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Incandescent Lamps for Motion Picture Service
By A. R. Bennington
ARRANGEMENT OF LIGHT SOURCE
In the development of an incandescent lamp for motion picture service, it is essential to concentrate the light source as much as possible because no condensing system can collect and utilize the rays of light from an extended source. The problem that comes up is, therefore, that of getting as much filament as possible within a given area and arranging this filament in such a way that there will be maximum illumination in the direction in which it can be utilized.
There are in general two methods which suggest themselves for the arrangement of the filament. One method consists in arranging the coils in series and placing the various sections close to each other and practically parallel. This series of coils may be arranged either in planes at right angles to the line of projection, or in a more or less complete arc of a circle. Arranging the coils in the arc of a circle has the advantage of placing each element of the filament equally close to the focal point of the optical system. However, it has the disadvantage that no element of the filament can be placed exactly at the focus and therefore no element is operating at the best possible efficiency for the system. The arrangement of the filament sections in a plane or planes perpendicular to the line of projection has been found to be more satisfactory than the circular arrangement, as it makes possible a more uniform field or beam than can otherwise be obtained. There are, however, spaces between the filament sections which are not sources of light and the result of this is that the screen may have light and dark areas corresponding to the filament areas.
The second method of arranging the filament is to connect the sections in parallel with very small space between them. This arrangement permits of a larger amount of filament within the given area but it has the disadvantage of requiring a high current which is liable to cause heating of the lamp seal as well as the wires and contacts. Any sagging of the coils tends to short-circuit to a greater or lesser extent portions of each section, as the current passes from a turn of one coil to a turn of another coil without passing around the helix, thus reducing the lamp voltage and the candle power as well. If the coils are spaced far enough apart so that there is no danger of the sections short-circuiting by contact with each other, there is little or no advantage of the parallel arrangement of coils over the series arrangement.
In order to obtain some of the advantages of practically an unbroken wall of filament and also retain the advantages of series connected coil sections, an arrangement of the filament has been developed which places the coil sections in two parallel rows, with the coils in the back row opposite the spaces between coils in the front row. This arrangement of the filament presents a solid wall along the axis
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