The optical properties of thin films of amorphous silicon nitride with embedded nanoparticles are determined in the infrared using spectroscopic ellipsometry. In the spectral range of study (950-3500 cm(-1)), the material presents a considerable number of absorption bands, and consequently, a large number of parameters are necessary for the complete description of its optical behaviour. This fact enhances the possibility of reaching good numerical solutions without or with incomplete physical meaning. Particularly, we observe that the common approach consisting of optimising all the parameters in a single step may neglect some of the absorption bands that are evidenced by the experimental data. We propose a fitting strategy based on the progressive fitting of the data, introducing at each step new absorption bands and thus extending the fitted spectral range. This strategy is able to assure a good numerical solution with a correct description for all the absorption bands considered. (c) 2006 Elsevier B.V. All rights reserved.