A method of optical characterization of a layer of material in which the spectrum of index n* (.lamda.) is characterized by a limited number of "nodes" that are points with coordinates (.lamda..sub.i, n.sub.i, k.sub.i) or (.lamda..sub.i, n*.sub.i), with n.sub.i=n(.lamda..sub.i), k.sub.i=k(.lamda..sub.i) and n*.sub.i=n.sub.i+jk.sub.i, where j.sup.2=-1, and an interpolation law between the "nodes," which can be, for example, linear, cubic, of "spline" type or polynomial (of any given degree). This interpolation law allows the calculation, from the "nodes," of the refraction indexes and the extinction coefficients for the wavelengths located between the "nodes."