| 摘要 |
An f theta lens system is disposed between a rotating polygonal mirror for deflecting a light beam and a surface being scanned, for focusing the light beam deflected by the rotating polygonal mirror on the surface. The f theta lens system places the reflecting position of the rotating polygonal mirror and the surface being scanned in conjugate relationship with respect to an auxiliary scanning direction, and has an f theta function, the f theta lens system comprising two lenses. The surfaces of the first and second lenses are defined as first through fourth surfaces successively from the rotating polygonal mirror toward the surface being scanned, the first surface being a spherical surface, the second surface being a cylindrical surface, the third surface being a cylindrical surface, and the fourth surface being a toric surface. The f theta lens system satisfies the conditions: 0.35<R4x/R1x<0.85, and -0.120<(1/R3y)+(1/R4y)<-0.095 where the Rix is the radius of curvatuve of the ith lens (i=1 through 4) in a plane in which the light beam is deflected by the rotating polygonal mirror, RiY is the radius of curvature of the ith lens in the auxiliary scanning direction, with the focal length fM of the f theta lens system in the main scanning direction being 100. |
