The reflection coefficient is calculated for radiation incident normally on a nonabsorbing, optically inhomogeneous, plane parallel layer bounded on either side by nonabsorbing semi-infinite media. The theory is applied to determine the reflectance of a layer in which the index of refraction varies nearly linearly with the thickness.
JOSEPH F. HALL, JR., "Reflection Coefficient of Optically Inhomogeneous Layers," J. Opt. Soc. Am. 48, 654-657 (1958)