Patent ID: 7327507

Claim:
An optical beam scanning device, comprising: a rotating polygon mirror for scanning a beam from a pre-deflection optical system in a main scanning direction; and two mirrors for imaging the beam scanned by the rotating polygon mirror on an image plane, and assuming that a linear expansion coefficient of the first mirror at the rotating polygon mirror side is α 1 and a linear expansion coefficient of the first mirror at the image plane side is α 2 , respective powers in the main scanning direction are φ 10 and φ 20 , and respective powers in the sub-scanning direction are φ′ 10 and φ′ 20 , an optical path length between the two mirrors is d 2 , and an optical path length between a reflection point of a deflector and the first mirror is d 1 , having the relationships: |α 2 −α 1i |>|α 1 −α 1i | and |α 2 −α′ 1i |>|α 1 −α′ 1i | here, α 1i =−α 2 (−1 +d 2 φ 10 ) 2 φ 20 /φ 10 α′ 1i =−α 2 (− d 2 −d 1 +d 2 d 1 φ′ 10 ) 2 φ 20 /( d 1 2 φ′ 10 ).