Optical scanning apparatus with polygon mirrors reflecting light beam at

Abstract

An optical beam scanning apparatus comprises: an beam source emitting an optical beam; a deflector deflecting the optical beam around a center line with a deflection angle .theta. radians in a main scanning direction perpendicular to the center line at a constant angular velocity d.theta./dt; a lens system, having a focal length f in the main scanning direction, for focusing the optical beam on a recording plane perpendicular to the center line to form and scanning a beam spot on the recording plane at a constant speed in the main scanning direction, wherein the lens system refracts the optical beam from the deflection means such that the constant speed is larger than f.multidot.d.theta./dt. An emitting angle from the lens system is larger than the deflection angle by the deflector. The deflector comprises a polygon mirror unit. The lens system may have a function to compensate a surface inclination in the polygon mirror unit. That is, the lens system includes toric surfaces including at least a rotation symmetric aspheric surface and at least a spherical surface or one of the plurality refracting surfaces has a toric surface having an aspheric surface and other refracting surfaces have rotation symmetric aspheric surfaces. A collimating lens unit may be provided between the beam source and the deflector. Units necessary for recording data may be further provided.