Results of theoretical calculations of the phase incursions of flat and focused optical wave reflected by the surface with variable refractive index is discussed. The possibility of determining the spatial distribution refractive index from the wavefront measurements is shown.
Andrey A. Goloborodko,
"Optical testing of reflective surfaces", Proc. SPIE 9066, Eleventh International Conference on Correlation Optics, 90660Z (17 December 2013); doi: 10.1117/12.2048394; https://doi.org/10.1117/12.2048394