| 摘要 |
PROBLEM TO BE SOLVED: To improve image quality by shielding return light without inclining a beam detection sensor in respect to a laser beam by providing a beam detecting means with an aperture part for spatially limiting the passage of a light beam, constituting that means so as to limit the reflected light of the light beam passed through the aperture part. SOLUTION: A slit 21 is provided with an aperture part 21a made eccentric to an optical axis C in main scanning direction. The surface of the optical axis C is not opened. As incident light, the laser beam advancing on an optical path A is passed through the aperture part 21a at a position eccentric from the optical axis C, refracted on a BD lens 22 later and made incident to a BD sensor 23 arranged on the optical axis C at an angleθ1. The return light reflected on the BD sensor 23 is advanced on an optical path B while having an angleθ2 on the opposite side of incidence. Since the reflection angleθ2 at such a time is equal with the incident angleθ1 since the surface of the BD sensor 23 is right opposed to the optical axis C. The return light is refracted on the BD lens 22 and shielded later by a blocking part 21b on the opposite side of incident time in respect to the optical axis C and does not reach a BD mirror 7.
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