Patent ID: 7489407

Claim:
A method, comprising: using an interferometer to produce an output beam comprising a phase related to an optical path difference between a path of a first beam and a path of a second beam, wherein the first beam contacts a measurement object and either the measurement object or the interferometer are coupled to a stage that is moveable within a reference frame; monitoring variations in the phase while both varying an orientation of the stage with respect to at least one degree of freedom in the reference frame and keeping a reference mark on the stage in a common position with respect to the reference frame; and determining information based on the monitored variations, the information being related to a contribution to the optical path difference caused by a deviation of the path of the first or second beam from a nominal beam path, wherein the information is determined based on additional monitored variations of the phase associated with varying an orientation of the stage with respect to the reference frame while keeping a second alignment mark on the stage at a common position with respect to the reference frame.