| 摘要 |
PURPOSE:To correct curvature of field due to a shift in entrance pupil position resulting from the rotation of a rotary polygon mirror sufficiently in a main-scanning and a subscanning direction and to prevent the surfaces tilt of the rotary polygon mirror by providing 1st - 4th lens surfaces in order from the side of the rotary polygon mirror toward a scanning surface and setting those surfaces so as to hold specific requirements. CONSTITUTION:This lens system has a function for putting the reflection position and scanning surface of the rotary polygon mirror in almost conjugate relation geometrically and optically in the subscanning direction and also has an ftheta function in the main scanning direction. Then the 1st - 4th lens surfaces are provided from the side of the rotary polygon mirror to the scanning surface and hold the specific requirements shown by inequalities I and II, where Rix is the radius of curvature of each lens surface in a deflection surface, R>=y is that in the subscanning direction, and n1 and n2 are the refractive indexes of the 1st and 2nd lenses. Further, a deformed toric surface is a curved surface satisfying an equation III, where Riyo is the radius of curvature on the optical axis in the subscanning direction, Hi is the distance from the optical axis in the main scanning direction, and the sign is '-'when Rix > 0 and '+' when Rix < 0. Consequently, the curvature of field due to the shift in entrance pupil position resulting from the rotation of the rotary polygon mirror is corrected sufficiently in the main-scanning and subscanning directions and the surfaces tilt of the rotary polygon mirror are prevented. |
