The objectives of the present invention are to prevent or inhibit the deterioration effects such as light transmission, diffraction, reflection, spectrum generation, and interference, and these combinations, and by so doing, decrease the frequency of maintenance operations such as window replacement and to reduce the costs for such operations. This invention is characterized by steps of creating a near vacuum zone with a presence of active energy to excite an oxidation reaction of carbon wherein the near vacuum zone faces the lighting surfaces of the optical system; generating negative ions or radicals in the near vacuum zone such as unstable chemical seeds containing oxygen atoms, such as OH radicals, OH ions, ozone, O.sub.2 ions, O-radicals; and removing or reducing the accumulated carbon which deposits on the lighting surface, by reacting the deposited carbon with the negative ions or radicals. More specifically, the method according to this invention is characterized by the step of supplying active energy while supplying a flow of gases containing oxygen atoms such as water gas or oxidizing gas (for example, water vapor, oxygen, hydrogen peroxide, ozone or mixtures of said gases with inactive gases (including air)) into the near vacuum zone, thereby removing or reducing the accumulated carbon which deposits on the lighting surface by exciting the oxidation reaction of the accumulated carbon with the supplied active energy.