An instrument for the measurement of ball diameters in the 0.5-20 mm range in a gauge block interferometer is realized. The measurement principle is that the ball is positioned between an optical flat and a calibrated gauge block. The total length (ball + gauge block) is measured in a gauge block interferometer. Special precautions are taken which enable a parallel adjustment of the gauge block relative to the optical flat and a proper measurement of the deformations due to the measuring force. The parallelism is adjusted using two piezo translators. The parallelism is adjusted while viewing the interference patterns on the gauge block and the optical flat. The measuring force can be varied in the 0.03 - 2.35 N range. The applied force is calibrated. Experiments show that commonly used formulas to calculate the ball indentation as a function of the applied force are approximately correct; this instrument however provides a direct means to measure this dependence and to apply a proper extrapolation to zero measuring force. The design is rather compact (90x70x150 mm) so it will fit in commonly used gauge block interferometers. The uncertainty which can be achieved is less than 0.1 micrometer.
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