1. Field of the Invention
The present invention relates to an optical scanner using a rotary polygon mirror.
2. Description of the Related Art
In an optical scanner, a parallel light beam from a light source device is deflected on a rotary polygon mirror and is converged as a light spot on a scanned face by an image forming lens system to perform an optical scanning operation. The optical scanner is widely known in association with an optical printer, a digital copying machine, etc.
Recently, a high quality of an image in an image outputting device using such an optical scanner is strongly required. Further, there are various kinds of proposals for stabilizing a diameter of the light spot scanning the scanned face in the optical scanner.
The diameter of the light spot is mainly varied by a shift between the scanned face and an image forming position of the light spot caused by field curvature. Therefore, stabilization of the spot diameter depends on how to preferably correct field curvature in the image forming lens system. A so-called face inclination is caused as a problem when the rotary polygon mirror is used as an optical deflector. To solve this problem, the image forming lens system is constructed by an anamorphic optical system having different refracting powers in a main scan-corresponding direction and a cross scan-corresponding direction. In this case, the main scan-corresponding direction is a direction parallel to a main scanning direction and corresponding to this main scanning direction on a virtual optical path provided by linearly developing an optical path from a light source to the scanned face along an optical axis of the optical system. The cross scan-corresponding direction is a direction parallel to a cross scanning direction and corresponding to this cross scanning direction on this virtual optical path. The image forming lens system may have an f.theta. function to perform the optical scanning operation using the light spot at an equal speed. However, it is difficult to preferably satisfy both field curvatures in the main scanning direction and the cross scanning direction while requirements for various kinds of optical performances such as the anamorphic optical system, the f.theta. function, etc. are satisfied.
An optical scanning optical system considering such a situation is shown in Japanese Patent Application Laying Open (KOKAI) No. 3-54513. In this optical system, a linear image extending in the main scan-corresponding direction is formed in the vicinity of a deflecting reflecting face of the rotary polygon mirror. An image forming optical system for converging a light beam deflected on the deflecting reflecting face onto the scanned face is constructed by a spherical lens system and a cylindrical mirror. A radius of curvature of the cylindrical mirror in the cross scan-corresponding direction is changed in the main scan-corresponding direction so that field curvature in the cross scanning direction is corrected.
With respect to the rotary polygon mirror, no rotating axis of the deflecting reflecting face is in conformity with an axis of the deflecting reflecting face. Therefore, the relation in position between the deflecting reflecting face and a linear image extending in the main scan-corresponding direction is asymmetrically changed with respect to an image height 0 of the light spot as the deflecting reflecting face is rotated. This problem is called sag.
No problem of sag is considered in the optical scanning optical system shown in the above Japanese Patent Application Laying Open (KOKAI) No. 3-54513. Therefore, field curvature can be preferably corrected on one main scanning side with respect to the image height 0 of the light spot, but is increased on the other main scanning side.