Lavers with a continuous variation in the optical index with thickness were characterized by spectroscopic ellipsometry. These oxynitride films were grown by electron cyclotron resonance plasma-enhanced chemical vapor deposition using silane as a silicon precursor and oxygen and nitrogen as plasma gases. Specific linear and parabolic index profiles were fabricated by computer-controlled gas flow. Reduction of ellipsometry data measured ex situ on these films was performed with polynomial analysis. The validity of this approach was checked by a depth analysis via chemical etching. An investigation of the limits of the method is presented.