Patent ID: 8164046

Claim:
An illumination system having a surface, the illumination system comprising: a) an array of reflective or transparent beam deflecting elements, wherein each beam deflecting element is configured to produce on the surface a projection light spot at a position that is variable by changing a deflection angle produced by the beam deflecting element, b) a spot shape measuring unit which is configured to measure the shapes of spots which are produced on the spot shape measuring unit by the beam deflecting elements, wherein the spot shape measuring unit is arranged outside of every possible path projection light is allowed to take between the array and the mask, and c) a control unit which is configured to control the beam deflecting elements, wherein the control unit is configured such that, at a given instant during an exposure operation of the apparatus, at least one beam deflecting element directs projection light exclusively on the spot shape measuring unit, and at least some beam deflecting elements direct projection light exclusively on the surface, and the illumination system is configured to be able to illuminate a mask in a microlithographic projection exposure apparatus.