We use the Newton-Raphson method to analyze the optical spectra of indium-tin-oxide and indium-tin-oxynitride films, by deriving the complex refractive index and, thereby, the plasma wavelength and the relaxation time. Reflectance and transmittance as well as partial derivatives of them, necessary for the application of the method, are introduced in analytical form reducing complexity and improving speed. The films we have investigated were deposited on glass by sputtering at different RF power levels. After deposition, they were subjected to annealing at 600 degrees C and were measured prior to and after that treatment. The results obtained are physically meaningful and lead to useful inferences about the quality of the material in the two different types of the investigated films. (C) 2008 Elsevier B.V. All rights reserved.