertures in a line, and so the illumination is to some extent specular. Moreover, unless the negative being printed is neutral in spectral transmission, its density as appreciated by the positive emulsion will differ from its visual density by reason of the difference in spectral response of the film and the eye. The ratio of printing density ' We exclude from consideration sensitometcrs using extremely low intensities of illumination. ** "Visual" means in every Case "visual diffuse.'