Abstract: A method of compensating for sample misalignment in an optical measurement apparatus (40), comprises the steps of: determining an expected response from a detector (58) in said optical measurement apparatus given a particular set of parameters defining a path that light can take through the optical measurement apparatus from a source (42), via a sample (50), to the detector (58); measuring a response from the detector for the sample under test; and refining the set of parameters until the expected response and the measured response converge so as to determine the set of parameters giving rise to the measured response.

Abstract: A method and means for determining the thickness, or curvature, of a thin film or stack of thin films disposed on the surface of a substrate having a curvature comprising generating a beam of radiation, focusing the beam through the one or more films onto a surface of the substrate, measuring the intensity across the reflected beam as a function of the angle of incidence of a plurality of rays derived from the focussed beam, determining the path of each of the plurality of rays and determining the thickness, or curvature of the film, or films, from the angular dependent intensity measurement.