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THE MOTION PICTURE PROJECTOR 183
attached to the opaque sections, in order to create a forced draft to assist further in this cooling action.
The two shutters shown at 'A' and 'B' are essentially similar in operation (it should be noted that in each case the blade is vertical when in the mid-position across the lens face) and are only cut to different shapes because the centres of rotation are not both directly beneath the centre of the lens. It is more likely that a shutter of type 'A' would be used in position 'S3' or '82', whilst type 'B' would probably be used in position 'S^.
Although the 'barrel' shutter shown at 'C occupies valuable space when placed between the film and the lens it has one very great advantage. When rotating in the direction indicated, the upper left-hand blade will be about to close across the film aperture at the same instant as the lower right-hand blade is also closing across this aperture. The 'wasted angle', during which 'cut-off' is completed is therefore reduced by this shutter to approximately 50 % of that required when disc shutters are used. Unfortunately, it is usually difficult to maintain the time during which the shutter is completely open equal to the closed period, which must be obtained to reduce noticeable flicker, without using a shutter of rather large diameter and, in consequence, some barrel shutters are located not in the position shown but between the condenser and the film gate. Of course, the efficiency of barrel shutters would only be exactly 50% that of disc shutters if the beam of light passing through the system at this point were parallel and, since it is always a rapidly converging beam, the efficiency is not quite so great.
At this point it is well to consider the placement of the condenser, film and objective lens in order to obtain maximum efficiency from the light-source. Figure 75 shows three arrangements whereby useful light is lost, and one arrangement in which the maximum efficiency is obtained. In the arrangement shown at 'A' rays of light from the arc are reflected by the parabolic mirror and collected by the condenser so that they all come to a focal point at the rear surface of the objective lens. However, since the film gate is placed too near the objective lens, the cone of light will not be sufficiently large to completely cover the aperture and will result in very low screen illumination at the picture edges. The arrangement shown at 'B' is similar to that at 'A', but in this case the film gate is placed too near to the condenser lens and, therefore, part of the cone of light is cut off by the aperture plate and never reaches the film. This results in an evenly illuminated picture but of lower intensity than is possible. Arrangement 'C shows the film gate correctly