| 摘要 |
PURPOSE:To detect an error in face overturn of a mirror face, by calculating the position of the center of gravity of a part of expansion of a light quantity distribution by a formula, so as to determine the amount of shift of a computed gravity center position. CONSTITUTION:A laser beam from a laser oscillator 1 is made to fall on each mirror face of a polygon mirror 3, and a reflected beam of light therefrom is sensed by an image sensor 4. In the image sensor 4, a sensed light quantity distribution of expansion of the reflected beam of light along the direction intersecting, perpendicularly intersecting in particular, the scan direction of the polygon mirror 3, is obtained by a plurality of light-sensing elements in the number of (n) arranged in said direction. Then, the position X' of the center of gravity of the sensed light quantity distribution is determined from the formula of X'[where D(i) is a sensed light quantity output of an i-th light-sensing element], it is interpolated in the length between the light-sensing elements, and an angle of face overturn of the given mirror face is led out.
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