Sunlight In A Bulb -- Sulfur Lamp Good For High-Tech Work, Has No Parts To Burn Out

Physics. Building on what has been learned about artificial lighting, a Maryland company has developed a prototype bulb that produces light similar to the type that comes from the sun. Some federal officials believe the bulb could represent a technological breakthrough. --------------------------------

The Department of Energy is looking at the future in a whole new light.

Forget incandescent bulbs, fluorescent tubes and even metal halide lamps, says Lee Anderson, DOE's program manager for lighting research. Much of tomorrow's illumination, he and others believe, will come from electrodeless devices that have no parts to burn out - and which produce the kind of intensely white light suitable to the increasingly detailed work done in high-tech industry.

"American workers need better lighting," Anderson said, "not only for attention to detail in microcircuits and small mechanisms but in quality control."

The brightest prospect of that kind is a revolutionary prototype bulb developed by Fusion Lighting of Rockville, Md., in conjunction with DOE: a tiny closed quartz sphere containing argon gas and a pinch of elemental sulfur. When zapped with ordinary kitchen-grade microwaves, the bulb gives off intensely bright and relatively cool rays that are remarkably similar to sunlight.

In unveiling test installations of the device recently at DOE headquarters and the National Air and Space Museum, Assistant Energy Secretary Christine Ervin called it "a major technological breakthrough."

For all its novelty, however, the sulfur bulb still makes light the old-fashioned way: by temporarily altering the energy level of electrons in atoms. Electrons can occupy any of a number of different energy states while orbiting the atom's nucleus. Raising an electron to a higher level takes additional energy, just as moving a worker from her normal office on the second floor to temporary quarters on the fifth floor requires power from the elevator or the muscles.

Blasting an atom with extra energy from outside - heat in an incandescent lamp, solar particles in the northern lights, microwaves in the new sulfur bulb - will cause electrons to pop up to higher energy levels. This is an unnatural condition, and they soon drop back to their normal states. As they do, they shed their excess energy in the form of photons: the individual units of light.

The color (wavelength) of the light depends on the kind of atom or molecule that is excited and the way its electrons are arranged.

Sticking a bit of common salt - sodium chloride - in a flame, for example, will turn the flame yellow. Spraying the Earth's upper atmosphere with electrons and protons blown off the sun (which is what happens in the aurora borealis) will cause nitrogen to glow violet and blue, and oxygen to flash crimson or whitish-green. Running an electrical current through a tube of neon turns it bright red.

A wide variety of energy sources can be used to produce this phenomenon, including fire. When a candle is lighted, the carbon and hydrogen atoms in the wax are vaporized and combine in rapid combustion with oxygen in the air.

The violent energy of the reaction excites electrons, which give off photons. A less messy way to achieve the same effect, Thomas Edison found, was to run a current through a wire filament and let the heat generated by electrical resistance turn the filament incandescent. In that case, the electrons of a tungsten wire are excited to the point at which they emit the familiar warm yellow-red illumination.

But the source does not have to be incandescent or even hot. In a fluorescent light, for example, small amounts of mercury are suspended in a cold gas. As the current passes down the tube, it ionizes the gas, which in turn imparts energy to the mercury, whose electrons give off photons in the ultraviolet range. The UV rays then strike a special coating of phosphorescent compounds on the inside of the tube. When excited by UV radiation, the phosphorescent compounds (usually magnesium, zinc and cadmium) shed their excess energy in the form of visible light.

Sodium-vapor or mercury-vapor outdoor lights work in a similar, though higher-intensity, fashion. In metal halide lamps - popular for lighting sports arenas and commercial interiors - an electric arc is run through a quartz tube containing a mix of metals such as sodium, indium, thallium, scandium or tin compounded with iodine. The combination of emissions from these metals produces a pleasing, near-white light.

In each case, the elements of the lamp have to be kept in an oxygen-free environment so that they don't burn. The environment of choice has been one of the "noble gases" - helium, neon, argon, krypton or xenon - that are notoriously unreactive. A frequent choice is argon, an inert element that seeps from below the Earth's surface and makes up about 1 percent of air.

In the version of the Fusion Lighting lamp used to light a 240-foot-long area outside DOE headquarters, a bulb about the size of a golf ball is filled with argon at one-tenth atmospheric pressure and approximately the amount of sulfur in a safety match. When the mix is irradiated with electromagnetic waves at 2.4 billion cycles per second (about what the average home microwave oven puts out), the argon heats up and vaporizes the sulfur, which forms into two-atom molecules.

As the molecules' excited electrons drop back to their ground states, the Fusion researchers found, they generate an unusually large quantity of photons in an uncommonly wide variety of types.

The result is a spectrum, or range of wavelengths, very close to those in white sunlight, which is the combination of all colors generated by incandescence on the sun's surface.

The sulfur bulb gets so hot that it has to be rotated at 300 to 600 revolutions per minute to prevent the quartz from melting, which it would do "in about 2 seconds" if uncooled, says Fusion Lighting Vice President Michael Ury. (Early prototypes also required two fans per bulb; later versions have eliminated that need.)

One of the bulb's chief advantages is that it has no electrodes. Eventually, Anderson said, any electrode will break down or burn out, and multipart tubes often develop flaws in their glass-to-metal seals; consequently, the amount of light emitted declines over time. In general, Anderson believes, "electrodeless lighting is the thing of the future."

And in the case of the sulfur bulb, it is also, ironically, a thing of the past: Perhaps the first electric lamp - as constructed in 1650 by German physicist Otto von Guericke - was based on sulfur.

As Michael Allaby writes in "Fire: The Vital Source of Energy": "When a globe filled with sulfur was rotated fast and he held his hand against it, the sulfur glowed. He did not know it, but the friction of his hand was imparting an electric charge to the glass, which was exciting electrons in the sulfur atoms. In effect, he had discovered electric light."