Anyone who has ever tried to change a lit light bulb has touched upon the problem with today’s lighting technology. Fingers get scorched because about 95 percent of the electricity entering an incandescent bulb is wasted as heat. Fluorescent bulbs do much better, converting 20 to 30 percent of electricity into light. But even that pales beside the potential that LEDs offer. In theory, at least, an LED can convert almost 100 percent of its electrical power into light. Developers have not yet attained such perfection, but the efficiencies of the best white-light LEDs being produced are already about halfway between those of incandescent and fluorescent fixtures.And although conventional lighting has already pretty much maxed out in efficiency, that’s not the case with solid-state lighting. LEDs, which work on an entirely different physical principle, have at their core neither the tungsten filament of a conventional light bulb nor the gas of a fluorescent tube. Rather, they use crystalline layers that convert electrical input into optical output at a color determined by the exact composition of the material.
The colored LEDs that are the main ingredients of white-light sources have already made a dramatic impact. Take the ordinary red stoplight. In a conventional setup, one big, inefficient incandescent light bulb sits behind a red filter; the bulb guzzles about 150 watts of electricity and lasts about a year before it burns out, sometimes snarling traffic until the local highway department rushes out with a bucket truck to change it. In a growing number of traffic lights, though, a dozen or so red LEDs sit behind a clear lens, consume about 15 watts, and control traffic for five or more years before requiring replacement.
According to the California Department of Transportation, replacement of conventional traffic-light bulbs with LEDs-red, yellow, and most recently, green-has trimmed at least $10 million from the state’s annual electric bill. And nationwide, according to Strategies Unlimited, a market research firm in Mountain View, CA, LED traffic lights are becoming commonplace: as of 2002, 39 percent of red lights and 29 percent of green lights used LEDs.
But coaxing LEDs to produce white light is a much tougher problem. LEDs inherently produce a single color. Producing what the eye perceives as white light requires the generation of many hues together (white itself is not actually a “color”). The easiest way to make white is to start with one LED-either blue or ultraviolet-and add materials called phosphors. The phosphors absorb those high-energy photons and generate lower energy ones, such as yellows and reds. Most of today’s white LEDs pair a blue LED with a yellow phosphor. But the resulting white lacks a full spectrum of colors and is therefore less pleasing to the eye than incandescent light; as with fluorescent tubes, which also use phosphors, skin can look a bit sickly. The newer white LEDs are better: they pair an ultraviolet-emitting LED with phosphors that put out light at a range of colors, which mix to make white. Any device that uses phosphors, however, loses some efficiency in the process.
Quality of lighting aside, developers must solve an even more fundamental problem before LEDs can replace the ubiquitous white light bulb: the white LEDs now being produced or developed all cost many times as much as an ordinary light bulb. “Nobody will pay 20 bucks for a light bulbeven if it lasts 500 times longer and uses half the electricity,” says Frank Steranka, vice president for research and development at Lumileds.
Overcoming the hurdles of efficiency, color, and cost is what the global race is all about. And advances on these fronts will soon drive wider adoption of LED lighting technology, says Robert Steele, director of optoelectronics at Strategies Unlimited. “In the next five years, they’ll continue penetrating a variety of specialty markets like decorative lighting and entertainment lighting. In five to 10 years, they will start to penetrate the commercial and industrial market,” Steele says. Next stop perhaps: your kitchen or living room.