Patent ID: 7612892

Claim:
An apparatus comprising (a) an imaging system including a primary optical path for imaging an image onto an object and an optical element arranged in the primary optical path, the imaging system defining an object plane and an image plane; (b) a stage configured to support a surface under investigation in the primary optical path, (c) a measurement optical path, the measurement optical path including: (i) an input path for directing radiation from a measurement radiation source onto the surface under investigation supported by the stage, the input path extending at least partially through the primary optical path, including the optical element arranged in the input path, (ii) an output path for receiving reflected radiation from the surface under investigation, the output path including the optical element arranged in the output path (iii) wherein the primary optical path includes an insertion location wherein the input path enters the primary optical path and is directed at least partially through the optical element arranged in the primary optical path; and (d) a control system including a detector, the detector optically coupled to the output path, and configured to receive the reflected radiation from the surface under investigation, the reflected radiation including topographic information related to the topography of the surface under investigation, wherein a processor is programmed to calculate a surface topography map, and the control system further configured to produce output for controlling the position of the stage using the surface topography map, so that the surface under investigation is in a predetermined relationship to the image plane and the object plane of the imaging system.