| 摘要 |
PURPOSE:To flatten an image surface by providing an image forming lens with a negative coefficient of tertiary sagittal curvature of field and setting the absolute value of the coefficient of tertiary sagittal curvature of field larger than the absolute value of a coefficient of tertiary meridional curvature of field. CONSTITUTION:An optical scanning system is equipped with a rotary polygon mirror 2 and a lens system for scanning arranged between the rotary polygon mirror and a scan surface; and the lens system for scanning is composed of the image forming lens 3 and a cylindrical mirror 7, the negative coefficient of tertiary sagittal curvature of field is provided as the design reference of the image forming lens, and the absolute value of the coefficient of tertiary sagittal curvature of field is set larger than the absolute value of the coefficient of tertiary meridional curvature of field. The distortion characteristic of the image forming lens 3 is defined by s=[(image height-ftheta)/ftheta)X100%, where (f) is the focal length of the image forming lens 3, theta is the angle of deflection of a beam incident on the image forming lens 3, and (s) is the degree of distortion of the image surface. Then, the degree (s) of distortion of the image surface is suppressed to <=1%, so almost no curvature of field appears. |
