Patent ID: 8749864

Claim:
An image reading optical system, comprising: an imaging optical system which is used for imaging a slit area of a document on an image sensor and includes an optical element having different cross section shapes in a main scanning direction as a longitudinal direction of the slit area and in a sub-scanning direction perpendicular to the main scanning direction; an aperture stop disposed in an imaging optical path from the document to the image sensor; and an optical phase changing filter which is disposed adjacent to the aperture stop and includes a phase lead area having a phase lead action of leading a phase of a wavefront of an incident beam, and a phase delay area having a phase delay action of delaying the phase of the wavefront of the incident beam, with reference to a wavefront phase at a center of the incident beam, wherein the optical phase changing filter includes a surface shape component that is symmetric only with respect to a predetermined plane including a surface normal at the center of the incident beam and one of the main scanning direction and the sub-scanning direction, and wherein, with respect to a surface that includes the surface normal at the center of the incident beam and is perpendicular to the predetermined plane, one side is the phase lead area, and another side is the phase delay area.