| 摘要 |
PURPOSE:To convert a composite focal distance to a short focus with regard to a sub-scanning direction, to reduce the deviation of a beam spot, and to remarkably relax the assembly and adjustment accuracy, etc., by forming an annular surface having a negative power in the sub-scanning direction, on one face of an image forming lens system. CONSTITUTION:As for an image forming lens 14, a face of its image forming surface side is formed roughly in the shape of a spherical surface and has an ftheta lens action, and also, a face 14a of a light source side is formed on a face having the respective prescribed radiuses of curvature being different in the main scanning direction and the sub-scanning direction, and also, formed to an annular face having a negative power in the sub-scanning direction. In such a way, with regard to the sub-scanning direction, a rear side principal point by synthesizing the image forming lens system 14 and a convex cylinder lens 15 moves to a scan image forming surface 16 side, and a composite focal distance is converted to a short focus. Therefore, the deviation of a beam spot on the scan image forming surface 16, generated by an influence of an inclination of a rotary polygon mirror 12 and an axial shift, etc. is reduced, a working error of the rotary polygon mirror 12, a deflection and an inclination of the motor shaft, and the assembly and adjustment accuracy of the rotary polygon mirror and the driving shaft can be relaxed remarkably, and the cost can be reduced remarkably. |
