Abstract

This paper describes a method for measuring the absolute flatness of flats.¹ The flat was measured at three positions: nominal, displaced in x, and displaced in y. From these three measurement positions, three phase maps were obtained. To avoid the tilt error during the translation or shearing operation, the tilts of the three phase maps were removed individually. The three phase maps were then used to obtain the shearing interferograms in the x and y directions. Using this technique the error of the reference surface error was removed. From the two shearing interferograms, the profile of the flat was reconstructed.² The theoretical derivation and comparison of different methods, and the experimental results are presented.

© 1991 Optical Society of America