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.