Electrically powered light sources can be divided into three broad classes: (1) solid materials heated to high temperature such as incandescent lamps, (2) direct-conversion solid-state devices such as light-emitting diodes, and (3) gas-discharge systems such as fluorescent and metal halide lamps. Gas-discharge lamps operated at radio frequencies are efficient sources with interesting properties, whereas there is no advantage in operating incandescent or solid-state light sources at radio frequencies. Radio frequencies range from about 10 kHz to 30 GHz.