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310 RICHARDSON'S BLUEBOOK OF PROJECTION
dent that the narrower the shutter blades are the greater is the amount of light that will pass through to the screen. What then determines the necessary width of the master blade, and therefore of the cut-off blade?
You will recall that the master blade must be wide enough to "close the lens" while the film is in motion over the aperture, although, as indicated, a slight opening is permissible at the start and stop of the intermittent sprocket. It is evident, then, that the shorter the time the intermittent movement is in motion the shorter the time the lens must be covered. (71) Put another way, the faster the speed of the intermittent movement and, therefore, the intermittent sprocket action, the less time must be given to covering the lens and therefore more light will pass through to the screen.
The projectionist can do nothing to alter the intermittent speed. Most movements are now what are known as "three-to-one." One complete rotation of the cam carrying actuating pin D, Fig. 134, is one complete cycle of action. Divide that action into four equal parts : the intermittent movement star, and therefore the intermittent sprocket mounted on its shaft, is in action moving the film down through one of those parts, and standing still through three of them. (72) "Three to one" has been shown to be as fast a speed as the film sprocket hole edges and intermittent movement parts will stand. If it were faster, the movement would wear out too rapidly and the film sprocket hole edges tear up and break apart. For best results the film lies over the aperture three times as long as the film is in motion past the aperture.
Old and New in Shutters
(73) The old practice was to locate the rotating shutter on the screen side of the projection lens, having no effect whatever upon the heat which was concentrated continuously upon both the film and the projector mechanism cooling plate. Where a high amperage was used (or a reflector type lamp with no condenser to absorb and dissipate a portion of the heat) the film and the mechanism were both subjected to very high tempera