A simple method for testing the 90° angle of a reflecting prism by placing two prisms in contact with one another on a standard test plate and counting the number of fringes is outlined. An angle accuracy of less than a second for the angle of a 90° reflecting prism has been obtained.
A. M. Ghodgaonkar,
R. D. Tiwari,
"Simple Method For Testing Of The 90Â° Angle Of A Reflecting Prism," Optical Engineering 21(6), 216064 (1 December 1982). https://doi.org/10.1117/12.7973034