The object of the present work is investigation of the role of the actual conditions of illumination on the magnitude of photo-darkening for a specific material (the incident photon flux and the penetration depth of absorbed photons). It was found that the photo-induced changes in the thin As<sub>2</sub>S<sub>3</sub> film depend on the light intensity and they are affected by heating of the material at higher laser intensities. The influence of the deposition rate on the film’s microstructure was investigated. The conditions for deposition of coatings with columnar structure were determined. The photo-induced changes of the optical properties of the obliquely deposited thin As<sub>40-x</sub>GexS<sub>60</sub> films are examined. It was established that the microstructure of thin As-Ge-S films can a play significant role in photo-induced volume changes.