Volumetric phase-measuring interferometer for three-dimensional coordinate metrology

We present a volumetric interferometer that has been specially designed to improve the measurement accuracy of the three-dimensional coordinate metrology by reducing excessive Abbe offsets. The interferometer is composed of two main parts; one is a three-dimensionally movable target and the other is a stationary two-dimensional array of photo-detectors. The target made of point diffraction sources emanates two spherical wavefronts, of which interference intensity is monitored by the photo-detectors. Applying phase shifting allows the phases of the photo-detectors to be precisely obtained, and the measured phases are fitted to a geometric model of multilateration so as to determine the xyz-location of the target by minimizing least square errors. A prototype built on a CMM demonstrates that the proposed interferometer is capable of measuring the xyz-coordinates of the probe with a volumetric uncertainty of less than 1.0 μm.