Patent ID: 7057742

Claim:
An interferometer for comparing an object surface to a reference surface for measuring departures of the object surface from a nominal geometric surface form of the reference surface comprising: an illuminating system that illuminates the object surface with an object beam and the reference surface with a reference beam along separate optical pathways such that the object beam does not pass through the reference surface on route to the object surface; an imaging system that produces overlapping images of the object surface and the reference surface with the object and reference beams containing interferometric data based on path length differences between the object and reference beams reflected from a plurality of corresponding points on the object and reference surfaces; a frequency regulator that varies a frequency of the object and reference beams at which the interferometric data is produced over a range of different beam frequencies; a data acquisition system that gathers the interferometric data from the plurality of corresponding points within the overlapping images of the object and reference surfaces as the interferometric data varies through conditions of constructive and destructive interference as a result of the variation in the beam frequency; a processor that converts the variations in the interterometric data as a function of the variation in beam frequency into measures of the path length differences between the object and reference beams from the plurality corresponding points on the object and reference surfaces and converts the path length differences between the object and reference beams from the plurality of corresponding points on the object and reference surfaces into measures of departures of the object surface from the nominal geometric surface form of the reference surface; and the reference surface being a substantially diffuse surface that scatters rays of the reflected reference beam through a plurality of different angles that contribute to the imaging of the reference surface by the imaging system.