Patent ID: 7471372

Claim:
Interferometer system for measuring changes in altitude between a first surface and a second surface, the system comprising: a first member that has the first surface; a second member mounted for plane movement relative to the first member in at least two orthogonal directions and in a direction perpendicular to the at least two orthogonal directions; a reflector that has the second surface, the reflector being provided to the second member for movement therewith, the second surface lying in a plane substantially parallel to the plane in which the second member moves; and at least one interferometer apparatus, the interferometer apparatus including: a source that generates a first beam and a second beam; a first path along which the first beam travels to and from the first surface; and a second path along which the second beam travels to the second surface, the second beam strikes the second surface at normal incidence and returning therefrom to be combined with the first beam to provide a signal containing information that varies in accordance with the changes in altitude between the first surface and the second surface, wherein: the second path includes a first portion that is parallel to the plane of the movement of the second member and a second portion that is perpendicular to the plane, and the second beam strikes the second surface in the second portion of the second path.