Common-path interferometers have been used to perform phase visualization for over 40 years. A number of techniques have been proposed, including dark central ground, phase contrast (pi/2 and pi), and field-absorption interferometers. The merits of the interferometers have been judged ad hoc by either tests with a small number of phase objects or by computer simulation. Three standardized criteria, which consolidate the work of others, are proposed to evaluate common-path interferometers: fringe visibility, fringe irradiance, and fringe accuracy. The interferometers can be described as one generic class of Fourier-plane filters and can be analyzed for all input conditions. Closed-form expressions are obtained for visibility acid irradiance under the forced condition that little inaccuracy is tolerated. This analysis finds that the pi-phase-contrast interferometer is a good choice if the optical phase disturbance is at least 2 pi; for Smaller disturbances, the pi/2 filter selected by Zernike is near optimum. It is shown mathematically that the resulting fringe visibility is highly object dependent, and good results are not ensured. By allowing the optical beam to be 50% larger than the phase object, the interferometer performs well under all conditions. With this approach and a combination pi-phase/field-absorption filter, interference fringe visibility is greater than 0.8 for all phase objects. (C) 1995 Optical Society of America
