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464 XIII. PROJECTION AND PROJECTORS
by one cam, but the second cooperating cam does not permit the entry of the claw into the perforations. As a result the film does not move. This type of claw movement is referred to in the trade as a "skip" movement.
The single-blade shutter used in 16-mm machines harks back some 40 years or more in its design; no doubt the study given to the flicker and the brightness characteristics of television systems should soon begin to react favorably upon the design of 16-mm projection transport systems. Flicker and brightness characteristics of 16-mm projectors are deserving of much intensive study, since, as one possible means of improving light efficiency and reducing flicker, it would seem that the fade-in and fade-out dissolving technique might be applied in shutter designs to eliminate the abrupt light interruption that occurs while the film is stationary in the aperture. Something of this sort has been accomplished with continuous projectors. These machines of the rotating prism type have been best suited to projection with objective lenses where long focal lengths are necessary. Appreciable increases in illumination and reductions in flicker are possible with a fade-in fade-out design. As commercial 16-mm sound quality improves, it should be possible to obtain relatively high light efficiencies and very low flicker with reduced film speeds (say, 16 frames per, second) should this seem desirable as a cost-reducing measure in the future ; this would seem applicable, especially to high-quality silent projectors.
The sprockets, sprocket guards, and rollers of the projector should be designed to cause minimum wear and tear on the film. One important requirement is that the film shall be supported at all points in its path through a machine only at the safety areas of the film. A safety area is any portion of the surface of the film that does not bear picture image that is projected or sound image that is scanned. One safety area occurs at the sound track edge of a sound film, another occurs between the sound track and the picture, and the third occurs around the sprocket holes. When film wraps around a sprocket, it may either be pulled around or it may be kept in place by a film guard. The pulling method appears to produce the least film damage in commercial designs.
The film gate usually consists of two main plates, one attached to the body of the machine near the lamp and the other a movable pressure plate that is pressed against the body plate by means of springs. Gate pressure is usually adjustable ; incorrect gate pressure either materially reduces film life or it results in a jumpy picture. Gate pressure should