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228 RICHARDSON'S BLUEBOOK OF PROJECTION
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 temperatures, as a consequence of which the film buckled, and sometimes the projector mechanism frame was warped out of shape.
Later this type of shutter was replaced by the superior rear shutter now in very general use. It is located between the light source and the film, and since it cuts off a little more than 50 percent of the light (74), it cuts off an equal amount of heat from both the film and the mechanism. This shutter is available in two radically different types, each of which will be considered separately
The Disc Type Rear Shutter
This type is, in effect, the old in-front-of-the-lens shutter somewhat enlarged in diameter and relocated just back of the projector aperture, (75) approximately five inches away. At this distance it must, with some types of beam converging element (lens or mirror), cut a light beam of considerable diameter, since the beam diameter in any direction is increased as the distance*
*The distance of the shutter from the aperture differs on different makes of projectors, though not very much. These quoted here are taken from the Super Simplex.