A general technique for determination of residual wedge angles of high optical quality parallel plates using a Fizeau interferometer is discussed. In this technique the parallel plate to be tested for degrees of parallelism is used to introduce angular tilt in the Fizeau fringes of an air cavity of a known wedge angle. The parallelism is calculated from the angular tilt of the fringes. The technique is most suitable for highly parallel transparent plates with very low residual wedge angles.
"Simple technique for measurement of residual wedge angle of high optical quality transparent parallel plate," Optical Engineering 42(11), (1 November 2003). https://doi.org/10.1117/1.1613961