Patent ID: 8339577

Claim:
An illumination system having a pupil surface, the illumination system comprising: an essentially flat arrangement of beam deviating elements configured to variably illuminate the pupil surface of the illumination system, each beam deviating element configured to allow deviation of a projection light beam incident thereon in response to a control signal applied to the beam deviating element; and a measurement illumination instrument configured to direct a measurement light beam independent of the projection light beam onto a beam deviating element; a detector instrument configured to record the measurement light beam after deviation by the beam deviating element; and an evaluation unit to determine the deviation of the projection light beam from measurement signals provided by the detector instrument, wherein: the beam deviating elements comprise tiltable mirrors, and each tiltable mirror has a reflecting mirror surface; a plurality of measurement light beams can be directed onto an individual reflecting mirror surface, and a curvature of a reflecting mirror surface can be ascertained by the evaluation unit on the basis of the different deviations which are experienced by the measurement light beams on the reflecting mirror surface, and the illumination system is configured to be used in a microlithographic projection exposure apparatus.