| 摘要 |
PROBLEM TO BE SOLVED: To prevent the reduction of reliability of a scanning start signal caused by the color aberration of an image forming lens. SOLUTION: Laser lights L1 and L2 emitted from semiconductor lasers 1 and 2 are synthesized by a beam synthesizer 3, polarization-scanned by a rotary polygon mirror 4, divided into scanning lights L3 and L4 after passing through an image forming lens 6 and an image is formed on a photodetector drum 5. A part (detected beam) L1 of a synthetic beam L0 is divided into respective beam componentsλ1 andλ2 by a detection mirror unit 9 composed of a dichroic mirror 9a and a reflection mirror 9b, made incident on a beam sensor 13 and converted into a scanning start signal.
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