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MANAGERS AND PROJECTIONISTS 650
possible condition the greater the percentage of the light we will be enabled to get through to the screen, and the better will be the condition as regards flicker.
LIGHT BEAM DIAMETER.— From the foregoing it will be observed that the diameter of the beam of light at the point at which it is cut by the revolving shutter is a matter of much importance, since the wider the light beam the greater length of time will be required for the shutter blade to cut through it, therefore the wider must be the master blade of the shutter.
To grasp the meaning of this, remember that the master blade of the shutter travels at a uniform rate of speed, consuming, let us assume, 5/80 of a second in making one complete revolution. It will therefore require a longer space of time for the edge of the blade to cut through a beam two and one-half inches in diameter than it will to cut through a beam one inch in diameter. Also it is evident that since the speed of the blade, as measured in inches per second, increases directly as the distance from the center of the shutter shaft is increased, the greater the distance from the center of the shutter shaft to the center of the light beam, the less time will be consumed by the edge of the master blade in cutting across a beam of given diameter. Therefore the greater the distance from the center of the shutter shaft to the center of the light beam the less important increased diameter of the light beam becomes.
Figure 228 shows the effect of added distance center of shutter shaft to center of light beam. At 2 45/64 of an inch a 1.5 inch circle occupies 32 degrees and 12 minutes. At 3 17/64 inches the same diameter circle covers only 26 degrees and 28 minutes, while if the distance be 3% inches the same circle occupies only 22 degree and 54 minutes. A 1.5 inch circle represents what is popularly known as a "quarter size lens." From this we see that added distance from center of shutter shaft to center of light beam makes for better conditions as to width of master blade of revolving shutter.
No matter what the distance may be, however, the .less the diameter of the light beam the quicker the blade will cut across it, therefore since the master blade must cover three-quarters of the beam when the film begins to move, and continue to cover three-quarters of it until the film has stopped, it is apparent that the less the diameter of the beam the narrower may be the master blade, therefore it follows that: