Optical scanning device and image forming apparatus using the same

An optical scanning device, comprising:

• light source means;
• an optical deflector having a deflecting surface;
• an input optical system having an input optical element and 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 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 input optical element of said input 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,
• wherein, when a thickness of said input optical element in the sub-scan section and at a position where a first marginal light ray of the light beam passing through said input 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 dm1 (mm), while a thickness of said input 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 dm2 (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 input optical system is scanningly deflected by the deflecting surface of said optical deflector and is then incident at a center of the scanned surface 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.