For direct density determination by so-called absolute procedures, the mass and the volume of a body must be determined. The paper gives an overview of dimensional measurements of silicon spheres used as density standards. Relative uncertainties for diameter measurements of less than 10−8 can only be achieved with interferometric procedures. After introducing optical interferometry the two main types of interferometers used are described in detail: plane-face interferometers and interferometers with spherical beam path. At present, uncertainties of 1 nm can be achieved for a diameter determination of silicon spheres, which corresponds to a relative volume uncertainty of ΔV/V = 3 × 10−8.

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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.

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