Patent ID: 6859295

Claim:
A scanning optical system for emitting a laser beam scanning in a main scanning direction on a scan target surface; comprising: a light source that emits the laser beam; a deflector that dynamically deflects the laser beam emitted by the light source within a predetermined scanning range; and an imaging optical system that converges the deflected laser beam on the scan target surface to form a beam spot scanning in the main scanning direction, wherein said imaging optical system comprises: a first plastic lens having a front surface located on a light source side and a rear surface located on a scan target surface side, said first plastic lens being configured such that origin points of the front and rear surfaces of said first plastic lens are located on opposite sides of a principal axis with regard to an auxiliary scanning direction which is perpendicular to the main scanning direction, an amount of shifting of each of the origin points of said first plastic lens with respect to the principal axis is less than half of a diameter of mirror-finished area of corresponding one of the front and rear surfaces of said first plastic lens, and both of centers of curvature of the front and rear surfaces of said first plastic lens on the respective origin points are positioned on the light source side of said first plastic lens; and a second plastic lens having a front surface located on the light source side and a rear surface located on the scan target surface side, at least one of the front and rear surfaces of said second plastic lens having an anamorphic aspherical surface, at least one of origin points of the front and rear surfaces of said second plastic lens being located on the same side of the principal axis as the origin point of the rear surface of said first plastic lens in the auxiliary scanning direction, an amount of shifting of each of the origin points of said second plastic lens with respect to the principal axis being less than half of a diameter of mirror-finished area of corresponding one of the front and rear surfaces of said second plastic lens in the auxiliary scanning direction, wherein said second plastic lens is located on the scan target surface side with respect to said first plastic lens, wherein the principal axis corresponds to an extension of a center axis of the laser beam lying between said deflector and said imaging optical system when the center axis is viewed along a line perpendicular to the auxiliary scanning direction.