Patent ID: 8164738

Claim:
An illumination optical system arranged to illuminate an illumination target plane by using light from a light source, the illumination optical system comprising: a prism unit arranged to refract the light; an optical integrator arranged to form a plurality of light sources with light emerging from the prism unit; and an optical system arranged to introduce light emerging from the optical integrator to the illumination target plane, wherein the prism unit includes a pair of conical refractive surfaces having a conical concave refractive surface and a conical convex refractive surface, and a pair of pyramidal refractive surfaces having a pyramidal concave refractive surface and a pyramidal convex refractive surface, and at least one of the concave refractive surface and the convex refractive surface of one pair is disposed between the concave refractive surface and the convex refractive surface of the other pair, wherein a first subunit includes the conical concave refractive surface and the pyramidal concave refractive surface, and a second subunit includes the conical convex refractive surface and the pyramidal convex refractive surface, wherein at least one of the first subunit and the second subunit is movable in a direction of an optical axis of the illumination optical system such that an interval between the first subunit and the second subunit is variable, and wherein, when the interval between the first subunit and the second subunit varies, a distance between the refractive surfaces of the first subunit and a distance between the refractive surfaces of the second subunit remain substantially unchanged.