Patent ID: 8149386

Claim:
An illumination optical system comprising: a pair of fly-eye mirrors configured to receive light from a light source; a first condenser configured to condense light from the pair of fly-eye mirrors; a reflection type integrator configured to receive light from the first condenser, the reflection type integrator including a plurality of cylindrical reflective surfaces having parallel generating line directions; an aperture stop arranged perpendicular to the generating line direction; and a second condenser configured to superpose on an illuminated surface luminous fluxes from a plurality of cylindrical reflective surfaces of the reflection type integrator, wherein a pair of fly-eye mirror includes a first fly-eye mirror that includes a plurality of element mirrors, and a second fly-eye mirror that includes a plurality of element mirrors each corresponding to one of the element mirrors of the first fly-eye mirror, and wherein the number of element mirrors of at least one of the first fly-eye mirror and the second fly-eye mirror is different between in the generating line direction and in an arrangement direction of the plurality of cylindrical reflective surfaces.