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1949 AIR COOLING OF FILM 645
usually visible only to a critical observer, but in-and-out of focus and blistering are two phenomena which make it impossible to obtain a satisfactory screen image; they take the control of image quality completely out of the hands of the projectionist. Accordingly, it may be said that the first four of these phenomena are necessary or at least harmless occurrences, while the fifth is borderline, and the last two must be avoided.
Some confusion has existed in the descriptions of film behavior because of the use of several names for describing the same effect. This discussion follows the terminology of Carver, Talbot, and Loomis,2 in order to keep separate the terms that describe physical behavior of film from those that describe thermal behavior.
(1) Negative Drift
It was shown by Carver, Talbot, and Loomis1 that film in the aperture is almost never flat and that its position bears no relationship to the curl or other physical shape it may have either entering the top of the projector gate or leaving the bottom of the gate. Film in the aperture under the influence of the light beam behaves as though the emulsion surface were expanding with reference to the base dimension, so that each frame is distorted into a pincushion shape with the emulsion surface on the convex side; since the edges are held, the center of the frame is displaced toward the arc.* This is a perfectly normal phenomenon occurring in all cine projection. High-speed pictures taken into the aperture show the center of the frame beginning to move toward the light source (which in our convention is called motion in the negative direction) from the first instant of its exposure to light. During the passage of the flicker blade, a partial recovery is effected, and with the second exposure to light the negative drift resumes and is carried to its maximum amplitude. Just how far negative the film drifts depends upon the intensity of incident radiation; the greater this intensity the greater will be the film motion.
* This conclusion assumes standard emulsion position for theater projection. If the emulsion position is reversed and the film is threaded emulsion to lens, as in some process projectors, and much 16-mm projection, it is still the emulsion surface that expands with reference to the base; accordingly the center of the frame is now displaced toward the projection lens. This paper assumes theater standard emulsion position throughout; if the results are to be applied to projection with the nonstandard emulsion position, the direction of film motion with respect to the projector will be reversed for negative drift, focus drift, image flutter, in-and-out of focus, and positive drift.