The Lorentz dispersion theory and the Kramers-Kronig transformation were widely used to determine optical constants and oscillator parameters from reflectivity spectrum. Each range of data correspond to a different oscillation. Since the experimental data is in the limited range, this technique can be used to subtract information from unmeasured range. The inter-band optical properties in UV region have been investigated. Hydrogenated amorphous silicon nitride thin film was used as an example. The reflectivity spectrum of a-SiNx:H was used to obtain the energy band gap in both standard and this technique. The measurement is in the UV-visible range. The optical energy gap value was found to be about 5.2 eV for the sample illustrated, and these values are consistent for both techniques. (C) 2004 Elsevier Ltd. All rights reserved.