Patent ID: 7773197

Claim:
An illumination optical apparatus configured to irradiate a light beam from a light source onto a surface to be irradiated, the illumination optical apparatus comprising: a first group of prisms which has a plurality of prisms; a second group of prisms which has a plurality of prisms; and a switch of a group of prisms which is capable of switching the first and the second groups of prisms so as to position one of the first and the second groups of prisms in an optical path, wherein an annular ratio of the light beam emitted from one of the first and the second groups of prisms is changed by moving at least one of a plurality of prisms included in one of the first and the second groups of prisms positioned in the optical path in an optical axis direction, wherein an upper limit of the annular ratio of the light beam emitted from the first group of prisms is equal to or higher than a lower limit of the annular ratio of the light beam emitted from the second group of prisms and is lower than the upper limit of the annular ratio of the light beam emitted from the second group of prisms, wherein the first group of prisms includes a first concave prism and a first convex prism whose inclined surfaces are arranged so as to face each other, wherein the second group of prisms includes a second concave prism and a second convex prism whose inclined surfaces are arranged so as to face each other in opposite directions, and wherein at least one of the concave prism and the convex prism included in one of the first and the second groups of prisms positioned in the optical path is movable in the optical axis direction.