Interferometry system are disclosed that include a detector sub-system including a monitor detector, interferometer optics for combining test light from a test object with primary reference light from a first reference interface and secondary reference light from a second reference interface to form a monitor interference pattern on a monitor detector, wherein the first and second reference interfaces are mechanically fixed with respect to each other and the test light, a scanning stage configured to scan an optical path difference (OPD) between the test light and the primary and secondary reference light to the monitor detector while the detector sub-system records the monitor interference pattern for each of a series of OPD increments, and an electronic processor electronically coupled to the detector sub-system and the scanning stage, the electronic processor being configured to determine information about the OPD increments based on the detected monitor interference pattern.