First they took over our radios. Then our televisions, stereos, watches, toys, telephones, computers, and just about everything else. So is it any wonder that semiconductors are threatening to transform the lighting industry? That's the lesson of this week's Lightfair International at the Convention Center, where light-emitting diodes, or LEDs, are the obvious stars of the show.

These electronic devices already have a bright history. LEDs have come to dominate in niches such as automobile brake lamps, emergency lights, and decorative displays - in Philadelphia, witness the colored LEDs outlining Boathouse Row, flashing civic pride on the facade of the Cira Centre at 30th Street Station, or tinting buildings in the nightly light show on Avenue of the Arts.

But are LEDs ready for prime-time use in homes and businesses? That's the hope of many of the 500 companies touting their wares at this year's Lightfair, the annual trade show of the lighting industry - even if it's a pitch greeted a bit more skeptically by bulb buyers.

"The vast majority of our homes and public spaces will be lit by LEDs in seven to 10 years," predicts Ted Russ, chief business development officer for Lighting Science Group Corp., a Satellite Beach, Fla., company that specializes in LED lighting.

Russ says Lighting Science, founded in 2003, has grown from about 75 employees a year ago to about 400 today at its Florida facility and a plant in Monterrey, Mexico. He foresees similar growth for LED technology, suggesting it will parallel the steady increases in computing power seen for decades in the microprocessor industry.

So far, most of the demand for Lighting Science's LEDs comes from commercial or institutional users. But the company is dipping its toe into the residential market: For the last month, it has been selling a 13-watt Definity lightbulb at Home Depot stores that it says produces light equivalent to a standard 60-watt incandescent bulb.

Unlike many of the company's LED lamps, the new bulb is "omnidirectional" - something done effortlessly by the traditional incandescent developed more than 125 years ago, which may be one reason that the bulbs of Thomas Edison's era have reigned so long despite their inefficiency.

To achieve a similar effect, the Definity bulb uses a design trick: A ring of 13 one-watt LEDs surrounds a "translucent reflector. Light shines through it as well as reflects off of it," explains Lighting Science's Larry Fallon.

The Definity saves about 80 percent of the electricity used by a traditional incandescent, Russ says, and it does so without raising concerns about mercury, a toxic element found in the compact-fluorescent lightbulbs that have become common as policymakers and power companies push to reduce the amount of electricity used for lighting.

But for now, it does so at an eye-popping price: about $37 per bulb, Russ says.

Lighting Science and its competitors all say that the up-front cost is worth it, at least for some users.

Toronto's Revlite Technologies, which began making automotive LEDs in the 1990s, aims its new LEDs at commercial applications such as hotels and retailers, with lamps that replace standard halogen bulbs but use about 75 to 80 percent less energy.

Jonathan Wylde, Revlite's R&D manager, says the bulbs should last up to 50,000 hours, about three times as long as fluorescents and as much as 50 times as long as many incandescents. For installations where lights stay on 12 to 24 hours a day, the benefits are obvious.

Even for $50 or more apiece, "it doesn't take long before the bulbs pay for themselves," Wylde says, especially when you factor in the labor cost of replacing burned-out fixtures.

Revlite has done some residential installations - including the home of TV journalist Lisa Ling, Wylde says. But he says that at today's prices, such uses are more about making statements than about finances, because residential lights are typically used just a few hours a day. "It would take years before you're paid back," he says.

An LED could save hundreds of dollars in power costs over its life span - more than enough to pay for itself. But if you use the bulb, say, only about three hours a day, it could take nearly 25 years to reach the 25,000-hour benchmark, and a staggering 50 years if it lasts the 50,000 hours that some manufacturers claim.

"The return on investment is there," Russ says. "But no one runs their house based on payback."

So how do LEDs look to lighting consultant Al Borden, a principal in The Lighting Practice, a Center City lighting-design firm?

"It's definitely the hot product," says Borden, who visited Lightfair on Wednesday and has used LEDs in projects such as the Avenue of the Arts light show and a gallery at the Library of Congress, where LEDs will be used to illuminate historic murals.

Borden says the long-lived lights make the most sense in institutional settings, where lighting needs are least likely to change. But for now, he remains skeptical - not just because of the price but because of the complexity of the emerging technology, where a single bulb can contain 50 to 100 components.

"People think of it like a lightbulb, and it couldn't be more different," Borden says. "It's a little bit like installing a computer in your ceiling."