This paper focuses on studying the optical properties of the semiconductors chalcogenide thin films [(As2S3):(AS2Se3)]1-x: Snx (where x=0.03; 0.04; 0.05 at.%). The optical transmission spectra and the modifications of optical parameters (optical band gap Egopt, absorption coefficient α, refractive index n) under light irradiation of the amorphous [(As2S3)0.5:(As2Se3)0.5]1-x:Snx thin films with different amount of Sn are investigated. The optical constants were calculated due to program PARAV-1.0 [1] based on method suggested by Swanepoel and Tauc method for estimation the optical energy band gap Egopt. The relaxation of photodarkening effect in amorphous [(As2S3)0.5:(As2Se3)0.5]1-x:Snx thin films under the light exposure also was investigated. The previous investigations [2, 3] show that the tin impurity introduced in the As-Se glass network influence the degree of modification of optical constants under light irradiation and heat treatment. The amorphous [(As2S3)0.5:(As2Se3)0.5]1-x:Snx thin films exhibit photoinduced effects under the light irradiation with photon energy above the optical band gap (hν≥ Egopt), that make its more perspective materials for registration of optical information. The experimental results regarding influence of the light irradiation (15 min - is a time of saturation) on the optical properties of amorphous [(As2S3)0.5:(As2Se3)0.5]0.96:Sn0.04 thin films are presented below. The shift of the absorption edge after light exposure to lower energy region was observed, i.e. the effect of photodarkening take place (Fig.1 and 3) like in some As-S-Se compounds. The fig.2 shows the dispersions curves with decreasing of the refractive index n under light exposure. For the glass composition x=0.04 of the Sn impurities the change of the optical band gap Egopt was determined from 1.92±0.02 eV to 1.86±0.02 eV. The similar calculations of the optical constants were done for the amorphous films of glass compositions x=0.03 and x=0.05. The experimental results are interpreted in framework of the model of molecular structure of chalcogenide glasses doped with tin impurities [3].figureFig. 1. The transmission spectra before and after light exposure. Fig.2. The dispersion curves n=f(λ) before and after light exposure.Fig.3. The absorption spectra in the Tauc co-ordinates (α hν)1/2 = f(Eg) before and after light exposure.[1] A. Ganjoo, R. Golovchak, Journal of Optoelectronics and Advanced Materials, 10(6), (2008), 13281332. [2] M.S. Iovu, S.D. Shutov, M. Popescu, Journal of Non-Crystalline Solids, (2002), 299-302, 924-928. [3] P. Boolchand, D.G. Georgiev, M.S. Iovu, Chalcogenide Letters, 2(4), (2005), 27-34.