Optical beam scanning device having two sets of fθ mirrors where the mirror base and mirror face have differing coefficients of linear expansion

摘要

An optical beam scanning device includes a rotating polygon mirror for scanning a beam from a pre-deflection optical system in a main scanning direction and two mirrors 23, 25 for imaging the beam scanned by the rotating polygon mirror 5 on an image plane. The device is arranged so that a first mirror 23 at the rotating polygon mirror side 5 has a negative power in the main scanning direction and a second mirror 25 at the image plane side has a positive power in the main scanning direction. Thereby, the degrees of freedom of shape and arrangement of process unit or the like can be made higher and an apparatus can be downsized by securing the distance between the last imaging element and the image plane while suppressing the increase in the optical path length between the reflection surface 5A of the rotating polygon mirror 5 and the image plane.