The study was devoted to optical characterization of non-stoichiometric silicon nitride films prepared by reactive magnetron sputtering in argon-nitrogen atmosphere onto cold (unheated) substrates. It was found that these films exhibit the combination of three defects: optical inhomogeneity (refractive index profile across the films), uniaxial anisotropy with the optical axis perpendicular to the boundaries and random roughness of the upper boundaries. The influence of the uniaxial anisotropy was included into the corresponding formulae of the optical quantities using the matrix formalism and the approximation of the inhomogeneous layer by a multilayer system consisting of large number thin homogeneous layers. The random roughness was described using the scalar diffraction theory. The processing of the experimental data was performed using the multi-sample modification of the least-squares method, in which experimental data of several samples differing in thickness were processed simultaneously. The dielectric response of the silicon nitride films was modeled using the modification of the universal dispersion model, which takes into account absorption processes corresponding to valence-to-conduction band electron transitions, excitonic effects and Urbach tail. The spectroscopic reflectometric and ellipsometric measurements were supplemented by measuring the uniformity of the samples using imaging spectroscopic reflectometry.