Absolute volume determination of a silicon sphere with the spherical interferometer of PTB

Abstract

For an absolute measurement of a density standard, mass and volume are measured separately. The absolute determination of the volume is still the quantity limiting the uncertainty. Lowest uncertainties are achieved by interferometric measurements of the diameter of spheres made from a silicon single crystal. So far, measurements have been based on a plane interferometer. At PTB, we demonstrate a new method of determining the diameter using a spherical interferometer, which yields more and particular information about the resulting volume.

Volume determination of two spheres of the new28Si crystal of PTB

“…The volume determination of the silicon spheres, 28Sikg01a and 28Sikg01b from the new crystal Si28-23Pr11, was performed at PTB by means of a special spherical interferometer [1]. The measurement of a sphere consists of pairs of alternating measurements, the basic principle of which is explained in the following (details can be found in [2]).…”

Contributions of precision engineering to the revision of the SI

“…This is different for a spherical Fizeau interferometer, in which spherical wavefronts propagate to the surface of the sphere. In this case the interferogram provides information about a manifold of sphere diameters within the aperture of the spherical Fizeau objective [137,12], see Fig. 32 for the first sphere interferometer developed at the PTB.…”

Diameter comparison of a silicon sphere for the International Avogadro Coordination project

“…Details of the optical interferometers of the three NMIs are described elsewhere [5][6][7]. Thus, only the main features and differences are briefly summarized here.…”