British Kinematography (1950)

48 BRITISH KINEMATOGRAPHY Vol. 16, No. 2 III. COOLING OF SLIDES IN STILL PROJECTORS The cooling of slides in a still projector is different from film in a moving projector in that the slide has to remain exposed for a long period and the thermal conditions reach equilibrium. The heat is absorbed from the beam of light at a constant rate whatever the temperature of the slide, but as the slide warms up the efficacy of the cooling increases. After a few minutes the rate of losing heat will become the same as the rate of absorbing it, and the temperature will rise no further. This temperature must not be so high as to damage the slide. Cooling by Natural Convection of Air Most simple projectors rely on natural air cooling of the slide, and this method is frequently used successfully for dissipating the heat absorbed by a glass filter, such as Chance ON20 glass. The air adjacent to the glass is heated and rises, thereby causing a natural cooling draught. In a slide the heat is mainly developed in the dense parts of the picture, but with natural cooling it travels to some extent all over the slide, the whole of it becoming hot. The maximum temperature which gelatine will withstand for 30 minutes without turning brown is about 180° C.10 and this appears to be the limiting factor, although on some slides the glass will break before the gelatine chars. With natural convection of air, the rate of cooling is theoretically equal to 00004 Tl'2b/h0'25 watts per sq. cm.11 where T= temperature of slide above temperature of air (°C) h = height of slide (cms.). This presupposes no obstruction to the air from the slide holder, and also no cover glass ; the total area is twice the area of the slide, because heat will be lost from both sides. If a cover glass is used, the cooling will be less effective because of poor heat transfer from the emulsion to the cover glass. For cut-film sandwiched between two glasses, the cooling will be reduced still further. Particularly in the case of glass heat-absorbing filters, which can run at a higher temperature than a slide, a significant amount of heat will also be lost by radiation : — Rate of cooling=5-7.s.(ri4— r24).10~12 watts per sq. cm. where £=emissivity =0-9 for glass T\= temperature of glass (°K) T2= temperature of surroundings (°K). It is found by experiment that the maximum safe intensity of heating of a 3| in. square slide without a cover glass is 0-25 watts per sq. cm. A glass heat-absorbing filter will withstand about one watt per sq. cm. with natural air-cooling. Forced Air Cooling of Slides Where a greater light output is required it is necessary to use forced cooling of the slide, air cooling being far the most convenient. A similar technique might be used to remove the heat from a glass heat-absorbing filter, or even a colour filter used in colour stage effects. In such a system the dense parts of the picture become hot and the clear parts remain fairly cool. Heat is not conducted well from one part of the slide to another, and it is calculated, and confirmed experimentally, that if a large dark area is adjacent to a large clear area, the temperature has fallen to a third at a distance of about 0-5 cm. from the dark area. As a result of this uneven heating and consequent differential expansion, large stresses may be set up in the glass, and failure occurs by fracture of the glass, usually along the boundaries between dense and clear areas.