British Kinematography (1950)

Feb., 1950 Ross : heating of films and slides 47 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. (i) 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 much disturbance to printing and processing equipment. (ii) Running the film faster would give it less time in the gate, but reference to Fig. 6 shows that a three-times increase in speed, and therefore three-times film costs, would only permit a two-times increase of light output. (iii) Since the rise of temperature of the rear surface of the base while being exposed is less than 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. (iv) The maximum amount of heat which could be extracted from the emulsion side by forced air cooling is limited by the greatest air velocity which can be obtained. Even with a 300 m.p.h. blast of air on the film, only an extra 10 per cent, of light could be used. Air cooling of moving film is therefore worthless. A small air-jet may, however, have another use, in blowing away the steam which is formed when the small amount of water always present in gelatine is driven out by the heat. (v) 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 2£-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. 5 through the film base, except that the heat would penetrate l|-times as far through the water. Even so, only about 0002 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. (vi) 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. (vii) 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 emulsion to the outer surface of the liquid, where the evaporation to the air is taking place. Due to the poor thermal conductivity, not a great deal of heat is transferred, and instead of evaporating smoothly the liquid might boil off, which might appear on the screen. A high velocity blast of air might assist in preventing this. It is estimated that a two, or perhaps four-times increase of light might be obtained, (viii) Doubling the thickness of the emulsion would only permit a small increase of light, because such a small proportion of the heat is in the emulsion. (ix) Because the basic difficulty arises from the poor conductivity of the film base, some improvement would result ift he thermal conductivity or thermal capacity of the base material could be significantly increased. Such a change to the base material appears most unlikely. It may be concluded that there are several ways in which the light might be increased about four-times above the present maximum (after filtering), but each would introduce severe practical difficulties. Even by a combination of the methods at present foreseen it would hardly be possible to obtain a ten-times increase.