ABSTRACT
This chapter reviews some of the two-wave interferometers. The two-wave interferometers produce an interferogram by superimposing two wavefronts, one of which is typically a flat reference wavefront and the other a distorted wavefront whose shape is to be measured. The basic configuration of the Twyman–Green interferometer, invented by F. Twyman and A. Green. The reflecting surface must be of the highest quality — flat, with an accuracy of about twice the required interferometer accuracy. Another source of wavefront errors in the spherical cavity configuration, when testing a high-aperture optical element, may be introduced by large axial displacements of the concave surface under analysis with respect to the spherical reference sphere. In lateral shearing interferometry, the interference pattern is formed with two mutually laterally displaced copies of the wavefront under analysis.
