First high-precision characterisation of the absolute diameter and complete topography of a 30 mm silicon sphere

Abstract

Spherical reference standards are widely used in metrology as roundness and volume references, and are especially employed in coordinate measuring technology for validating measurement systems. Silicon spheres are of particular interest because their diameter and radius topography can be determined interferometrically with very high accuracy. Within the kilogram realisation, two sphere interferometers were developed at PTB specifically for characterising 93 mm silicon spheres. A sphere of this diameter is rather unsuitable as a reference for many applications. By modifying one of the two interferometers, it becomes possible for the first time to measure spheres with smaller diameters as well. This publication describes the modifications to the experimental setup, as well as the alignment and measurement strategy. The complete and absolute determination of the topography of a natural silicon sphere with a diameter of 30 mm was performed using sphere interferometer 1 with an uncertainty of only 5 nm ( k=2), and its results are now being applied in the substitution method on a coordinate measuring machine.