| 摘要 |
PURPOSE:To prevent a deflecting and reflecting surface from being burnt by composing an image formation lens system of an ftheta lens consisting of plural lens elements, forming an infinite-curvature surface whose radius of curvature is infinite in a main scanning direction as one of their surfaces, and providing a concave cylinder lens surface on this surface. CONSTITUTION:One element 3 between the plural lens elements of the ftheta lens constituting the image formation lens system has a spherical surface on its light source side and infinite-curvature surface whose radius of curvature is infinite in the main scanning direction on its image formation surface side, and the other lens element 14 has a concave cylinder surface on the light source side and a spherical surface on the image formation side; and a convex cylinder lens 15 has a convex cylinder surface on the light source side and a plane on the image formation side. The focal length in a subscanning direction is shortened by the concave cylinder lens 14 constituting the ftheta lens and the convex cylinder lens 15 arranged between the ftheta lens and a scanning image formation surface 16, so the deviation of a beam spot on the scanning image formation surface due to the influence of the slanting and axial shift of a rotary polygon mirror is reduced. Consequently, the burning of the reflecting surface is reduced. |
