| 摘要 |
PURPOSE:To measure the rotation with high accuracy by photodetecting successively two pieces of laser light beams by two pieces of photodetectors opposed to the surface of a rotary polygon mirror on a plane being parallel to a rotation axis of the polygon mirror, and inputting an obtained trigger signal to a counter. CONSTITUTION:Beams 5a, 5b emitted from two laser light sources 2a, 2b are reflected by turn-back mirrors 7a, 7b, respectively, come to reflected beams 7c, 7d, then beams irradiate the surface of a rotary polygon mirror 1, and come to reflected beams 6a, 6b. These beams 6a, 6b are photodetected by photodetectors 3a, 3b, respectively. In this case, the detectors 3a, 3b are provided on a plane being parallel to a rotation axis A of a rotary polygon mirror motor M. Accordingly, in the case when the mirror 1 is displaced, the beams 6a, 6b are photodetected against the surface of the mirror 1 only when the detectors 3a, 3b are positioned in the front, respectively. In such a way, only when the detectors 3a, 3b are positioned in the front against the prescribed surface of the mirror 1, the respective trigger signals are obtained, and by measuring a time difference of those two trigger signals, the time when the mirror 1 is rotated by a prescribed angle can be detected. |
