Patent ID: 8638420

Claim:
An illumination optical system for illuminating a surface to be illuminated with light from a light source, comprising: an optical integrator arranged in an optical path between the light source and the surface to be illuminated and comprising a plurality of wavefront dividing elements two-dimensionally arrayed; and a light collecting optical system arranged between the optical integrator and the surface to be illuminated, which guides the light from the optical integrator to the surface to be illuminated, wherein each of the plurality of wavefront dividing elements includes at least one entrance-side optical face and at least one exit-side optical face, each of the plurality of wavefront dividing elements is so configured that a ray group obliquely incident to a center on an optical axis of an entrance face of each wavefront dividing element is emitted in parallel with the optical axis from the wavefront dividing element, in at least one wavefront dividing element out of the plurality of wavefront dividing elements, at least one curved optical face of the at least one wavefront dividing element is formed as inclined around an axis along a predetermined direction passing the optical-axis center of a predetermined entrance-side optical face of the at least one wavefront dividing element and being perpendicular to the optical axis, and illumination regions on the surface to be illuminated, which are illumination regions respectively illuminated with lights from the plurality of the wavefront dividing elements, are partially overlapped with respect to each other.