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254
GILLETTE
September
of the light-output curve must be multiplied by the corresponding transmission values of the two shutter curves. This step is illustrated in Fig. 7.
Here curve 1 shows the effective light output for the first of the two scannings of the line. Curve 2 shows the same quantity for the second scanning. Curve 3 is the sum of curves 1 and 2 and represents the total light which is effective in exposing the corresponding area of the film. As expected, curve 3 is identical with the original lightoutput curve of the phosphor.
Fig. 8 shows the same group of curves, but they represent erroneous timing in that the closing of the shutter has been retarded 100 microseconds with respect to the second scanning of the line. This pro
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Fig. 8 — Effective light output — retarded closing.
duces a defect in the total output curve which has been emphasized by showing with a dashed line the desired shape. The resulting exposure error is proportional to the area between curve 3 and the dashed line.
The same calculation was carried out for several values of shutter error and the various exposure errors were determined. The results are shown in Fig. 9. Here the horizontal co-ordinate is the shutter error in microseconds and the vertical is the exposure error which results. If again one -averages the exposure over a small area and multiplies by the photographic gamma it is seen that a timing error of around 30 microseconds will produce a screen-brightness defect of about 6 per cent. Since this defect would be quite visible it follows that timing which is accurate to 0.1 per cent is not good enough.