Optical measurement of narrow gap between surfaces of test object and transparent element

摘要

A beam of polarised light (5) is directed at an oblique angle against a surface (25) of a transparent element. The beam has a plane of incidence defined by a polarisation base vector p, and a base vector s at ninety degrees with respect to the vector p. Polarisation of both s and p types are present in the beam incident to the transparent element surface. The light beam is reflected back from the transparent element surface, and the reflected beam (6) contains both s and p type polarisation. The two polarisation components have opposing relative phase positions and associated amplitudes. An interference of the reflected polarisation components is performed to determine the relative phase position and amplitudes. From this information the gap can be determined.