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252
GILLETTE
September
that the inverse power law is not a satisfactory approximation to the facts, particularly in the tail of the curve where the intensity values are very low and have not been checked experimentally.
PERSISTENCE IN RELATION TO THE MECHANICAL SHUTTER
In considering the action of a mechanical shutter, the situation will be found to be quite different. Electronic blanking is not used and all of the lines of the television scan are shown on the cathode-ray tube. Thus lines which have actually been scanned on the tube prior to the beginning of the exposure interval may contribute significantly to the exposure of the film by virtue of the phosphor's persistence.
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MICROSECONDS
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Fig. 6— Mechanical-shutter action.
The mechanical shutter is designed so that its closing edge crosses any point in the shutter plane exactly Vso of a second after that same point was crossed by the opening edge. For a particular line which is scanned just before the shutter opens, the first part of its light output is blocked by the shutter, and only a segment of the tail of the output curve contributes to the exposure of the corresponding element of the film. However, since each line is scanned repeatedly at intervals of Vso of a second, the same line is scanned again just before the shutter closes, and this time the initial portion of the light output reaches the film and only a segment of the tail of the curve is blocked by the shutter. If everything is working to perfection, the contributions from the two scannings of the line add up to give a total