Introduction to LEDs

Statistics from the U.S. Department of Energy show that each year an astounding 68,000 gigawatt-hours of electricity goes toward illuminating streets and parking lots at night. (By one estimate, 60 million cobraheads line American roadways.) The vast majority of these area- and streetlighting fixtures use high-pressure sodium (HPS) bulbs, a relatively efficient source that can generate 100 lumens per watt.

However, the rapid development of solid-state lighting — and light-emitting diodes (LEDs) in particular — promises to take energy efficiency to a new level. Some “power” (high-output) LEDs can produce 150 lumens per watt and soon might top 200. Moreover, they’re compact, can last up to 15 years or more, and (unlike HPS and other typical streetlight bulbs) can be dimmed or turned off/on at will. Best of all, LEDs are inherently directional: they must be pointed toward their target. According to Robert Parks, IDA’s Executive Director, “LED lighting has the potential to revolutionize outdoor lighting in a profoundly positive way.”

The comparison above, provided by researcher Chris Luginbuhl, shows spectra of the most common outdoor-lighting sources. Note how low- and high-pressure sodium (LPS, HPS) lamps leave much of the visual spectrum unaffected. By contrast, metal-halide (MH) and light-emitting diode (LED) lamps emit over a wide swath of visual wavelengths.

Officials at the U.S. Department of Energy (DOE) and Environmental Protection Agency intend to introduce many commercially viable LED-based lighting products as soon as practicable. Right now these agencies are readying the criteria that will qualify an LED fixture for Energy Star certification.

Although still relatively expensive, LEDs are likely coming to a streetlight near you. The first large-scale installations are already being tested and installed in China, North America, and Europe. For example, in 2009 officials in Los Angeles agreed to install 140,000 LED streetlights over the next five years.

If there’s a downside to this technology, it’s that the most-efficient LEDs emit a strongly blue cast, with up to 30% of their photons having wavelengths of 500 nanometers or less. To the eye, this light looks harsh. Photobiologists and eye doctors have expressed concern about unleashing so much blue light, however well directed, into the nighttime environment. If not filtered, it might prove harmful to the circadian function of animals and humans. As the graph below shows, much of the emission from blue-rich LEDs falls directly in the range where human circadian sensitivity is greatest. (For more details, see the IDA's report on blue-rich nighttime lighting.)

To get “warmer” light, an LED’s output can be passed through a phosphor that reradiates it at longer wavelengths — but in doing so it can lose up to a third of its lumens. This lumen gap is narrowing, however, as manufacturers strive to strike a balance between optimum efficiency and environmental sensitivity.

For more information, see the U.S. Department of Energy's website on LED basics.