The present invention is directed a method of flame retarding a polymeric substrate using a specific group of azo and peroxide derivatives as flame retardants, to flame retardant compositions as well as to novel azo compounds usable as flame retarding compounds. When the subject is irradiated with radiation, the radiation conversion panel stored as a roll in the casing is unrolled and pulled out of the casing, and the radiation conversion panel is extended flatwise against the subject. The invention applies in particular in terminals operating in the Ka band.