High-Accuracy Characterisation of Spheres Using PTB’s Sphere Interferometer with an Enhanced Stitching Procedure

Abstract

Against the background of the activities of the National Metrology Institutes for the redefinition of the SI unit "kilogram" the sphere interferometer of PTB has been developed for the measurement of the volume of spherical material measures [1,2]. Provided that the shape deviation is small the mean diameter is sufficient for this purpose as long as only the volume is of interest [2]. Four years ago we have presented a stitching approach which has extended the applicability of the sphere interferometer beyond the determination of diameters [3]. The described method allows the reconstruction of radii based on the measurement results from the sphere interferometer and yields the real shape of the sphere under test. By a comparison with independent results from tactile roundness measurements the accuracy of the approach was estimated to be lower than 5 nm [4].