A micrometric non-contact profiler for optical quality surfaces

A non-contact technique for obtaining accurate profiles of optical quality surfaces with micrometric accuracy has been developed. The technique is bas ed on the Ronchi test principle, that is, on the study of the interaction o f a wavefront reflected on the surface to be profiled with a square-wave tr ansmittance ruling. From the resultant fringe pattern and some basic geomet rical optics principles it is possible to measure the local normal to the s urface being tested at a set of given points. This local normal map may the n be integrated, yielding the surface profile. By use of a theoretically ex pected surface shape, the main parameters of the surface may then be determ ined by surface fitting of the measured data to that expected surface shape . Results of the profilometric measurements both of a spherical and of a to roidal surface are presented. The measured profiles are validated by compar ison of the radii of curvature obtained using a high precision radioscope w ith the ones obtained by surface fitting of the measured profiles to their expected surface shapes. Additionally, subtracting the best-fit theoretical surface from the measured profile allows the observation of surface deviat ions from the theoretical shape to within some tenths of a nanometres.