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636 KOLB December
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 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 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