Abstract: A coherence shifting interferometry system determines the thickness or refractive index of an optically transmissive medium. The system utilizes an optical energy source for generating a source beam. The source beam is divided into two beams by reflecting one portion of the beam off of a reflective surface, e.g., a front surface, of an optically transmissive test sample having the unknown thickness or refractive index. A second portion of the beam is transmitted through the sample and reflected off a refractive interface in the sample, e.g., a back surface. The two reflected beams are combined into a composite beam. The composite beam is then directed to a Mach-Zehnder type interferometer, where a portion of the combined beam traverses a fixed optical path length, and another portion traverses a variable optical path length.