A method is described whereby the complex index of refraction can be determined for a strongly absorbing substrate material by measurement of reflectance maxima and minima at a given wavelength, as a function of the thickness of a wedged nonabsorbing thin film layer deposited on the substrate. The procedure for determination of the complex index n - ik employs the reflectance extremum afd correlated film thickness to construct graphically the complex reflectance amplitude of the substrate material. Subsequently, the Fresnel equations for reflectance amplitude can be used to calculate the values of n and k. Examples of the successful use of the method to determine the far-IR indices of fused silica and sapphire are given. The limitations of the method are discussed, and examples of the expected errors for n and k for the two specific cases calculated are given.