| 摘要 |
PURPOSE:To suppress the change in the deflection point of a polygon mirror and to well correct the curvature of field by providing a condensing position changing element which brings the condensing position in the peripheral part of an image plane substantially near to a scanning lens side than the condensing position in the central part of the image plane. CONSTITUTION:This device has a laser diode 10, a collimating lens 11 which collimates the divergent light emitted from the laser diode 10 into collimated beams of light, a turnback mirror 12 which reflects the collimated luminous flux, a cylindrical lens 13 as the imaging lens to form line images by this luminous flux, a prism block 20 having a slit mirror provided in alignment to the line image position of the luminous flux, the polygon mirror 30 which reflects and polarizes the luminous flux reflected by the slit mirror 21, and an ftheta lens 40 which condenses the reflected luminous flux by the polygon mirror 30 and forms a spot in the image plane. The prism block 20 provided in the optical path does not act on the luminous flux on the main scanning plane and generates the effect of the focus movement by the incident angle on the sub- scanning surface. The curvature of field is corrected by the condensing position changing element in this way. |
