Patent ID: 8791974

Claim:
An optical scanning apparatus, comprising: a light source including a plurality of light emitting parts; a deflector including a deflecting surface and configured to deflect a plurality of light beams respectively emitted from the plurality of light emitting parts for scanning by the deflecting surface in a main scanning direction; a first optical system configured to cause the plurality of light beams to enter the deflecting surface of the deflector at an oblique angle within a sub-scanning section perpendicular to the main scanning direction; and a second optical system configured to focus the plurality of light beams deflected by the deflecting surface of the deflector on a surface to be scanned to form a plurality of spot images defining a plurality of scanning lines on the surface to be scanned, wherein: the plurality of light emitting parts are arranged away from each other in a sub-scanning direction perpendicular to the main scanning direction and to a direction of an optical axis of the second optical system; the second optical system includes an optical element including at least one optical surface having a non-arc shape within the sub-scanning section, which asymmetrically varies in the main scanning direction, so that a wave optics interval in the sub-scanning direction between the plurality of scanning lines based on barycentric positions of the plurality of spot images on the surface to be scanned is aligned along the main scanning direction; when the non-arc shape of the at least one optical surface of the optical element within the sub-scanning section is defined by S = Z 2 r ′ 1 + 1 - ( 1 + k ) ⁢ ( Z r ′ ) 2 + ∑ k = 1 16 ⁢ ⁢ ∑ j = 0 16 ⁢ ⁢ G jk ⁢ Y j ⁢ Z k , ⁢ r ′ = r ⁡ ( 1 + ∑ i = 2 10 ⁢ ⁢ D i ⁢ Y i ) where S is the non-arc shape of the at least one optical surface of the optical element within the sub-scanning section, Y axis is an axis perpendicular to the optical axis in the main scanning direction, Z axis is an axis perpendicular to the optical axis in the sub-scanning direction, r is a curvature radius of the at least one optical surface on the optical axis within the sub-scanning section, r′ is a curvature radius of the at least one optical surface within the sub-scanning section, D i is a variation coefficient of the curvature radius r′, G jk Y j is an aspheric coefficient, and the non-arc shape S includes a term of a fourth-order or higher; and when a relationship between an aspheric coefficient of any order among the aspheric coefficients included in the non-arc shape of the at least one optical surface of the optical element and a position in the main scanning direction is represented by a first function and a relationship between an imaging magnification of the second optical system in the sub-scanning direction and the position in the main scanning direction is represented by a second function, each of the first function and the second function has an extremum within an effective scanning area of the at least one optical surface.