Patent ID: 7768542

Claim:
A multi-beam optical scanning device, comprising: a light source device including a plurality of light emitting members having a spacing in a main-scan direction; a rotary polygonal mirror configured to scanningly deflect a plurality of light beams emitted from said light emitting members; a first optical system configured to image, in a sub-scan section, the plurality of light beams from said plurality of light emitting members on a deflecting surface of said rotary polygonal mirror; and a second optical system configured to image the plurality of light beams scanningly deflected by the deflecting surface of said rotary polygonal mirror on a surface to be scanned, wherein, in the sub-scan section, the deflecting surface of said rotary polygonal mirror and the surface to be scanned are in a conjugate relationship with each other, wherein each of the plurality of light beams incident on the deflecting surface of said rotary polygonal mirror is incident from an oblique direction in the sub-scan section, relative to a plane perpendicular to a rotational axis of said rotary polygonal mirror, wherein, when a direction toward a downstream side in a movement direction of the surface to be scanned is defined as a plus direction in the sub-scan direction while a direction toward an upstream side in the movement direction of the surface to be scanned is defined as a minus direction in the sub-scan direction, each of the plurality of light beams incident on the deflecting surface of said rotary polygonal mirror is incident from an oblique minus direction in the sub-scan direction relative to a plane perpendicular to the rotational axis of said rotary polygonal mirror, wherein, under the above condition, the imaging magnification in the sub-scan section of said second optical system on an optical axis, between the deflecting surface of said rotary polygonal mirror and the surface to be scanned, is larger than the imaging magnification in the sub-scan section of said second optical system between the deflecting surface of said rotary polygonal mirror and the surface to be scanned in the end portion at the scan start side of the light beam scanning the surface to be scanned, and wherein the imaging magnification in the sub-scan section of said second optical system on the optical axis, between the deflecting surface of said rotary polygon mirror and the surface to be scanned, is smaller than the imaging magnification in the sub-scan section of said second optical system between the deflecting surface of said rotary polygonal mirror and the surface to be scanned in the end portion at the scan end side of the light beam scanning the surface to be scanned.