Fluorescent lamps are high-efficiency lamps that produce visible light as a result of an interaction of the ultraviolet (UV) energy they produce with the phosphor coating on the insides of their glass envelopes. The lamp envelope contains traces of an inert gas and a drop of mercury. When the lamp is turned on, the mercury is vaporized, and this vapor and the inert gas are ionized by electron flow between electrodes at each end of the lamp to produce UV emission.

The UV energy excites phosphor powders that have been applied as thin layers on the inside of a glass envelope, a tube or bulb. The phosphors transform the UV emission into visible light whose color temperature and characteristics depend on the composition of the phosphors in the layers. Fluorescent bulbs or tubes can operate with either "hot" or "cold" electrodes called cathodes. A cutaway view of a hot-cathode, preheat-starting fluorescent tube is shown in Fig. 8-5.

Figure 8-5: Cutaway view showing the construction of a hot-cathode, preheat-starting fluorescent tube.

After a vacuum is pumped in the tube and before it is sealed, a drop of liquid mercury is inserted and the inert gas, usually argon, is admitted. When the lamp is turned on, the voltage between the electrodes is high enough to vaporize the mercury and ionize the mercury-gas mixture. The UV emission from the gas discharge is almost entirely due to the mercury vapor.

After gas ionization occurs, a much lower voltage...