British Kinematography (1950)

Feb., 1950 ross : heating of films and slides 43 II. HEATING OF FILM IN MOVING PROJECTORS* We may now consider the heating of film in a moving projector. Providing allowance is made for the different gate-sizes and speeds of travel, the same argument will apply to 35 mm. and sub-standard projectors. First, it is clearly desirable to reduce as much as possible the temperature of the metal parts of the gate, the pressure pad and film guides. This particularly helps to prevent emulsion pick-up. Spill-light and light round the edges of the aperture fall on these parts all the time, and heat is generated. It is largely the designer's concern, but much can be done by maintaining a good polish to metal surfaces so that most of the light and heat is reflected instead of being absorbed. However, some means must be provided for taking away the heat which passes into any metal part, by water-cooling or air-cooling (natural or blown) or, what is probably best where it is practical, by conduction of the heat from these small metal parts into the massive metalwork of the body of the projector. Where, on the other hand, some metal part such as a pressure pad absorbs heat and yet cannot be rigidly mounted on the main metalwork, it must be cooled in some manner to keep down its temperature. Placing the shutter between the light source and these metal parts will halve the heating effect, but cannot do more. Measurements of Temperature If attempts are made to measure the temperatures of such parts, it is extremely important that the measuring device be kept out of the light-beam,, or it will be heated up and a false reading obtained. It should also absorb only a small quantity of heat, particularly if the part being measured is made of material with low thermal conductivity, such as fibre, plastic or glass. It is essential that good contact be maintained between the part and the measuring device. A method which could sometimes be used is to embed a thermo-couple in the part. An alternative is to run the projector for some minutes and then to turn off the light and measure very quickly the temperature of the part by means of special waxes of graded softening temperatures, or by the sense of touch. Such methods, although not particularly accurate, can reveal a great amount of information if carefully used. Any attempt to measure the " gate temperature " directly by holding an instrument in the gate aperture is bound to fail, because the temperature reached depends primarily on the extent to which the instrument absorbs the radiation, and on the rate of losing heat from it by conducting or cooling. At best this method can only give a comparison between projectors. Heating of Film when Exposed to Light Secondly, and by far the more important, the film is heated in the gate by the absorption of light and heat in the silver of the emulsion, or by dyes in the case of colour film. This problem has been solved by a theoretical investigation made by Mr. Brian S. Kellett, when with Sir Robert WatsonWatt and Partners, scientific consultants to the J. Arthur Rank Organisation, and this section of this paper is written with his co-operation. Consideration is given to a particular frame during the short time when it is in the gate. In the darker parts of an ordinary print, almost all of the light and heat is absorbed by the silver particles, which are distributed all through the thickness of the emulsion layer, being slightly more concentrated near the outer surface of the layer, as shown in the greatly enlarged cross-sectional view of the film, *While this paper was in the press, Dr. F. J. Kolb published work on this subject, referring particularly to film in the gate of theatre projectors. The present results, although entirely independent, are substantially in agreement with Dr. Kolb's, with the exception of the importance attributed to air cooling of film. See J. Soc. Mot. Pic. Eng., Dec. 1949, p. 635.