Comparison of asphere measurements by tactile and optical metrological instruments

A comparison of topography measurements of aspherical surfaces was carried out by European metrology institutes, other research institutes and a company as part of an European metrology research project. In this paper the results of this comparison are presented. Two artefacts were circulated, a small polymer coated aspherical lens with a clear aperture of about 12 mm, and a large conical convex lens with a clear aperture of 300 mm developed for the ESO Very Large Telescope. The participating laboratories were allowed to follow their own measurement strategies. Both tactile and optical measuring instruments were used, as well as single point and imaging techniques. The measured data were compared with respect to the root-mean-square (RMS), peak-to-valley and Zernike polynomial representations of the measured deviations from the nominal shape. The comparison shows for five out of eight measuring instruments/methods a very good agreement of the measured topographies within 14 nm (RMS)