It has roughly the dimensions of a Tootsie-Pop, but don't let the diminutive size fool you. Sparked by microwaves, a new sulfur bulb packs a lighting wallop. One 5,900-watt bulb generates the same amount of light as hundreds of today's industrial high-intensity mercury-vapor lamps--using 20 to 30 percent less energy.
"Its a major breakthrough in lighting," says Christine Ervin of the Department of Energy. Unlike conventional lighting, sulfur bulbs have no electrode to wear out. That means the bulbs could operate as long as the microwave generators (10,000, to 15,000 hours). While this is comparable with today's high-intensity lamps, those typically dim to half their original output by that time because their phosphors degrade. "We don't know how long it will last ultimately," says Kent Kipling of Fusion Lighting of Rockville, Md., which developed the bulb under a DOE contract.
The sulfur bulbs were combined with plastic "light pipes" from 3M for testing at two installations in Washington, D.C.--DOE's headquarters at the Forrestal Building and the Smithsonian's Air & Space Museum. DOE says the two test installations cost less than half the price of the conventional lighting systems they replaced.
The sulfur bulb's hollow quartz sphere contains a small amount of inert argon gas and yellow sulfur powder. To generate light, the bulb is spun at 600 rpm and is simultaneously bombarded with microwaves in a compact generator cavity originally developed for ultraviolet industrial technology. Heated by the argon, the sulfur powder boils into a vapor and emits a bright white light. This light is projected by a reflector into long plastic pipes lined with a semi-reflective film with either a mirror or a second bulb at the far end. The light zig-zags along the pipe, with some of it direct through the film for illumination of an area.
The sulfur technology may sound simple to develop. But like many movie stars, the bulb is a ten-year "overnight" success. For example, it requires a special microwave cavity, which had been refined over decades for industrial uses. "We must have spent more than 100 man-years to get to this point," says Michael Ury, one of the bulb's inventors.
Sulfur had also never been tried in lighting before, says Ury, because it oxidizes metals--and so would degrade the electrodes used in conventional bulbs. "We were always trying new ideas, crazy ideas," he says. "I guess we just wanted to be different." Serendipitously, Ury's colleague James Dolan picked the right rpm to spin the bulb on the first try. "If we didn't get everything together like that we might have missed it," says Ury.
In addition to its energy efficiency and long life, the sulfur bulb offers other benefits. Because its bright white light is similar to natural light, it is more aesthetically pleasing than the blue-white color of high-intensity mercury-vapor lamps or some fluorescent lamps. Plants may prefer it as well: NASA has a two year contract with Fusion to develop its light for growing plants in space. And the new bulb produces far less of the fabric-fading ultraviolet rays generated by other industrial lamps--a bonus for fragile museum exhibits as well as home furniture. The disposal of mercury-containing lamps also has been an environmental problem.
While sulfur bulbs will first illuminate large spaces like shopping centers, aircraft hangars, and factories, home applications are also under development. Commercial products could appear within two years.