The experimental investigations of the influence of the light irradiation on the optical parameters of one of highest sensitive chalcogenide glass [(As ₄₀S ₆₀)0.5:(As ₄₀Se ₆₀)0.5]0.5: (As ₅₀Se ₅₀)1.0 was investigated. For the amorphous As ₄₅S ₁₅Se ₄₀ thin films, prepared by "flash" thermal evaporation, from the transmission spectra the value of the refractive index n was found n=2.921 at λ=650 nm, and the optical band gap E _g was determined from the Tauc plot E _g=1.929 eV. The kinetics of photodarkening in amorphous As ₄₅S ₁₅Se ₄₀ thin films is described by stretch exponential function and the effect at illumination with laser beam λ=540 nm is more pronounced that in amorphous As ₂S ₃ and As ₂Se ₃ films. The maximum of the diffraction efficiency η of the holografical gratings recorded during the exposure of laser beam at λ=540 nm is reached at about t=250 s.