The interferometric method of optical testing presents considerable advantages in view of the unequivocal nature of the information given and the self-calibrating nature of the test. However, difficulties are presented in practical applications of interferometers of the Twyman-Green type because of the necessity of providing a reference beam.
Two types of interferometer are described in which the reference beam follows the same path as the test beam, thus avoiding this difficulty. Some test results are given, illustrating a few of the applications of these instruments.
J. DYSON, "Common-Path Interferometer for Testing Purposes," J. Opt. Soc. Am. 47, 386-387 (1957)