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 It also provides functionality for the differential refractive index (the refractive index for the boundary when you have a material inside another material).
 
 [1] http://henke.lbl.gov/optical_constants/getdb2.html
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 [2] B. L. Henke, E. M. Gullikson, and J. C. Davis, “X-ray interactions: photoabsorption, scattering, transmission and reflection E= 50-30,000 eV, Z= 1-92”, Atomic data and nuclear data tables, vol. 54, 1993.
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