Patent ID: 8451522

Claim:
An optical scanning apparatus comprising: a light source unit; an incident optical system for guiding a light beam emitted from the light source unit to a deflecting unit; and an imaging optical system for causing the light beam deflected for scanning on a deflecting surface of the deflecting unit to form an image on a surface to be scanned, wherein: the light beam emitted from the light source unit enters the deflecting surface of the deflecting unit in an oblique manner with respect to a plane perpendicular to a rotational axis of the deflecting unit in a sub-scanning section; the imaging optical system consists of a single imaging optical element; each of an incident surface and an exit surface of the single imaging optical element is a surface in which a tilt angle of a sagittal line changes from on axis toward off axis in a sub-scanning direction, the tilt angle of the sagittal line indicating a gradient of a surface normal to the sagittal line on a meridian line with respect to a main scanning section; the incident surface and the exit surface of the single imaging optical element each have the same sign for a difference between an axial tilt angle of the sagittal line indicating a gradient of the surface normal to the sagittal line on the meridian line with respect to the main scanning section and an off-axial tilt angle of the sagittal line indicating a gradient of the surface normal to the sagittal line on the meridian line with respect to the main scanning section; if an incident direction of the light beam entering the deflecting surface of the deflecting unit in an oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is negative, a value of the difference is negative; if the incident direction of the light beam entering the deflecting surface of the deflecting unit in the oblique manner with respect to the plane perpendicular to the rotational axis of the deflecting unit in the sub-scanning section is positive, the value of the difference is positive; and the incident surface and the exit surface of the single imaging optical element each have the same sign for a rate of change of the tilt angle of the sagittal line, which changes from on axis toward off axis in the sub-scanning direction.