Patent ID: 7933003

Claim:
A method of measuring a surface using an interferometer, in which there is relative motion between the surface and the interferometer, the motion having a total velocity V tot which includes a transversal or traversing component V t and a longitudinal component V l , and a transversing component in a transversal direction relative to the longitudinal component V l , the method comprising: directing an object beam of coherent light to a measurement position at the surface, whereby there is relative motion between the surface and the measurement position; arranging an array of detectors on the interferometer in a line extending generally in the transversal direction, the detectors being arranged to detect light rays with different angular directions, representing different sensitivity directions; producing a reference beam of coherent light which is at least partly coherent with the object beam; combining the reference beam with the reflected object beam from the surface to produce a cross interference in the speckle pattern providing information about the relative motion of the surface and the interferometer; detecting the speckle pattern and the cross interference pattern with the detectors; determining which detector in the array has zero or minimum sensitivity to the total velocity V tot of the motion; thereby identifying the detector associated with a sensitivity direction that is normal to V tot while other detectors are associated with other sensitivity directions and sense a smaller or larger part of the total velocity V tot ; monitoring a temporal change in the detector which has zero or minimum sensitivity to the total velocity, thereby ascertaining the change in direction of V tot over time, brought about by changes in V l ; and determining temporal changes in V l .