Patent ID: 8842291

Claim:
A method for measuring angular deflections of a target, the method comprising: splitting an input electromagnetic beam into a first beam and a second beam, the location of the splitting defining a first position; propagating the first beam along a first portion of a first propagation path, the first portion of the first propagation path extending from the first position to a reflecting surface of a target, the first beam being incident on the target reflecting surface approximately normal to the target reflecting surface and reflecting from the target reflecting surface; propagating the reflected first beam along a second portion of the first propagation path, the second portion of the first propagation path extending from the target reflecting surface to a position-sensitive photodetector, the first beam being reflected an even number of times during its propagation along the second portion of the first propagation path; propagating the second beam along a first portion of a second propagation path, the first portion of the second propagation path extending from the first position to the target reflecting surface, the second beam being incident on the target reflecting surface approximately normal to the target reflecting surface and reflecting from the target reflecting surface, the first and second beams being at least partially superimposed at the target reflecting surface; propagating the reflected second beam along a second portion of the second propagation path, the second portion of the second propagation path extending from the target reflecting surface to the position-sensitive photodetector, the second beam being reflected an odd number of times during its propagation along the second portion of the second propagation path, the first and second beams interfering at the position-sensitive photodetector so as to produce interference patterns; interpreting the interference patterns to determine angular deflections of the target reflecting surface.