Patent ID: 8179561

Claim:
An optical scanning device, comprising: light source means; an optical deflector having a deflecting surface; an incident optical system having an incident optical element and being configured to project a light beam emitted from said light source means onto the deflecting surface of said optical deflector; and an imaging optical system having an imaging optical element and being configured to image a light beam scanningly deflected by the deflecting surface of said optical deflector, on a surface to be scanned, wherein the light beam incident on the deflecting surface of said optical deflector is obliquely incident in a sub-scan section with respect to a normal to the deflecting surface, wherein said imaging optical element of said imaging optical system has at least one optical surface which is decentered in the sub-scan section, wherein said incident optical element of said incident optical system has at least one optical surface having an aspherical surface shape being asymmetric, in the sub-scan section, with respect to a principal ray of a light beam passing through an optical element, and wherein, when a thickness of said incident optical element in the sub-scan section and at a position where a first marginal light ray of the light beam passing through said incident optical system, which first marginal light ray is closer to an optical reference axis than the principal ray of that light beam is, passes, is denoted by dm 1 (mm), while a thickness of said incident optical element at a position where a second marginal light ray further remote from the optical reference axis than the principal light ray of the light beam is, passes, is denoted by dm 2 (mm), a relation dm1<dm2 is satisfied, where the optical reference axis is defined as an axis which, when a principal ray of a light beam emitted from said incident optical system is scanningly deflected by the deflecting surface of said optical deflector and is then incident at a center of the surface to be scanned with respect to a main-scan direction, passes through a deflection point for the principal ray of the light beam on the deflecting surface in the sub-scan section and which is perpendicular to the deflecting surface.