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).
Precisely known artifacts are required to characterize the accuracy of asphere and freeform measuring instruments. To this end the best knowledge of the surface characteristics in conjunction with a low measurement uncertainty are necessary. Because this is a challenging task for typical freeform surfaces used in optical systems, the concept of "metrological" artifacts is introduced. We have developed a multispherical freeform artifact for performance tests of tactile touch probe and contact-free optical measuring systems. The measurement accuracy of the complete form and the deviation from calibrated spherical sections can thus be determined. The radius calibration of multiple spherical sections is performed with an extended radius measuring procedure by interferometry. Evaluated surface forms of different measuring methods and the radii determined can be compared to each other. In this study, a multispherical freeform specimen made of copper, with two differing radii, has been measured by two optical measuring methods, a full field measuring tilted-wave interferometer and a high accuracy cylinder coordinate measuring machine with an optical probe. The surface form measurements are evaluated and compared, and the radii determined are compared to the results of a radius measurement bench.
The concept of traceability is presented for the interferometric form measurement of optical surfaces. The calibration chain for interferometric flatness measurement is evaluated in detail, showing that only a few influence quantities are significant. For spherical surfaces, the complexity increases as the measurement separates into sphericity and radius measurement. Traceability in asphere metrology is much more complex, and some aspects are discussed in terms of the example of the Tilted-Wave Interferometer concept.
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