Patent ID: 7130056

Claim:
A system for acquiring position information relevant to a specific axis comprising: a movable apparatus having first and second reflective faces at a same side associated with a parallel to said specific axis, said first reflective face being at an angle to said second reflective face and said first and second reflective faces being non-parallel to said specific axis; an interferometer positioned to direct a first beam for impingement of said first reflective face and to direct a second beam for impingement of said second reflective face, said interferometer including a beam combiner aligned with a detector and aligned to combine said first and second beams; and beam-steering members located with respect to said interferometer and said first and second reflective faces to manipulate said first and second beams to reach said beam combiner without a beam path segment that varies in length in unity with displacements of said movable apparatus along said specific axis.