Interferometric method for measuring optical distance

A system for determining an absolute optical distance between two reflecting surfaces that make up an interferometric cavity, wherein the system comprises:

• (a) an interferometer having an optical axis and configured to define the interferometric cavity with a test part;
• (b) a dynamic illumination system configured for moving an illumination spot in a back focal plane of the interferometer along a trajectory that has a nonzero radial component relative to the optical axis to cause different radial positions for the illumination spot along the trajectory;
• (c) a detector configured to acquire images of interferograms of the interferometric cavity as the dynamic illumination system moves an illumination spot in a back focal plane of the interferometer along the trajectory that has the nonzero radial component relative to the optical axis;
• and
• (d) an electronic controller coupled to the dynamic illumination system and the detector and configured to analyze the interferograms to determine a functional dependence of an optical path length difference (OPD) for the interferometric cavity versus spot radial position and extract the absolute optical distance between the two reflecting surfaces from based on the functional dependence.