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PKOJECTOR MECHANISM 463
of these moves the claw in and out of the perforations, and the other advances the film one sprocket hole spacing — about 0.300 in. — at a time. The intermittent mechanism of the projector is a critical part, since imperfect functioning may cause pictures to jump on the screen, excessive wearing of the film, tearing of the perforations, and losing of the loop below the picture gate with consequent serious film mutilation. A 3-toothed claw is to be preferred to a 2-toothed claw because of the better chance of a film with a damaged perforation going through the film gate without interruption. Because of the complexity of the motions of the intermittent parts of the machine, such parts may wear very rapidly and lose their accuracy of movement if poorly designed, poorly manufactured, or made of unsuitable materials.
It is necessary that the light beam from the projector be cut off while the film is moving through the film gate. This is accomplished by a simple rotating sectored disk, called a shutter,* that is rotated by the projector gear train in synchronism with the movement claw of the film gate. Most shutters cut off the light between the projection lamp and the film gate, thereby eliminating heating of the film while it is in motion. The shutter interrupts the light beam not only while the film is being moved through the gate, but also when the film is stationary in the aperture. This latter interruption occurs for the purpose of reducing flicker, which would be intolerable if the additional light interruption did not occur. At conventional brightness, machines designed for sound projection speed (24 frames per second) and for combinations of sound speed with silent speeds provide two interruptions per frame, one during the film-moving period and the second during the period when the film is stationary in the aperture. Machines designed for silent speed alone (16 frames per second) are preferably designed with three interruptions per frame, one during the film-moving period, and two during the period when the film is stationary in the aperture. To obtain the maximum amount of light from the projector lamp, manufacturers seek to keep the light on the screen as long as possible consistent with minimum flicker. Incidentally, when the film is stationary in the aperture but the light is cut off, the movement claw moves downward, being actuated
* To reduce flicker still further, 35-mm machines of recent design such as the Simplex E-7 have two rotating shutters, one between the lamp house and the aperture, and the other between the objective lens and the screen. With such shutters, the image may be said to be swept on the screen in one direction, and off the screen in the other. Light efficiency is improved and flicker reduced compared with the single-blade shutter type.