after years of resisting what they had dismissed as a fringe technology, giants like General Electric and Philips have begun making LEDs.Though the United States Department of Energy calls LED a pivotal emerging technology, there remain significant barriers. Homeowners may balk at the high initial cost, which lighting experts say currently will take 5 to 10 years to recoup in electricity savings. An outdoor LED spotlight today costs $100, as opposed to $7 for a regular bulb.Another issue is that current LEDs generally provide only directional light rather than a 360-degree glow, meaning they are better suited to downward facing streetlights and ceiling lights than to many lamp-type settings.And in the rush to make cheaper LED lights, poorly made products could erase the technologys natural advantage, experts warn. LEDs are tiny sandwiches of two different materials that release light as electrons jump from one to the other. The lights must be carefully designed so heat does not damage them, reducing their lifespan to months from decades. And technological advances that receive rave reviews in a university laboratory may not perform as well when mass produced for the real world.Britains Low Carbon Trust, an environmental nonprofit group, has replaced the 12 LED fixtures bought three years ago for its offices with conventional bulbs, because the LED lights were not bright enough, said Mischa Hewitt, a program manager at the trust. But he says he still thinks the technology is important.Brian Owen, a contributor to the trade magazine LEDs, said that while it is good that cities are exploring LED lighting: They have to do their due diligence. Rash decisions can result in disappointment or disaster.At the same time, nearly monthly scientific advances are addressing many of the problems, decreasing the high price of the bulbs somewhat and improving their ability to provide normal white light bright enough to illuminate rooms and streets. For example, many LEDs are currently made on precious materials like sapphire. But scientists at a government-financed laboratory at Cambridge University have figured out how to grow them on silicon wafers, potentially making the lights far cheaper. While the original LEDs gave off only glowing red or green light, newer versions produce a blue light that, increasingly, can be manipulated to simulate incandescent bulbs. And researchers at dozens of universities are working to make the bulbs more usable.This is the second in a series of articles about stopgap measures that could limit global warming. Future articles will address appliance-efficiency standards, transportation, reducing global-warming gases other than carbon dioxide and other efforts.Previous Article in the Series This is a technology on a very fast learning curve, said Jon Creyts, an author of the McKinsey report, who predicted that the technology could be in widespread use within five years.So far, the use of LEDs has been predominantly in outdoor settings. Toronto, Raleigh, Ann Arbor and Anchorage not to mention Tianjin, China, and Torraca, Italy have adopted LEDs for street and parking garage lighting, forsaking the yellow glow of traditional high-pressure sodium lamps. Three major California cities Los Angeles (140,000 streetlights), San Jose (62,000)