Imaging optical element and optical scanning apparatus including the same

An optical scanning apparatus, comprising:

• a light source comprising a plurality of light-emitting parts;
• a deflection unit configured to deflect a plurality of light fluxes emitted from the light source;
• and
• an imaging optical system configured to condense the plurality of light fluxes deflected by the deflection unit onto a scanned surface, wherein:
• the imaging optical system comprises an imaging optical element;
• the imaging optical element comprises an optical surface whose shape within a sub-scanning section has a non-circular shape;
• assuming that a coordinate along a main scanning direction is Y and a coordinate along a sub-scanning direction is Z and that, within the sub-scanning section, a curvature radius on an optical axis of the optical surface is r, an eccentricity is k, a coefficient of variation in the curvature radius of the optical surface is Di, and an aspherical surface coefficient is GmnYm, when a shape S of the optical surface within the sub-scanning section is defined by an expression:
• S
• =
• Z
• 2
• r
• ′
• 1
• +
• 1
• -
• (
• 1
• +
• k
• )
• ⁢
• (
• Z
• r
• ′
• )
• 2
• +
• ∑
• n
• =
• 1
• 16
• ⁢
• ⁢
• ∑
• m
• =
• 0
• 16
• ⁢
• ⁢
• G
• m
• ⁢
• ⁢
• n
• ⁢
• Y
• m
• ⁢
• Z
• n
• ,
• ⁢
• r
• ′
• =
• r
• ⁡
• (
• 1
• +
• ∑
• i
• =
• 2
• 14
• ⁢
• ⁢
• D
• i
• ⁢
• Y
• i
• )
• the expression comprises a term in which n is an odd number equal to or larger than 3;
• the aspherical surface coefficient of at least one of the terms in which n is an odd number equal to or larger than 3 is changed asymmetrically along the main scanning direction;
• and
• when wavefront aberration amounts in positions Zmax and Zmin of end portions of the light flux along the sub-scanning direction are set as W(Zmax) [λ] and W(Zmin) [λ], respectively, in an arbitrary position in the optical surface along the main scanning direction,
• |
• W
• ⁡
• (
• Zmax
• )
• -
• W
• ⁡
• (
• Zmin
• )
• 2
• |
• ≤
• 0.3
• is satisfied.