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

the overheating of film, consideration may be given to possible methods of cooling film (after filtering the light, of course). Unfortunately, it is the case that air cooling of film in a moving projector does not permit any great increase of light output. It is shown later that there is a theoretical limit to the maximum amount of heat which can be extracted from a surface, such as the emulsion; this is determined primarily by the speed of sound in air, and by the maximum temperature which the film will withstand. Under best conditions, air cooling can remove about 7 w/sq cm, but the acceptable rate of heating of film in a moving projector is about 50 w/sq cm (mean net flux). Air cooling, therefore, in itself can only permit an increase in light output of about 15%,* although Kolb has shown2 that highvelocity air jets serve a useful, but different, function in helping to hold the film flat in the gate. Future Developments. Should the need ultimately arise, it is, however, possible to foresee moving projectors with several times as much light output as those of today, perhaps using a "blown-arc" or large arcs and different optical systems. Looking into the future, we may tentatively consider some of the means of preventing the additional light from damaging the film, several of them being well known. In every case, of course, it will be necessary to filter the light. (1) Using 70-mm film would permit four times the gate area but probably only about three times as much light, since grave difficulties might be experienced with maintaining the film flat in the gate; there would also be * The relevant experimental results published by Dr. Kolb for film in a moving projector are in the right-hand curve of his Fig. 8 (ref . 2, page 654). This curve shows a maximum increase of light output due to air cooling of 30%, and does not substantiate his claim for a possible increase of 50%. much disturbance to printing and processing equipment. (2) Running the film faster would give it less time in the gate, but reference to Fig. 4 shows that a three-times increase in speed, and therefore threetimes film costs, would only permit a two-times increase of light output. (3) Since the rise of temperature of the rear surface of the base while being exposed is about 1/100 of that of the emulsion side, it will be quite useless to try to reduce the over-all temperature by cooling the rear surface. (4) It might be possible to surround the gate with a cell with glass windows, filled with liquid, so that the film is immersed in the liquid while it is in the gate. Water is the best cooling medium, and a two and a half-times increase of light could be obtained. Due to the poor conductivity of liquids the temperature distribution through the water would be similar to that shown in Fig. 3 through the film base, except that the heat would penetrate a little further through the water. Even so, only about 0.002 in. thickness of water is absorbing any significant amount of heat. There would be the real complication in drying the film before spooling it, even if another liquid were used. (5) Cooling with a rapidly moving stream of liquid would be more effective. If glass plates were placed close to the film, to form narrow channels for the cooling fluid which would be pumped through at high velocity, it might be possible to obtain about a four-times increase in the light output. (6) On several occasions a liquid has been applied to the picture area of the emulsion so that it is evaporated away while in the gate, some of the heat developed in the emulsion providing the latent heat of evaporation instead of heating the base. Only the liquid on the emulsion side of the film assists in preventing damage to the base. The difficulty is that the heat has to travel through the layer of liquid from the Hugh McG. Ross: Cooling of Film 545