Patent ID: 7027162

Claim:
A measurement device, comprising: an interferometer, the interferometer comprising a polarizing first beam splitter, a first quarter-wave retardation plate, a second quarter-wave retardation plate, a reflector, and a fringe counter, the interferometer capable of receiving a radiation beam, the radiation beam comprising a first beam component and a second beam component, the first beam component having a first polarization and the second beam component having a second polarization, the second polarization orthogonal to the first polarization; an autocollimator, the autocollimator comprising a second beam splitter, a first lens, and a detector, the detector having at least a first sensing surface; the first beam splitter reflecting an outgoing target portion of the first beam component through the first quarter-wave retardation plate and the second beam splitter to impinge upon a planar reflecting target, the target reflecting a return target portion of the first beam component back to the second beam splitter, the second beam splitter reflecting an autocollimator portion of the return target portion of the first beam component through the first lens, the first lens focusing the autocollimator portion of the return target portion of the first beam component onto a point on the first sensing surface, the detector generating a first output signal corresponding the location of the point on the first sensing surface; the second beam splitter, the first quarter-wave retardation plate, and the first beam splitter transmitting an interferometer portion of the return target portion of the first beam component to the fringe counter; and the first beam splitter and the second quarter-wave retardation plate transmitting the second beam component to the reflector, the reflector reflecting the second beam component back through the second quarter-wave retardation plate to impinge on the first beam splitter, the first beam splitter reflecting the second beam component to the fringe counter while merging the second beam component with the interferometer portion of the return target portion of the first beam component, the fringe counter generating a second output signal in response to interference between the interferometer portion of the return target portion of the first beam component and the second beam component.