A procedure is described for determining the inhomogeneities of a sample of optical material in terms of small changes in index of refraction at discrete points over the measured surface. The method is used in the visible- and near-ir regions of the spectrum. An He-Ne gas laser source was used for the ir measurements. The variation in index was mapped with contour lines enclosing areas of similar index variations. The largest index change in 1-cm distance was noted and recorded as the maximum variation per centimeter. Over twenty-five different samples of six different materials were examined. The results are presented in a chart indicating the range of maximum refractive index variations of the samples observed. Data are shown to illustrate the possibility of determining index values to higher precision than the material justifies.
F. W. Rosberry, "The Measurement of Homogeneity of Optical Materials in the Visible and Near Infrared," Appl. Opt. 5, 961-966 (1966)