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THE MOTION PICTURE PROJECTOR 173
Between the take-off sprocket and the gate the film is formed into a free loop, but immediately below the gate the film is drawn taut around an intermittent sprocket which, as its name suggests, rotates sufficiently to draw the film forward by a distance equal to the height of one picture, and then comes to rest to enable that picture to be projected. From this point the film is formed into a second free loop until it reaches the lower constant speed sprocket. The film is then passed through the sound reproducer which, at this stage, does not require description, and finally is wound up once more on the lower take-up spool, again housed within a fire-proof casing.
That period of the cycle during which the film is being moved downwards through the gate is not suitable for projection, since it would tend to blurr the definition of the remaining part of the cycle during which the film is stationary in the gate. A shutter is therefore employed in a manner similar to that used in the camera mechanism and, once again, serves to cut off the light from the film gate. However, this shutter is not necessarily always found in one position and, although shown in this illustration between the light-source and the film, may also be placed between the film and the lens or just in front of the lens.
By 1956 the Bauer Company produced a compact 2,000watt xenon discharge lamp for standard projection purposes, but the conventional light source is the high-intensity carbon arc, one arrangement of which is outlined in the diagram. To ensure that all the available light is directed to the film aperture a high quality concave mirror, mounted behind the arc, and a condenser lens placed between the arc and the film aperture, collect all that light falling within a large angle and concentrate it in a converging beam to a point at the rear surface of the objective lens. The necessary layout of the optical system to ensure maximum screen illumination will be discussed later. Owing to the intense heat produced by the arc it is enclosed within a large lamp -ho use, to the roof of which is attached a large air duct connecting it to some external wall of the operating box and so leading most of the fumes away from the operators.
The heat concentrated at the film gate is very intense and, because of this, fire precautions are taken to ensure that the apertures in the spool housings, through which the film enters and leaves the machine, will automatically close in the event of film becoming ignited at the gate. Most projectors are now fitted with fire extinguishing appliances which automatically come into operation if the film is caught in the aperture, the mechanism stops with the arc directed onto the film surface or if the safety shutters between