| 摘要 |
A beam spot detector is disposed on an extension of a surface to be scanned by a beam of light. The beam spot detector has a mask in front thereof, the mask having a side which the beam of light intersects perpendicularly and a side which the beam of light intersects obliquely. An output signal of the detector is differentiated at a differentiating circuit, and positive and negative peak values are held at a peak hold circuit. A CPU calculates diameters of the beam of light in both a main scanning direction and an auxiliary scanning direction from the peak values. If the beam of light is out of focus, then the CPU outputs adjustment signals. A counter calculates a width of the output signal of the beam spot detector and outputs the calculated width to a CPU. The CPU compares the received width with a reference to detect a displacement in the auxiliary scanning direction (displacement of registration) and outputs a correction signal.
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