Patent ID: 7433057

Claim:
In a metrology system including a part-positioning means and a wavefront-measuring gauge, a method for obtaining high-accuracy measurements of surfaces or wavefronts with underlying symmetry in a design prescription, the method comprising the steps of: a) acquiring a plurality of measurements at different relative positions of said part and gauge, and b) performing at least one of the following steps: i) determining said gauge's optimal focus position for each of said measurements using knowledge of the local curvature of each test wavefront in said design prescription and gauge calibration constants; ii) calibrating systematic gauge wavefront error in said measurements, by averaging said measurements having the same test conditions of said gauge and the positions of said measurements to average follow said symmetry, such that said gauge measurements at all of said positions would be identical if said part were perfectly matched to said design prescription; iii) reducing uncalibrated error by averaging said measurements obtained at the same nominal location of the test part, but where two or more of said measurements have different orientations relative to said gauge; and iv) reducing uncalibrated error by stitching said measurements together, where low spatial frequency information in said measurements is explicitly filtered out or compensated implicitly with free compensators.