Patent ID: 8780427

Claim:
An optical scanner comprising: a light source configured to emit a light beam; a light deflector configured to deflect the light beam from the light source in a main scanning direction; an incident optical system disposed between the light source and the light deflector and configured to converge the light beam from the light source into a linear image elongated in the main scanning direction and to bring the same to a focus on or in proximity to a deflecting surface of the light deflector; and a scanning lens configured to focus the light beam deflected by the light deflector on a target surface to form a dotted image, wherein the incident optical system includes an anamorphic condensing lens which has different powers with respect to the main scanning direction and a sub-scanning direction, wherein the anamorphic condensing lens has a diffractive lens structure at least in one lens surface thereof, and a length of an optical path increased by the diffractive lens structure φ [rad] is defined by an equation below by a function of a height h from an optical axis: φ( h )= M ( P 2 ·h 2 +P 4 ·h 4 + . . . ) where Pn is a coefficient of an nth-order term of the height h (n is an even number), and M is a diffraction order, wherein the anamorphic condensing lens satisfies the following relations: −216 ≦P 2≦−49 1110 ≦P 4·( hm max) 4 /( fm·NAm 4 )≦3800 10 ≦fm≦ 35 where hmmax [mm] is an effective diameter in the main scanning direction, fm [mm] is a focal length in the main scanning direction, and NAm is a numerical aperture in the main scanning direction, and wherein the anamorphic condensing lens is configured such that a wavefront aberration WFE 1 [λrms] in a first wavelength λ 1 [nm] satisfies the following relation: WFE 1≦0.01.