Southwell’s analysis of optical multilayers within the limits of very thin films has been extended to include absorption in the multilayer for predicting the effective values of the refractive index n e and extinction coefficient k e of mixed-composition binary homogeneous films over a wide spectral region, including the high-absorption (k > 10-2) region. It has been found that n e in general is a complicated function of the optical parameters (n 1, k 1, n 2, k 2) and volume fractions (f 1, f 2) of the component materials in a homogeneous layer, and the expression for n e becomes the same as that predicted by the Drude model in the spectral region where the layers are transparent. Moreover, according to the present analysis, the volume fractions of the product of the refractive index and the extinction coefficient of the component materials of a binary composite film are additive and the sum equals the product of the effective refractive index and extinction coefficient of the composite film.
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