Abstract
The self-calibration test known as the random ball test (RBT) is adapted and applied to instrument calibration for measurements of microrefractive lens figure error. The RBT exploits the symmetry properties of a microsphere, resulting in a low-uncertainty estimate of the instrument biases. One hundred surface patches on a 1-mm-diam steel sphere are imaged by commercial instruments then averaged together in software to determine the instrument bias for a 500-µm radius of curvature test piece. The results show biases on the order of a few hundred nanometers peak-to-valley for a scanning white light interferometer and a Twyman-Green interferometer.
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