Patent ID: 7599074

Claim:
Apparatus for measuring a change of angular orientation of a test object, the apparatus comprising; an optical source providing an incident beam of substantially monochromatic optical radiation to said test object at an angle of incidence θ i , wherein a diffraction grating disposed on said test object is illuminated by said incident beam and provides a first diffracted beam having a first diffraction angle θ d1 responsive to said incident beam; wherein for a reference angular orientation of said test object, said incident beam and said first diffracted beam define a first diffraction plane; a first optical detector receiving said first diffracted beam and providing a first output responsive to a position of said first diffracted beam on said first detector in said first diffraction plane; wherein a first change ε 1 of angular orientation of said test object in said first diffraction plane relative to said reference angular orientation induces corresponding changes Δθ i of said angle of incidence and Δθ d1 of said first diffraction angle; wherein Δθ i =ε 1 and wherein θ i and θ d1 are selected such that Δθ d1 is substantially greater than ε 1 .