| 摘要 |
PROBLEM TO BE SOLVED: To reduce the number of parts, to produce a photodetector at low cost and to shorten a working stage by including a rotary polygon mirror deflecting a laser beam in a direction different from its rotating direction and a photodetector means detecting the light beam reflected from the rotary polygon mirror. SOLUTION: A projected part 101 is formed on the rotary polygon mirror 10 and has a reflection surface inclined so that a part of the laser beam made incident on the mirror 10 may be made incident on the photodetector 24 (figure C) attached to the upper part of the mirror 10 (figure A). It is good to provide a cut part 111 obtained by partially cutting the rotary polygon mirror 11 in order to miniaturize the photodetector (figure B). The photodetector 24 is arranged at the projected part 101 in figure A or at the upper part of the rotary polygon mirror 25 having the cut part 111 in figure B. Since parallel laser beams made incident on the polygon mirror 25 are focused in one direction (subscanning direction) by a cylindrical lens 23, the detection effective surface of the photodetector 24 is set in a direction at a right angle to the above direction. Thus, the photodetector 24 detects that one scanning action is started. |
