Patent ID: 7929187

Claim:
An imaging optical system for image reading, in which an image of image information on a surface of an original is formed onto a line sensor aligned in a main scanning direction, and the line sensor reads the image information by changing a relative position of the original surface and the line sensor in the sub-scanning direction, wherein off-axial reflecting surfaces constituting the imaging optical system consist of a first off-axial reflecting surface and a second off-axial reflecting surface, and the first off-axial reflecting surface and the second off-axial reflecting surface are of asymmetry in the sub-scanning section with respect to a reference axis, wherein the first and second off-axial reflecting surfaces are, respectively, a plus deflecting surface and a minus deflecting surface, or a minus deflecting surface and a plus deflecting surface, disposed in the mentioned order from the original surface side in the optical path of the imaging optical system, where an off-axial reflecting surface that deflects a reference axis beam in a first direction with respect to a rotational axis which is a straight line perpendicular to the sub-scanning section in the sub-scanning section is defined as a minus deflecting surface and an off-axial reflecting surface that deflects the reference axis beam in a second direction, which is an opposite direction to the first direction, with respect to a rotational axis which is a straight line perpendicular to the sub-scanning section in the sub-scanning section is defined as a plus deflecting surface, and wherein the following condition is satisfied, 0.01<φ2/φ4<0.1, where φ2 and φ4 represent refractive powers of the first and second off-axial reflecting surfaces in the sub-scanning direction, respectively.