Three-beam interference has been investigated, in order to obtain an intensity distribution that enables more accurate determination of small phase changes by photometric null setting criterion. A phase change in the interfering beam makes the adjacent fringes unequal in brightness as well as in thickness so that the form discrimination property of the eye assists its brightness discrimination property, thereby increasing the accuracy of measurements to λ/450. A simple experimental arrangement to demonstrate the results of analytical study is presented.
R. C. Tyagi and K. Singh, "Improved Three-Beam Interferometric Method," Appl. Opt. 7, 1971-1974 (1968)