Three As_xTe_100−x films with different x and dissimilar average thickness $\overline{\mathrm{d}}$ are characterized mainly from one interference transmittance spectrum T(λ = 300 to 3000 nm) of such film on a substrate based on the advanced optimizing envelope method (AOEM). A simple dual transformation of T(λ) is proposed and used for increasing the accuracy of computation of its envelopes T₊(λ) and T₋(λ) accounting for the significant glass substrate absorption especially for λ > 2500 nm. The refractive index n(λ) of As₄₀Te₆₀ and As₉₈Te₂ films is determined with a relative error 300 nm on a substrate, compared to all the other methods for characterization of such films only from T(λ).