| 摘要 |
PURPOSE:To allow the good correction of the curvature of field in main and sub-scanning directions by constituting the lens of two elements in two groups, in which the 1st lens has a negative refracting power and the 2nd lens a positive refracting power within the parallel plane and determining the composite focal length within the orthogonal plane of polarization and the radii of curvature of the respective faces so as to satisfy specific conditions. CONSTITUTION:This device has the function to connect the reflecting position of a rotary polyhedral mirror 3 and the surface to be scanned to a conjugation relation with respect to the sub-scanning direction and has the ftheta function with respect to the main scanning direc tion. This lens is constituted 4, 5 of the two elements in two groups disposed from the side of the rotary polyhedral mirror 3 toward the side of the surface to be scanned in order of the 1st and 2nd groups. The shapes within the plane of polarization of these lens faces are successively arc, straight, straight, and arc from the 1st face to the 4th face. The 1st lens 4 has the negative refracting power and the 2nd lens 5 the positive refracting power within the plane parallel with the plane of polarization. The combined focal length within the orthogonal plane of polarization, designated as fs, and the radii of curvature of the 1st, 3rd, 4th faces within the orthogonal plane of polarization, designated as r'1, r'3, r'4, are determined to satisfy the conditions 2.20<{(1/r'1)+(1/r'3)-(1/r'4)}.fs<3.51. The curvature of field in the main and sub-scanning directions is well corrected in this way and the high- density optical scanning is possible. |
