Nighttime commuters driving over the reconstructed Interstate 35W bridge may notice a bluish glow coming from the ten pairs of street lights lighting their way. Mounted 40 feet above the traffic, the lights are not bulbs but rows of LEDs, or light-emitting diodes -- similar to those found in stoplights and laser pointers.
"This is the first interstate highway to be lit with LED lighting," said Kevin Orth, director of sales for Wisconsin-based BetaLED, which makes the lights. LEDs are coming to the streets of Eden Prairie, where officials are replacing the city's old street lights, and already illuminate the parking lot of a Cub Foods store in St. Paul's Phalen neighborhood, which last month became the second certified energy-efficient supermarket in the country.
For large projects like these, the long-run savings in energy and maintenance, as well as the environmental concerns, generally outweigh the short-run costs.
This growing use of LEDs by government and industry marks a move away from traditional incandescent bulbs and, more recently, the more-efficient fluorescent lights that have come on the market. Although LEDs cost more to manufacture than other lighting options, they consume a small fraction of the energy of even fluorescent bulbs and last 25 to 30 years.
Lighting still accounts for as much as 20 percent of electricity used around the world, so improving lighting technology by even a little bit can lead to great savings in energy and reductions in greenhouse gases.
New lighting standards
On June 29, President Obama unveiled new, stricter lighting standards to accompany the phasing out of incandescent bulbs (which already are banned in parts of Europe) and to provide $50 million in funding for the development of "solid-state lighting" technologies such as LEDs.
"We could save 50 percent of the energy that is currently being used for lighting by switching to LEDs," said Fred Schubert, an electrical engineer at Rensselaer Polytechnic Institute in Troy, N.Y.
The standard 60-watt light bulb gets its number from the amount of energy required to light it up. An incandescent bulb runs an electric current through a thin metal wire, but only about 5 percent of the current gets turned into light.
Fluorescent bulbs, which send electricity through a thin gas, are much more efficient, turning 20 to 30 percent of the energy into light; so a 13-watt fluorescent bulb is about as bright as a 60-watt incandescent. Fluorescent bulbs are often described not in watts but in efficiency, measured in lumens (brightness) per watt.
Compared with the typical fluorescent light -- which achieves about 60 lumens per watt -- LEDs in the lab have surpassed 120 lumens per watt. They pass electric current through a thin sandwich of a semiconductor such as silicon, causing electrons to move around and pop into vacancies left by other electrons -- like guests checking in and out of hotel rooms -- to send out tiny packets of light.
Despite their efficiency, LEDs have not yet found their way into broad use in home lighting, due in part to cost and because it has been difficult to get them to glow with the white light that people are accustomed to.
LEDs are more expensive than other lights, and bringing the price down has proven difficult. Every decade or so the price of LEDs falls by a factor of 10, while their performance increases by a factor of 10.
For now, LED bulbs aren't cheap -- Home Depot is selling on its website the LED equivalent of a 25-watt incandescent bulb for $10.99. On the other hand, it draws only 3 watts of electricity and is rated for 100,000 hours of use.
"The cost of the LED product is five to 10 times more expensive for the same [brightness]," said Maria Anc of Osram Sylvania, which will release its first 100-lumens-per-watt LED bulb for the home this summer. "What we need to bring down the cost is 10 to 100 times more production worldwide."
In search of warmer light
Another obstacle for LEDs is the quality of the light they produce, known as the "color rendering index" (CRI). Sunlight has a CRI of 100, fluorescent bulbs can reach 98, but LEDs struggle to achieve 70. The color of light generated by a single LED depends on the kind of material it is made of, and the typical way of making white LED light is to embed blue LEDs in yellow material that whitens the light. Even so, LED lighting has a reputation for being harsh and unpleasant.
An option for making LED light pleasant is being developed by Stephen Campbell, director of the University of Minnesota's Nanofabrication Center. His technology, recently unveiled at an electronics conference in San Francisco, runs electricity through tiny bits of materials as small as 20 atoms wide. The size of these "quantum dots" determines what color of light will shine out of it. By grouping dots of different sizes together, Campbell hopes to mix and match different colors to create white light. "We're making these out of silicon, which should be dirt-cheap and nontoxic," said Campbell.
"LEDs are everywhere, in car headlights, traffic lights, refrigerators, parking lots, TVs, and cell phones," said Brian D'Andrade, senior associate at the consulting firm Exponent. But incandescent lights are "hard to get rid of in residential homes," he said, and to compete LEDs will need to be able to put out bright, warm, white light at a reasonable price.
This story was provided by Inside Science News Service, which is supported by the American Institute of Physics, a nonprofit publisher of scientific journals.
