| 摘要 |
PROBLEM TO BE SOLVED: To prevent dust from accumulating on the reflection surface of a means to return an optical path having a structure for returning the optical path. SOLUTION: A light beam deflected by a polygon mirror 20 is made incident nearly perpendicularly on the incident plane 33a of a trapezoid return prism 33, having the incident plane 33a and two reflection surfaces 33b and 33c. The incident light beam is totally reflected toward the reflection surface 33c by the reflection surface 33b and also is totally reflected toward the incident plane 33a by the reflection surface 33c, so that the light beam is emitted from the incident plane 33a in a direction nearly opposite to that of an incident beam. The emitted light beam passes through a cylindrical lens 34 and is directed to a surface to be scanned 40 by being reflected by the cylindrical mirror 35, so that a scanning spot is formed on the surface 40. Dust will not adhere to the reflection surface 33c of the return prism 33 facing to a ceiling side, and the reflection surface 33b positioned on the ceiling side is covered with a dustproof cover 39, so that similarly the problem of dust sticking will not be caused.
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