Thin films with thickness of 400nm have been obtained from the Ga15Se81Ag4 ternary chalcogenide glass prepared by the melt quenching technique. The behavior of several optical constants has been studied from absorption and reflection spectra as a function of photon energy in the wavelength region 400–1200nm. The amorphous nature of the sample was examined by X-ray diffraction and non-isothermal DSC measurements. Thin films were illuminated by shining white light using 1500W tungsten lamp with different exposure time. The ambient temperature during the illumination process was controlled and kept at 348K, selected by DSC thermogram. Analysis of the optical absorption data shows that the rule of non-direct transition predominates. It is found that the optical band gap decreases by increasing the illumination time. It has also been observed that the value of absorption and extinction coefficients increases while the refractive index decreases by increasing the illumination time from 0 to 150min. The decrease in optical band gap is explained on the basis of the change in nature of the films, from amorphous to crystalline state, with increase of the illumination time.