As discussed in the chapter on Lighting Sources, there are two basic kinds of light sources. One is a filament (usually tungsten) that is heated by electrical current until it glows and emits light. The other type is a discharge source. These include fluorescents, HMIs, Xenons, mercury vapor, sodium vapor, and others. In all of these, an arc is established between a cathode and an anode. This arc then excites gases or a plasma cloud, inducing them to glow. All discharge sources run on alternating current.
Any arc-based bulb powered by alternating current has an output that rises and falls as the waveform varies. Alternating current rises and falls as it heats a tungsten filament as well, but the filament stays hot enough ...