An empirical dielectric function (EDF) has been used to represent the spectra in the effective optical constants of Si-rich nitride (SiNx) thin films processed by PECVD. A good correlation was found between the EDF parameters and the film composition results from Auger spectroscopy. It is shown that the EDF approach allows one to use a simple optical model of the film stack and extrapolate the spectra in the optical constants of SINx outside the measured wavelength range. This advantage gives a feasibility of using the EDF to characterize optical properties of thin films in the Deep-UV region, below 230 nm, which is outside the spectral range of most commercial ellipsometers.