Method for optical characterisation of at least one layer of material in an interval A of values taken by a function α of an optical wavelength λ, when λ varies in an interval of wavelengths, this layer being created on a substrate, this method being characterised in that it comprises the following stages: 1) we carry out a group of reflectometry and/or ellipsometry measurements over the interval A, this set of measurements leading to a measured spectrum, marked Ψ, and we choose the methods for calculating associated with the nature of the measurements and with the type of layer to be characterised; 2) we choose m initial values α 1...α m of the function α, belonging to this interval A, m being a whole number at least equal to 1, and we define an interval B as being the set of points α of the interval ranging from the smallest to the biggest number among α 1...α m, when m is greater than 1, and as being the interval A when m equals 1; 3) we choose m complex initial values of a complex refraction index n*=n+jk for the m points α i, i ranging from 1 to m; 4) when m is not 1, we choose an interpolation law which allows to calculate the refraction index n(α) of the material over the interval B, from the points (α i, n i), with n i =n(α i), i ranging from 1 to m, and when m equals 1, n(α) is taken equal to the number n 1 (α 1) over the entire interval B; 5) we choose M variable parameters, M being less than or equal to 2m+1; 6) we choose an error function Er(Ψ, Ψ) which characterises the difference between a measured spectrum Ψ and a theoretical spectrum Ψ; 7) using a minimising function of Er(Ψ, Ψ) with M parameters, we perform the following series of stages: a) by applying the interpolation law of (α i, n i) over the interval B, we deduce n(α), α belonging to B; b) by using n(α) and the thickness ε of the layer, and methods for calculating spectrums, we calculate a theoretical spectrum Ψ (n(α), ε); c) we compare Ψ and Ψ by using Er(Ψ,) and, if Er(Ψ, Ψ) is sufficiently small, i.e. less than a predetermined value e, or is minimal, we go to stage e), otherwise we go to stage d); d) we make the M variable parameters vary so as to tend to the minimum of Er (Ψ, Ψ), and we return to stage a); e) if Er(Ψ, Ψ) is less than e, we then obtain a set of M variable parameters, for which Er (Ψ, Ψ (n(α, M), ε)) is minimal and the refraction index is then taken equal to the last one obtained, and if Er (Ψ, Ψ) is greater or equal to e we go to stage 8). 8) we increase the number m of initial values of the function α and we return to stage 2). 1) we carry out a group of reflectometry and/or ellipsometry measurements over the interval A, this set of measurements leading to a measured spectrum, marked Ψ, and we choose the methods for calculating associated with the nature of the measurements and with the type of layer to be characterised; 2) we choose m initial values α 1...α m of the function α, belonging to this interval A, m being a whole number at least equal to 1, and we define an interval B as being the set of points α of the interval ranging from the smallest to the biggest number among α 1...α m, when m is greater than 1, and as being the interval A when m equals 1; 3) we choose m complex initial values of a complex refraction index n*=n+jk for the m points α i, i ranging from 1 to m; 4) when m is not 1, we choose an interpolation law which allows to calculate the refraction index n(α) of the material over the interval B, from the points (α i, n i), with n i =n(α i), i ranging from 1 to m, and when m equals 1, n(α) is taken equal to the number n 1 (α 1) over the entire interval B; 5) we choose M variable parameters, M being less than or equal to 2m+1; 6) we choose an error function Er(Ψ, Ψ) which characterises the difference between a measured spectrum Ψ and a theoretical spectrum Ψ; 7) using a minimising function of Er(Ψ, Ψ) with M parameters, we perform the following series of stages: a) by applying the interpolation law of (α i, n i) over the interval B, we deduce n(α), α belonging to B; b) by using n(α) and the thickness ε of the layer, and methods for calculating spectrums, we calculate a theoretical spectrum Ψ (n(α), ε); c) we compare Ψ and Ψ by using Er(Ψ,) and, if Er(Ψ, Ψ) is sufficiently small, i.e. less than a predetermined value e, or is minimal, we go to stage e), otherwise we go to stage d); d) we make the M variable parameters vary so as to tend to the minimum of Er (Ψ, Ψ), and we return to stage a); e) if Er(Ψ, Ψ) is less than e, we then obtain a set of M variable parameters, for which Er (Ψ, Ψ (n(α, M), ε)) is minimal and the refraction index is then taken equal to the last one obtained, and if Er (Ψ, Ψ) is greater or equal to e we go to stage 8). a) by applying the interpolation law of (α i, n i) over the interval B, we deduce n(α), α belonging to B; b) by using n(α) and the thickness ε of the layer, and methods for calculating spectrums, we calculate a theoretical spectrum Ψ (n(α), ε); c) we compare Ψ and Ψ by using Er(Ψ,) and, if Er(Ψ, Ψ) is sufficiently small, i.e. less than a predetermined value e, or is minimal, we go to stage e), otherwise we go to stage d); d) we make the M variable parameters vary so as to tend to the minimum of Er (Ψ, Ψ), and we return to stage a); e) if Er(Ψ, Ψ) is less than e, we then obtain a set of M variable parameters, for which Er (Ψ, Ψ (n(α, M), ε)) is minimal and the refraction index is then taken equal to the last one obtained, and if Er (Ψ, Ψ) is greater or equal to e we go to stage 8). 8) we increase the number m of initial values of the function α and we return to stage 2).