A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger soda lime or borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz arc tube. Mercury-vapor lamps are more energy efficient than incandescent lamps with luminous efficacies of 35 to 55 lumens/watt. Their other advantages are a long bulb lifetime in the range of 24,000 hours and a high-intensity light output. For these reasons, they are used for large area overhead lighting, such as in factories, warehouses, and sports arenas as well as for streetlights. Clear mercury lamps produce a greenish light due to mercury's combination of spectral lines. This is not flattering to human skin color, so such lamps are typically not used in retail stores. "Color corrected" mercury bulbs overcome this problem with a phosphor on the inside of the outer bulb that emits at the red wavelengths, offering whiter light and better color rendition. Mercury-vapor lights operate at an internal pressure of around one atmosphere and require special fixtures, as well as an electrical ballast. They also require a warm-up period of four to seven minutes to reach full light output. Mercury-vapor lamps are becoming obsolete due to the higher efficiency and better color balance of metal halide lamps.
Origins
Charles Wheatstone observed the spectrum of an electric discharge in mercury vapor in 1835, and noted the ultraviolet lines in that spectrum. In 1860, John Thomas Way used arc lamps operated in a mixture of air and mercury vapor at atmospheric pressure for lighting. The German physicist Leo Arons (1860–1919) studied mercury discharges in 1892 and developed a lamp based on a mercury arc. In February 1896 Herbert John Dowsing and H. S. Keating of England patented a mercury-vapor lamp, considered by some to be the first true mercury-vapor lamp. The first mercury-vapor lamp to achieve widespread success was invented in 1901 by American engineer Peter Cooper Hewitt. Hewitt was issued U.S. patent 682,692 on September 17, 1901. In 1903, Hewitt created an improved version that possessed more satisfactory color qualities which eventually found widespread industrial use. The ultraviolet light from mercury-vapor lamps was applied to water treatment by 1910. The Hewitt lamps used a large amount of mercury. In the 1930s, improved lamps of the modern form, developed by the Osram-GEC company, General Electric company and others led to widespread use of mercury-vapor lamps for general lighting.
Principle of operation
The mercury in the tube is a liquid at normal temperatures. It needs to be vaporized and ionized before the lamp can produce its full light output. To facilitate starting of the lamp, a third electrode is mounted near one of the main electrodes and connected through a resistor to the other main electrode. In addition to the mercury, the tube is filled with argon gas at low pressure. When power is applied, if there is sufficient voltage to ionize the argon, the ionized argon gas will strike a small arc between the starting electrode and the adjacent main electrode. As the ionized argon conducts, the heat from its arc vaporizes the liquid mercury; next, the voltage between the two main electrodes will ionize the mercury gas. An arc initiates between the two main electrodes and the lamp will then radiate mainly in the ultraviolet, violet and blue emission lines. Continued vaporization of the liquid mercury increases the arc tube pressure to between 2 and 18 bar, depending on lamp size. The increase in pressure results in further brightening of the lamp. The entire warm-up process takes roughly 4 to 7 minutes. The mercury-vapor lamp is a negative resistance device. This means its resistance decreases as the current through the tube increases. So if the lamp is connected directly to a constant-voltage source like the power lines, the current through it will increase until it destroys itself. Therefore, it requires a ballast to limit the current through it. Mercury-vapor lamp ballasts are similar to the ballasts used with fluorescent lamps. In fact, the first British fluorescent lamps were designed to operate from 80-watt mercury-vapor ballasts. There are also self-ballasted mercury-vapor lamps available. These lamps use a tungsten filament in series with the arc tube both to act as a resistive ballast and add continuous black-body radiation to that of the arc tube. Self-ballasted mercury-vapor lamps can be screwed into a standard incandescent light socket supplied with the proper voltage.
Metal halide A very closely related lamp design called the metal halide lamp uses various compounds in the form of metal halides with the mercury. Sodium iodide and scandium iodide are commonly in use. These lamps can produce much better quality light without resorting to phosphors. If they use a starting electrode, there is always a thermal shorting switch to eliminate any electrical potential between the main electrode and the starting electrode once the lamp is lit. (This electrical potential in the presence of the halides can cause the failure of the glass/metal seal). More modern metal halide systems do not use a separate starting electrode; instead, the lamp is started using high voltage pulses as with high-pressure sodium vapor lamps.
Self-ballasted lamps Self-ballasted (SB) lamps are mercury-vapor lamps with a tungsten filament inside connected in series with the arc tube that functions as an electrical ballast. This is the only kind of mercury-vapor lamp that can be connected directly to the mains without an external ballast. These lamps are about as efficient as incandescent and halogen lamps, but have the added benefit of longer lifespans. They give light immediately on startup, but similar to regular mercury lamps, need a few minutes to restrike if power has been interrupted. Because of the light emitted by the filament, they have significantly better color rendering properties than mercury-vapor lamps. Self-ballasted lamps are typically more expensive than a standard mercury-vapor lamp due to their more complex structure.
Operation
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