| 摘要 |
PURPOSE:To suppress an increase in the size of a rotary polygonal mirror formed by plastic molding and to maintain high image quality by setting the unused parts of the reflection surface of this rotary polygonal mirror in such a manner that the end side where the size of the light beam to be cast to the reflection surfaces increases is smaller than another end side. CONSTITUTION:A revolving shaft 3b of the rotary polygonal mirror 3 formed by plastic molding is arranged in a position 3b' moved by DELTA1 to an incident light beam LB side from a segment gg' extending the bisector of an angle W formed by the incident light beam LB and the reflected light beam B' at a deflection angle 0 deg. in a direction orthogonal with this segment, by which the unused parts 3c, 3d to be not optically used are disposed at least at one end of the reflection surface 3a of the rotary polygonal mirror 3. In addition, the size of the unused parts 3c, 3d is so set that the end side 3c where the size of the incident light beam LB projected onto the reflection surface 3a increases according to rotation of the rotary polygonal mirror 3 is smaller than the other end side 3d. |
