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PRINCIPLES OF CINEMATOGRAPHY
Fig. 166. — Projector sound-head design.
As is well known, the current generated in a photo-electric cell when illuminated by a constant intensity light source is, over a wide range, proportional to the strength of that source. If, therefore, the film shown in diagram 'A', Figure 166, is removed and the intensity of the exciter lamp is varied by some external controlling device, the current generated in the photo-cell can be measured and compared with the variations in the lamp. Such a current is, originally, very weak and must therefore be strengthened by a preamplifier (located within the projector body in 16-mm mechanism) before it can be passed through the considerable length of cable which usually connects the projector with the main amplifier. This amplifier is usually located with the speaker, but in some cases, can be separated therefrom. Obviously, any variations in the intensity of the exciter lamp will cause a variation in the output from the photo-cell and, therefore the supply to that lamp must be perfectly constant and, preferably, should be from a Direct Current source. If the film is then replaced in the position shown in diagram 'A' the light beam to the photo-cell will be modified in proportion to the effective transmission of the sound track on that film. Since it is necessary to reproduce the highest frequencies which have been recorded, the exciter lamp and associated optical system must be capable of isolating one cycle of the highest frequency likely to be encountered, that is, the area of the sound track which receives illumination must be exceptionally small the height of the area, measured along the film length, usually being of the order of one half-thousandths of an inch in 16-mm projectors.
Various methods have been used to obtain this very narrow 'slit' or scanning area and, in the design shown, the objective