Patent ID: 8109636

Claim:
An illumination optical system that guides light emitted from a light source to an image forming device, the illumination optical system comprising: a rod type integrator that uniformizes luminance distribution of light emitted from the light source; a relay optical system that guides light having the illumination distribution uniformized by the rod type integrator to the image forming device; and a shield board disposed on an optical path between the rod type integrator and the relay optical system, wherein the shield board has an opening which is smaller than the outlet end face of the rod type integrator and has similar shape to an image formation area on the image forming device, and is movable in a plane containing the opening, wherein the shield board and the image forming device have a conjugate relation, and the shield board acts as an object face and the image forming device acts as an imaging surface in the conjugate relation, and wherein an image of an opening of the shield board is projected on the image forming device at a similar magnification.