Tracking device using interference and reference signals, and controlling position of reference surface to maintain predetermined point in range of scanning

摘要

A method of tracking the position of an object surface (12) includes generating an interference signal between light beams of short temporal coherence length respectively comprising a primary beam (10) reflected or scattered from the object surface (12) and a reference beam (8). A reference surface (14) in the path of said reference beam (8) is scanned about a position at which the interference signal is generated, which position is thereby indicative of the position of the object surface (12). In one aspect, the position of the reference surface (14) is controlled (16, 18) to maintain a predetermined point in the range of the scanning at the indicative position.