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ti42 KOLB December
Additional evidence for believing that only the image absorbs energy is given by the behavior of dye-image films which are relatively transparent in the near-infrared region, even for relatively high absorption and density in the visible. Such dye-image films show less heating and less of the thermal effects than silver-image film does when projected at the same intensity. We have found this advantage of dye-image films to be roughly proportional to their transparency in the infrared.
What actual temperature the film reaches during projection is an interesting question that has never been answered satisfactorily. Obviously, the limitations of pulldown time, together with the limitations of the size of the film, make direct experimental measurement extremely difficult. A number of estimates have been made, however, and perhaps the best is that of Paschkis,10 who set up an electrical analogy to the heat-transfer problem and thereby estimated the temperatures at various positions across the film thickness as a function of time during the projection cycle. The accuracy of these values theoretically is limited only by the accuracy of the necessary assumptions that must be made in setting up the problem. For example, one must assume the proportion of incident energy absorbed by the film, the location of that absorption, the thermal constants of an emulsion containing the metallic sponge of a silver image, the nature of the heat loss from the film surfaces, and so forth. The results of his estimate are presented in Fig. 4, which indicates the temperature rise in the film cross section at intervals from first exposure to temperature equalization after pulldown. The actual temperature reached by thefilm during the projection cycle depends first upon its initial temperature when it enters the aperture, and second upon the amount of energy absorbed. The temperature rise above the initial temperature is directly proportional to the amount of energy absorbed, as is indicated in the ordinate of Fig. 4, which is labeled A77/Q, the temperature rise in degrees Fahrenheit for the absorption of 1 watt per square millimeter of radiant energy. It will be observed that this energy figure is not the "mean net energy" used throughout the rest of this paper, but the "instantaneous net energy" incident upon the film during the time that the beam is uninterrupted by the shutter blades.
Estimated temperature distributions in the emulsion layer — the peaks of the curves in Fig. 4 — depend somewhat upon where in the emulsion cross section absorption is assumed to take place. Even