Patent ID: 7330274

Claim:
A method comprising: using an interferometer in an interferometry system to produce an output beam comprising a phase related to an optical path difference between paths of two beams, at least one of which contacts a measurement object in the interferometry system, wherein the two beams are derived from an input beam and the interferometer comprises interferometer optics spaced from the measurement object; providing precalibrated information that accounts for contributions to the optical path difference caused by a deviation of at least one of the beam paths from a nominal beam path due to an imperfection in at least one of the interferometer optics or an imperfection in a light source used to produce the input beam; determining a position of the measurement object with respect to at least one degree of freedom based on information derived from the output beam and the precalibrated information; and moving the measurement object with respect to the interferometer based on the determined position.