the optical system , which is comprised of a thin layer of ion conducting , macromolecular material , or solid polymer electrolyte sandwiched between two electrodes presenting each a conducting deposition carried by a support plate such that at least one of the conductor depositions and the associated support plate are transparent , is heated by applying between the conductor depositions of said electrodes an electric voltage signal of which at least one portion includes an alternating component which has an amplitude between 0 . 05 and 100 volts and a frequency lower than 5 khz and preferably between 2 and 2000 hz so as to generate within said optical system an alternating ionic current susceptible of producing a heating of the ion conducting macromolecular material by joule effect . the optical system to be heated , which may be an electrochromium device , may have particularly a glazing type structure or a mirror type structure .