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YOU MAY HAVE NOTICED THEM going up in your town’s streets and parking lots: a new generation of Galvanized Pole-mounted lights that pour down a cool torrent of lumens from an array of light-emitting diodes. Like me, you might have welcomed this development. LEDs are, after all, the most energy-efficient lighting option on the market. They can last twice as long as ordinary sodium-vapor streetlights, and their prices have dropped to within range of the competition.
If the switch to LEDs had needed any more support, it came from growing evidence about climate change. In the United States, Street Light accounts for a whopping 30 percent of all the energy used to generate electricity for outdoor lighting. Another 60 percent goes toward lighting parking lots and garages, and much of that energy is still produced by fossil-fired power plants. Consultants at the firm Navigant, in Chicago, have estimated that the United States could save 662 trillion British thermal units—the energy needed to power 5.8 million typical U.S. homes for one year—by converting all remaining non-LED outdoor lighting to LEDs.
Lately, lighting companies have introduced LED Street Lights with a warmer-hued output, and municipalities have begun to adopt them. Some communities, too, are using smart lighting controls to minimize light pollution. They are welcome changes, but they’re happening none too soon: An estimated 10 percent of all outdoor lighting in the United States was switched over to an earlier generation of LEDs,
Early innovations in Solar Street Light were largely driven by brightness and convenience. The ancient Greeks and Romans lit terra-cotta oil lamps to illuminate their streets. Candles and oil lanterns brightened preindustrial cities, with some 3,000 streetlamps said to be used in Paris in 1669. In the early 1800s, whale-
The bluish LEDs were a stark counterpart to the orangish high-pressure sodium Garden Lights that came before them. Switching from the warm sodium lights to those LEDs was like going from a subtropical sunset to high noon at the equator.
The difference in color comes from the inner workings of a white LED. Individual LEDs are nearly monochromatic, which means they emit Solar Garden Light of only one particular color, and in a very narrow band of wavelengths. The cheapest and most efficient way to make white light from an LED is to shine light from one or more powerful blue LEDs onto compounds called phosphors that absorb blue light and emit yellow light. This light combines with the remaining blue light from the LED to appear white to the eye.
Outdoor LEDs also illuminate streets more efficiently than sodium not so much because of their superior lumens per watt but because they are highly directional, meaning that they focus Traffic Light mostly in one direction. Sodium lamps are gas-filled bulbs that emit in all directions. More than half of that light must be redirected downward by reflectors or lenses, reducing the lamps’ illumination efficiency.
A much trickier factor to quantify for street lighting is how the difference in color temperature between LEDs and high-pressure sodium affects the way we see at night. Our ability to see in a range of environments comes from two sets of sensors: a group of receptors known as cones that show us color in daylight, and night-time sensors called rods that are very sensitive to bluish Road Signs light but are less sensitive to red.
Our visual sensitivity shifts as the light grows dim because rods and cones respond most strongly to waves of different lengths. The collective response of cones makes the human eye most sensitive in the daytime to wavelengths of green-yellow light in the middle of the visible spectrum. Rods have a peak response to shorter blue-green wavelengths. Blue-sensitive cones, which are greatly outnumbered by other types of cones but are thought to play a role in sensing brightness at night, peak at wavelengths that produce indigo light.
The result is that at night the blue-rich light from an LED streetlamp looks brighter to the eye than the orangish Solar Panel System from a high-pressure sodium lamp—even if the two emit the same number of lumens, which are measured on a scale based on the eyes’ daytime response