 Once the plasma is fully formed, the plasma strips away the outer electrons from the mercury; when these electrons return to a lower energy level, visible and ultraviolet light is emitted.Some of the short-wave ultraviolet excites the phosphors, which then emits photons in the proper spectrum for tanning.In the older style (but still most popular) "choke ballast", each end of the lamp has its own cathode and anode, however, once the lamp has started, the plasma flows from one end of the lamp to the other, with each end acting as a single cathode or anode.