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462 XIII. PROJECTION AND PROJECTORS
jection in most machines; a 6-v., 1-amp. lamp or a 4-v., 3/4-amp. lamp is the common light source for the sound light beam. The 750-w. lamp is customarily energized directly from the mains, either alternating or direct current. The sound exciter lamp is usually energized from a supersonic electron tube oscillator that is built as a part of the audio amplifier assembly. The oscillator operates at a frequency of about 15 to 25 kilocycles. This arrangement of the supply for the sound exciter lamp minimizes a.-c. hum without the expense of heavy rectifiers and filters, or batteries.
The film is moved through the film gate by the movement claw wTith an intermittent motion, and a new frame of film enters the projector gate every 1/24 second when the machine is running at sound speed.
In the optical system of the picture portion of the projector, a lamp (usually 750 watts) directs light to the aperture of the gate through a set of condenser lenses. To the rear of the lamp in the lamp house is a small mirror that redirects the light falling on it to the aperture, increasing the aperture illumination. The projection objective focuses the film image in the aperture to the screen. All optical elements are customarily aligned along a straight-line optical axis. Lenses, the mirror, and the gate should all be accessible for ready cleaning because the efficiency of the projector is seriously impaired by even small amounts of dust and dirt that are picked up in a short time by the projector.
Projector Mechanism
In a modern 16-mm projector, the motion of changing from one frame of film to another at sound speed takes but one-ninth* of a movement cycle of 1/24 second. 1/24 second includes both the periods of film movement and of rest. Some of the older models of projectors had a longer movement period amounting to one-sixth of a movement cycle, but were not capable of as bright a picture as the one-ninth movement cycle machines due to the fact that light could be theoretically available on the screen for a maximum of five-sixths of a cycle, whereas with the one-ninth movement cycle type the light could be theoretically available for eight-ninths of a cycle.
The intermittent motion is accomplished in most machines by a 2 or 3-toothed claw that enters the perforations of the film and withdraws from them, moving the film through the gate one frame at a time. The motion of the claw is usually produced hy two synchronized cams. One
* Approximate value.