AbstractThe modifications of optical parameters (optical band gap E g, absorption coefficient α, refractive index n) under light irradiation by He-Ne laser of the amorphous thin films with different amount of Sn were measured and calculated. In the present work the transmission spectra of bulk and thin films of [(As 2 S 3) 0.5:(As 2 Se 3) 0.5 ] 1-x:Sn x (x=0; 1; 2 at.%) in the visible and middle infrared (IR) regions were studied. The red shift of the fundamental absorption edge under light exposure was observed, and the values of the optical band gap E g opt from the graphics in Tauc coordinates (α hν) 1/2 =A(hν - Eg) were obtained. The dispersion of the refractive index was examined. The relaxation of the relative optical transmission T/T 0 =f(t) under the light exposure (λ=633nm and λ=543nm) for amorphous [(As 2 S 3) 0.5:(As 2 Se 3) 0.5 ] 1-x:Sn x thin films also was investigated. The relaxation curves of photodarkening under light irradiation were processing using the stretched exponential presentation of the data: T(t)/T(0) = A 0 +Aexp[-(t-t 0)/τ] (1-β), where t is the exposure time, τ is the apparent time constant, A characterizes the exponent amplitude, t 0 and A 0 are the initial coordinates, and β is the dispersion parameter (0<β[removed]