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no light would be obtainable on the screen; as the shutter wings would block out the light each time it occurred.
ONE SHUTTER REVOLUTION too* ; 80° : too* , 60'
ONE SHUTTER REVOLUTION
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If now the shutter be moved 60 degrees on its shaft, the maximum light would be obtained, as then the shutter openings would register with the peaks of the current alternations. These conditions, of course, would prevail only if the shutter and current were perfectly synchronized. If the speed of the shutter were reduced to 20 revolutions per second, 4 peaks at each opening.
While the 60 cycle current prevails in practice the aforementioned speed and shutter openings do not ; consequently, at the normal shutter speed of 16 revolutions per second, the shutter openings would be out of step with the current frequency, with the result that some of the shutter openings would receive one, some two and some three impulses of light, causing a visible variation of light on the screen.
Since each projected image is illuminated for a time period equal to the sum of all the shutter openings during one revolution, the light impinged on each image would be the same if the light source were constant. With the alternating current arc, the light would vary with the number of peaks to each respective image, the variation and resulting flicker depending upon actual difference in frequency of shutter flicker, or in other words, depending on how much they are out of step.
The following diagrams show these variations under the normal conditions of a projection speed of 16 images per second, with a 60 cycle arc, and using a commercial shutter of two 80-degree wings with two 100-degree openings.
It is obvious from the above that a current flicker can be avoided in alternating arc projection, only by occasional accidental synchronizing for short periods of time, or by a carefully planned synchronization, together with special shutter design.