In this paper an autocollimation method of refractive index gradient measurement is described. The autocollimation method, in contrast to those applied up-to-date, makes it possible to measure the linear changes of refractive index. Small nonlinear changes of the refractive index in the region under test are approximated to the corresponding linear changes. From the theoretical considerations as well as by the measurements made it follows that the measurement accuracy is of the order of 10-7 cm-1. The method can be applied in the quality control of the applied glass.
Zygmunt Bodnar and Florian Ratajczyk, "On an Autocollimation Method of Optical Glass Heterogeneity Measurement," Appl. Opt. 4, 181-186 (1965)