Adequate modeling of high quality non-absorbing dielectric thin films requires the inclusion of the degree of bulk inhomogeneity as one of the parameters in the model describing the film. We show that in the case of a satisfactory choice of a thin film model, the main source of errors in the computed optical parameters of high quality films are systematic errors in spectrophotometric data. Based on practical examples and theoretical predictions we estimate that an accuracy of 0.5% in the determination of optical parameters of dielectric thin films should be considered as a good result when viewed in the context of the current state of the art in optical characterization based on spectrophotometric data. A level of accuracy significantly better than this would require an extraordinary effort.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.