A microlithographic projection exposure apparatus includes an optical surface and a measurement device which measures a parameter related to the optical surface at a plurality of separated areas on the optical surface. The measurement device includes an illumination unit which directs individual measuring light beams towards the areas on the optical surface. Each measuring light beam illuminates at least a portion of an area, which is associated with the measuring light beam, and at least a portion of an adjacent area which is not associated with the measuring light beam. A detector unit measures a property for each measuring light beam after it has interacted with the optical surface.