Re: How do sodium vapor lamps glow?

A sodium lamp produces light in much the same manner as a lightning bolt. But instead of a brief flash, the electric arc between the lamp's two main electrodes is continuous. The striking of this continuous arc is made possible by special starting gases (like xenon). A very high start-up voltage (between 2500 and 4000 volts) is also needed to start the arc. This voltage pulse is provided by a starter circuit separate from the lamp and ballast. The ballast assists in starting and in limiting the current across the arc once the arc has been struck. Sodium lamps do not provide the electrical resistance of filament lamps. If a ballast were not used, the arc would draw an unlimited amount of current and the lamp would quickly be destroyed.

When a sodium lamp is energized, the high voltage start-up pulse ionizes the starting gas in the arc tube, and a gas arc is established between the main electrodes. As soon as this arc is established, the start-up voltage pulse is switched off. Sodium and mercury quickly vaporize and join this arc stream, and the arc current increased and stabilizes with the help of the ballast.

The light that you see is produced by ionisation of the gas; gas electrons are knocked off the gas atoms, but they don’t go very far. They recombine with the gas atoms and, as they recombine, they emit light. The color of the arc depends on the gases inside the lamp. Sodium light is strongest in the yellow and orange range of the spectrum and weakest in the blue-green wavelengths. Mercury is usually added to the arc tube to help strengthen blues and greens. (We’ll talk about mercury again later.)

The arc tube is a slender cylinder approximately 1/4" to 3/8" in diameter. Sodium cannot be contained in a glass tube. The sodium would etch the glass and further degrade light output. A special ceramic material known as polycrystalline alumina (PCA) is used to construct the sodium lamp arc tube. PCA is virtually insensitive to sodium attack. PCA end caps are epoxied or glued to the arc tube to seal it. Each end cap contains an electrode. The sodium-mercury amalgam and starting gases are placed inside the arc tube before it is sealed closed.

Although the sodium is not consumed. Metal electrode material discharged from the electrodes during start-up and operation deposits on the arc tube ends in much the same way the tungsten filament of an incandescent lamp evaporates and blackens the bulb. This blackening of the arc tube will increase operating voltage. The operating voltage of sodium lamps increases about 1-2 volts per 1000 hours operated. Lamp voltage will rise until the ballast cannot provide enough voltage to operate the lamp. At this point the lamp will cycle on and off and it will be replaced and (hopefully) recycled.