The colored lights on the Cira Centre are LEDs. Ditto for Boathouse Row.
But where they really shine is in holiday lighting displays.
The breakthrough came in 2001, after a Yardley entrepreneur, Dave Allen, and his brother Mark, an engineer in California, figured out how to make LEDs work with a common household electric plug. Previously, they needed transformers.
The Allens' company, Fiber Optic Designs, licenses their technology to Colorado-based Holiday Creations.
Now holiday LEDs use a fraction of the energy needed for incandescents. Last year, the U.S. Department of Energy estimated that incandescent Christmas tree lights kept on 12 hours a day for 40 days would cost $23.95 in electricity. With LEDs: 54 cents.
Plus, LEDs don't get hot, which makes them safer around flammable greenery.
That also means the outside of the light can be made of unbreakable epoxy instead of glass.
And they last up to 30 times longer.
The pros have embraced holiday LEDs.
Last year, the national Christmas tree in Washington, the tree in New York's Rockefeller Center, and the Times Square New Year's ball all went LED.
This year, Philips Lighting is upping the ball's glitter quotient with more than 32,000 LEDs, digitally controlled to create 16 million colors - must be pretty subtle - and "billions" of effects.
GE Consumer and Industrial touts the Washington tree's 45,000 LEDs as "the most energy-efficient holiday display in our national history."
Closer to home, Longwood Gardens, whose annual display attracts 220,000 gawkers, has been gradually converting and now is 80 percent LED.
The King of Prussia mall just converted 300,000 of its half-million holiday lights. Marketing manager Mark Bachus saw some LEDs at a sister mall and loved how they made the decor "pop."