British Kinematography (1950)

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38 Vol. 16, No. 2 THE HEATING OF FILM AND SLIDES IN PROJECTORS Hugh McG. Ross, M.A* Read to the British Kinematograph Society on November 2, 1949. IN virtually every projector the designer and users are faced with the problem of the heating of the film or slide in the gate. This is particularly severe with high-power theatre projectors or process projectors used in film studios for front or rear projection shots, or when the output of sub-standard moving or strip-film projectors is increased. It is, however, valueless to attempt to measure the temperature in the gate aperture with the aim of keeping this below the temperature at which the film or slide is damaged. Rather, the cause of the heating must be considered. I. THE CAUSE OF HEATING The energy radiated from the light source of a projector may be analysed as in Fig. 1, showing the distribution of energy throughout a spectrum which extends from the ultra-violet region, through the visible spectrum, and into the Fig. i. Spectral distribution of energy from high-intensity carbon arc, and from tungsten projector bulb. Tungsten Bulb At 3ioo°K (SOO WATT 115 VOLT/ ZQ 22 2 4 Wavelength >* infra-red region. The curve for the high-intensity arc is considered to be fairly typical, although S.H.I, carbons may give a slightly greater proportion of visible light. The curve for the ultra-violet and visible wave-lengths was measured at the gate position of a Stelmar arc,1 while the part of the curve in the infra-red is based on the radiation of a black-body at 5500 °K2. This curve takes into account the radiation from the flame near the crater, as well as the radiation from the crater itself, this being correct for normal mirror or condenser lens optical systems. The curve for a tungsten filament lamp (derived from Ref. 3) is typical of a Class Al projector bulb as used in sub-standard equipment and film-strip projectors. The two curves are drawn to show approximately equal visual brightness, ^rgddemonstrate how the arc gives a lower proportion of heat, for the same visual c-rT^ct *Formerly J. Arthur Rank productions Ltd.