Spectrophotometry of layers on plane parallel substrates

Integral expressions are obtained for the reflectance and transmittance spectra of a structure that is formed by two thin layers deposited on opposite faces of a plane-parallel substrate and that is obliquely illuminated with partially coherent light. As a result of an asymptotic analysis of the integrals, approximate analytical formulas are found for calculating the indicated spectra, which are convenient for use to solve inverse spectrophotometry problems. An aluminum doped zinc oxide layer deposited on a glass substrate is studied. The spectra of the refractive indices and absorbances of the layer and the substrate, as well as the layer thickness, are restored by processing the reflectance and transmittance spectra of the structure measured for $s$- and $p$-polarized waves at two angles of incidence of the light on the structure. The found parameters of the structure are used in computational experiments to estimate the applicability limits of the formulated approximations.