International projectionist (Jan-Dec 1950)

Screen illumination in theater projection is limited by a maximum radiant-energy flux through the projector aperture. Excessive flux heats the film beyond its safe operating temperature, producing high-intensity effects in the film with loss of image quality on the screen and possible film damage. An increase of 30 to 60 per cent in permissible flux and in attainable screen illumination can be effected if film is cooled and its position controlled by high-velocity air jets directed at the film in the aperture. The high-intensity film effects are discussed and improvements in film behavior with air cooling are described herein. MAXIMUM screen illumination obtainable in motion picture projection has been limited by a number of practical factors. In recent years with the development of more brilliant light sources, faster optics, and improvements in mechanical design, it has become apparent that the film itself is one limiting factor, and that it does not perform satisfactorily beyond a certain maximum intensity of radiation. This limit has already been reached with the lamphouses and projectors currently offered for the first-run, de luxe theaters. Nevertheless, the realization that more light could be used to advantage has been increasing. Especially significant has AIR COOLING ness level would improve comfort and convenience as well as permit more illumination of the surroundings. Focus vs. Radiation Intensity The first discussion of the problems of film behavior under the high projection intensities necessary for maximum screen light was given by Carver, Talbot, and Loomis,1 who pointed out the in-and-out of focus phenomenon and showed its dependence upon radiation intensity. The work described in the present report is a natural sequel to this earlier discovery. Basically, the maximum permissible radiation on film is set by a maximum film temperature. Film is heated during of Motion Picture Film for Higher Screen Illumination By F. J. KOLB, Jr. Eastman Kodak Company been the growth in popularity of the outdoor theater, where the viewing conditions demand an unusually large screen area — by comparison with indoor standards— if the seating capacity is to be made large enough for the theater to operate profitably. This larger screen size should be accompanied by an increase in total screen light in order to keep the screen brightness at a satisfactory level. There have been other, although less pressing, reasons for increasing total screen light for indoor theaters, as well as for background projection, and for 16-mm projection. In these cases, little increase in screen size would be attempted, but an increase in the bright t J. Soc. Mot. Pict. Eng., Dec, 1949. projection by a net absorption of energy in the photographic image, and the highintensity projection effects result from the unknown, but fairly definite, temperature levels that the film reaches in the aperture. There are at least four ways of increasing the brightness of the projected image without increasing correspondingly the thermal misbehavior of the film: 1. Increase the Directional Effect of the Projection Screen A screen that is not a good diffuser can concentrate the reflected light into a beam whose angle is more nearly limited to that angle within which the 1 E. K. Carver, R. H. Talbot, and H. A. Loomis, "Effect of High-Intensity Arcs Upon 35-mm Film Projeciton," J. Soc. Mot. Pict. Eng., vol. 41, pp. 69-87; July, 1943. audience is seated, and thus there can be an increase in apparent screen brightness without an increase either in total light upon the screen or in total radiation incident upon the film. 2. Reduce the Heating Effect of Nonvisible Radiation Film is subjected to an amount of infrared radiation which, in the light from a modern high-intensity carbon arc, is approximately equal to the visible energy; all of this infrared can be as effective as the visible in heating the film but contributes nothing to the brightness of the projected image. Such infrared radiation can be reduced by filters which absorb the infrared and transmit the visible; a theoretically perfect filter (removing all of the infrared and transmitting all of the visible) would permit a doubling of the permissible screen illumination without increasing the resultant heating of the film. In addition to the heating produced by infrared radiation, there may be heating by other "useless" radiation. Any energy incident upon the film which does not contribute to the brightness of the projected image, tends needlessly to increase film temperatures. On some projectors, the shutter arrangement permits radiation to fall upon the film during the time that none is transmitted to the screen; in other equipment, the film is illuminated by a cone of light of greater aperture than the projection lens can accept. Finally, some "indirect 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. 3. 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 high-velocity air jet upon it so that the coefficient of heat INTERNATIONAL PROJECTIONIST January 1950