Patent ID: 8593645

Claim:
An apparatus, comprising: an optical surface; and a measurement device configured to measure a parameter related to the optical surface at a plurality of separated areas on the optical surface, the measurement device comprising: a) an illumination unit configured to direct individual measuring light beams towards the plurality of separated areas on the optical surface, each measuring light beam simultaneously illuminating: i) a portion of a first one of the plurality of separated areas on the optical surface which is associated with the measuring light beam; and ii) a portion of a second one of the plurality of separated areas on the optical surface which is not associated with the measuring light beam, the portion of the second one being adjacent the portion of the first one; b) a detector unit configured to measure a property for each measuring light beam after it has interacted with the optical surface; and c) an evaluation unit configured to determine the optical surface-related parameter for the portion of the first one based on the properties measured by the detector unit for: i) the measuring light beam that is associated with the portion of the first one; and ii) at least one measuring light beam that is associated with the portion of the second one, wherein the apparatus is a microlithographic projection exposure apparatus.