Patent ID: 8422027
Filing Date: 2013-04-16
Classification: G01B,G03F

Abstract:
1. Apparatus for detecting changes in the lateral position of an moveable object with respect to image plane or object plane in an imaging optical system, comprising (a) a primary imaging optical system configured to direct an image along a primary imaging path from an object plane to an image plane, (b) a measurement diffraction grating being integrally connected with the moveable object and configured to move with the the moveable object in directions lateral to the primary imaging path; (c) a beam generator arranged outside the primary imaging path, that generates a reference beam and a measurement beam; (d) the measurement beam directed at the measurement diffraction grating to produce a diffracted measurement beam, and the reference beam being directed at a component of the apparatus that cannot move laterally with the image plane or the object plane, to produce a diffracted reference beam, such that the reference beam is unaffected by changes in the lateral position of the moveable object; and (e) a detection system that directs at least a component of the reference beam and a component of the diffracted measurement at a detector configured interferometrically to detect phase changes between the reference beam component and the diffracted measurement beam component, to produce an output related to changes in the lateral position of the moveable object.