| 摘要 |
A system for optically detecting the presence of indicating markers on microfilm frames and for ignoring stray marks and smudges. As light shines through the microfilm and is detected by a photo-transistor, the system self regulates to provide an absolute baseline value, called a prescaler, for each frame of microfilm. Then, as each mark appears on the microfilm and interrupts the light path to the photo-transistor, the degree or extent of light blockage is measured as a deviation from the baseline for that frame. If the deviation exceeds a predetermined threshold, the mark is counted as a marker; otherwise the mark is ignored as a stray spot or smudge. The threshold is also self regulated based on a percentage of the amount that the light blockage of the marker in the immediately preceding frame deviated from the baseline for that frame.
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