| 摘要 |
PROBLEM TO BE SOLVED: To provide high plotting precision by suppressing the curvature of field in the sub-scanning direction to be small. SOLUTION: A first lens 5a having positive powers in both the main scanning and the sub-scanning directions and the cross sectional shape of a meniscus in the sub-scanning direction whose convex surface confronts the side of a polygon mirror 4 is added to a fθlens 5, being composed of three lenses by arranging, in order from the side of the polygon mirror 4, a second negative lens 5b, a third positive lens 5c and a fourth positive lens 5d, on the side of the polygon mirror 4 of the scanning lens. The second lens 5b is provided with a cylindrical surface having a negative power only in the sub-scanning direction and the fourth lens 5d is provided with a toric plane having a positive power. |
