Patent ID: 8520291

Claim:
An illumination optical system which illuminates an irradiated plane with illumination light provided from a light source, the illumination optical system comprising: a spatial light modulator which is arranged in an optical path of the illumination optical system and cooperates with part of the illumination optical system to form a desired light intensity distribution at a pupil position of the illumination optical system or a position optically conjugated with the pupil position; a detection unit which detects the light intensity distribution of the illumination light at a position in an optical path extending from the light source to the spatial light modulator, with the detection unit including a light receiving surface which receives some of the illumination light; a control unit which controls the spatial light modulator based on the light intensity distribution detected by the detection unit; an information acquisition unit which acquires information on the light intensity distribution at the pupil position of the illumination optical system or the position optically conjugated with the pupil position; and an optical path branching member which is arranged in an optical path between the spatial light modulator and the irradiated plane, with the optical path branching member branching off some of the illumination light from the optical path and guiding the branched off light to the information acquisition unit, wherein the control unit corrects the controlling executed by the control unit based on the light intensity distribution at the pupil position of the illumination optical system or the position optically conjugated with the pupil position that is acquired by the information acquisition unit.