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1949 AIR COOLING OF FILM 637
heating" may be experienced, if the spill-over illumination intercepted by aperture baffles, and so forth, is allowed to raise the temperature of the film trap and other metal surrounding the film path.
(8) Increase the Cooling of the Film
Energy absorbed by the film during projection goes only partly into raising the film temperature, since some energy is reradiated from the film to cooler surroundings, and some is lost by convection to the air. If the amount of energy that the film loses can be increased, there can be an increase in the amount absorbed with no rise in film temperature. One method of increasing the rate of heat loss from the film is to impinge a highvelocity air jet upon it so that the coefficient of heat transfer from film to air is considerably increased.
(4) Restrict the Film Motion Mechanically
As will be described later, some of the high-intensity effects limiting the radiation intensity on film appear as an undesirable film motion during the projection interval when film is supposed to be steady in the gate. In some cases, this motion can be restricted or prevented by mechanical means, such as the glass plates used on each side of the film in some viewers, or the forces exerted by streams of air directed at the film in the projector aperture.
This report covers only the application of the latter two methods of increasing permissible screen illumination, that is, increased cooling of the film, and mechanical restriction of film motion.
PREVIOUS WORK
Air cooling of film has long been an attractive possibility for extending the satisfactory operating range in projection, and there have been many attempts to develop theater and background projectors — for both motion pictures and slides — employing air cooling. For the most part, such equipment has used a rather large volume of air at low velocity, effective in a partial cooling of the metal structure of the gate but much less effective than high-velocity air in cooling the film itself. Also it is only recently that the movement of film during the instant of projection has been known, so that in this earlier work positioning problems were imperfectly understood.
Several years ago, the Research Council,12 in preparing its recommendations for the design of a process projector, called for air cooling as an additional safeguard for film to be used under the most severe