An interferometer is described that is capable of measuring the deviation of a target device from a plane with an accuracy of 0.1 µ, when the interferometer-target separation is as much as 10 m. This interferometer resembles one described in 1965 by Baldwin and Whitten in that they both use a Kösters prism and a Porro prism. The present use of a laser for the light source yields nonlocalized fringes at the output that are easily monitored by an electronic fringe detector.
Robert O. Naess, "A Measuring Interferometer for High Accuracy Alignment," Appl. Opt. 7, 2315-2317 (1968)