Scanning differential interferometer to measure index heterogeneity

The index heterogeneity of rectangular glass samples is measured to a repeatability of 2 x 10(-8) by a scanning differential interferometer. The noise-limited instrument resolution is 2 nm of optical path length. The surface figure is decoupled from bulk inhomogeneity by a thin film of index-matching liquid, which is located by surface tension between the interferometer cavity and the test sample. An algorithm based on Poisson's equation reconstructs the integrated optical path length profile from data in differential form with a minimal integration of noise.