Ternary semiconductor compounds of I-III-VI2 type and their solid solutions are promising materials for high-efficiency thin film solar cells. In particular, the quantum efficiency of CuIn1-xGaxSe2 based solar cells is reported to reach 19.9 % [1]. It is now believed that CuIn3Se5 or similar Cu-poor compounds such as CuGa3Se5, CuIn5Se8 or CuGa5Se8, might play an important role in the optimization of the efficiency of these thin film solar cells. The present contribution reports on the optical constants of CuIn3Se5 thin films deposited onto sodalime glass substrates by the flash evaporation technique with the addition of an oscillating sample-holder, in order to improve the uniformity in thickness and in composition of the films. As source material, direct synthesis from high pure elements in stochiometric quantities were used. The compositional measurements have been conducted by energy dispersive X-ray analysis. The optical properties of the films are characterized by spectral transmittance T and reflectance R at normal incidence (Fig.1a). Fitting the optical spectra with a model of flat layers [2] yields the optical constants (absorption coefficient (Fig.1b) for wavelengths between 350 and 1500 nm). An analytical model that predicts the broadening and energetic position of absorptance spectra for semiconductors with lateral band gap inhomogeneities was applied [3]. This model supplies information about the extent of the investigated band gap fluctuations from the broadening of absorption edge. The length scale of the fluctuations is determined by the position of the absorption edge. The fitting of the model to experimental data of CuIn3Se5 thin films allowed to obtain the value of the band gap energy, Eg=1.09, 1.06, 1.21 for CIS1, CIS2, CIS3 respectively. The results are in a good agreement with those (1.0-1.28 eV) reported in literature. Also from this model fitting standard deviations of the band gap fluctuations in the range of 90–120 meV were extracted. For a photovoltaic material, optical reflectance and absorbance are the cornerstones for optical losses and photo-generation, and therefore establish the link between optical properties and photo-response of a solar cell.figureFigure 1. a) CuIn3Se5 Thin film transmittance and reflectance from normal incidence, at 300K ; b) absorption coefficient α and model fitting for CuIn3Se5 thin films.