| 摘要 |
PROBLEM TO BE SOLVED: To automatically correct the positional deviation of a dot in a main scanning direction when a scanning optical system is changed because of the uneven rotation of a polygon motor or the like. SOLUTION: The polygon motor driving a polygon mirror 3 causes the fluctuation of a longitudinal line in the main scanning direction because of the uneven rotation, whereby the position of the dot happens to be deviated at the time of finishing write. The polygon motor is controlled in order to correct the deviation. A reference clock for measuring scanning time of every rotation of the polygon mirror is set first, and the revolving speed of the polygon motor in accordance with writing speed is set next. Then, the calculated value of the counted number of every rotation of the polygon motor is calculated. Then, the real scanning time of every rotation of the polygon motor is measured by the counted number of the reference clock. If a measured value is equal to a calculated value, the correction is not performed. If the measured value is not equal to the calculated value, a correction clock is generated to change the clock of the polygon motor. Thus, the rotating speed of the polygon motor is controlled to be constant.
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