Patent ID: 7542190

Claim:
A tandem light scanning unit (LSU) forming a substantially straight scanning line by scanning different light beams in a main scanning direction on different photosensitive drums, the tandem LSU comprising: a light source; a beam deflector rotating on a light-emitting path of a light source and substantially simultaneously deflecting different light beams obliquely incident at a predetermined angle to each other within a sub-scanning plane substantially parallel to a rotational axis of the beam deflector; and an optical scanning lens allowing the light beams deflected by the beam deflector to be formed on corresponding photosensitive drums, wherein the optical scanning lens includes at least one sub-scanning cross-section having an aspherical surface in a sub-scanning direction to reduce a curvature of the scanning line, wherein the aspherical surface has a shape defined by: Z = y 2 Rdy 1 + 1 - ( y Rdy ) 2 + ∑ m = 3 10 ⁢ A m ⁢ Y m + x 2 ⁢ c ⁡ ( 1 + ∑ m = 3 10 ⁢ B m ⁢ Y m ) 1 + 1 - ( 1 + k ⁡ ( 1 + ∑ m = 3 10 ⁢ D m ⁢ Y m ) ) ⁢ ( xc ⁡ ( 1 + ∑ m = 3 10 ⁢ B m ⁢ Y m ) ) 2 + ∑ m = 3 10 ⁢ C m ⁢ X m ) where, y and x are coordinates of a main-scanning direction and a sub-scanning direction, respectively, and Z is a depth of the optical lens' surface with respect to an optical axis direction on the assumption that an intersection point of the optical scanning lens and an optical axis thereof is an origin of a coordinate system; Y and X are defined by Y=√{square root over (y 2 )}, X=√{square root over (x 2 )}; R is a curvature radius in the main scanning direction; c is a curvature in a sub-scanning direction; Am, Bm, Cm, and Dm are coefficients selected for lens design; and k is a conic constant.