Second, improved sphere interferometer of PTB

A last precise determination of Avogadro’s constant in 2016 will be the crucial contribution to the redefinition of the SI unit kilogram, awaited for 2018. The measurement uncertainty of this fundamental constant is limited mainly by the interferometric volume measurement of a sphere from a single crystal of highly enriched 28Si. The sphere interferometer 1 (sphinx1) of PTB measures sphere diameters with uncertainties less than 2 nm. Unique feature of this interferometer is a complete diameter topography as the spherical layout allows the diameters of the sphere to be evaluated in the complete field of view of 60°. With appropriate reorientations the sphere can be characterized with some 500 000 diameter values. In the recent years, with great expense a new sphere interferometer was constructed and now accomplished, which significantly exceeds the optical performance of sphinx1. Reproducibility measurements over 10 years with sphinx1 and the new measurements with sphinx2 which agree in the sub-nm range give reason to expect that the XRCD measurements will, due to the reduced uncertainties, dominate the value for the Avogadro constant and the Planck constant, respectively.