| 摘要 |
PURPOSE:To eliminate variation in the intensity of scanning light due to noise generated by a light source itself, a decrease in its light output, etc., and to accurately detect the passing time of a light point distinctively from when there is no light by using the time when the light point passes a point slightly shifted from the midpoint between two photodetectors as reference time. CONSTITUTION:When the photodetectors 61 and 62 are arranged at distance equal to the diameter of a light beam, the output of the photodetector 61 is denoted as 71 and the output of the photodetector 52 is denoted as 72. The differential output between those two output is 73 and then the intermediate point between the two detectors is not found because the two photodetector outputs does not cross each other. For the purpose, the output 71 is delayed by a constant ttheta through a delay circuit to obtain an output 74, which is made to cross the output 72 with time. Consequently, the two detector output are extracted as shown by a waveform 75 and the time when the output 75 crosses zero is used as the reference time.
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