Infinitely thick substrate with uniform refractive index, ns, occupying half-space z > d.
Ambient medium is assumed to be air, having a refractive index, n0 = 1.
An inhomogeneous layer exists between the substrate and ambient medium that has a refractive index, n(z), that varies with z only.
Inhomogeneous layer is assumed to match the refractive indices of the substrate at z = d, and the ambient at z = 0; n(0) = 1, and n(d) = ns
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