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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.
Armed with statistics like these, and a mandate to cut energy use wherever they can, municipalities across the United States have installed more than 5.7 million outdoor LED street and area lights. Other towns and cities in Canada, Europe, and Asia have added millions more over the past decade. Amid this rush to adopt outdoor LEDs, the U.S. Department of Energy (DOE) stressed energy efficiency as the biggest advantage of the new technology while cautioning cities to also consider light output and color quality. But now that ordinary folks have got an eyeful of those new lights, some municipalities are coming down with a case of the early-adopter blues.
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, which included those problematic blue-rich varieties, at a potential cost of billions of dollars.
The episode invites a few questions: How did an energy-saving technology that looked so promising wind up irritating so many people? Why has it taken so long for the impacts of blue-rich lighting to become widely known? And why did blue-rich LEDs so captivate municipal lighting engineers long before better options reached the market?
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-oil lamps and lanterns began to give way to relatively inexpensive gas streetlights, which were first installed throughout London, Paris, and St. Petersburg, Russia.
Not until the 20th century did engineers start worrying about efficiency. Brilliant arc lamps were the original electric streetlights in the late 1800s, but it took more practical incandescent bulbs to persuade most cities to replace gas streetlights with electric ones. These were gradually phased out for even higher-efficiency successors: mercury-vapor lamps starting in 1948, and then high-pressure sodium in 1970.
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.
The resulting shade of white depends on the blend of blue from the LED and yellow from the phosphor. It’s measured on the color-temperature scale, which corresponds to the temperature (in kelvins) of a “black body,” which is an object that absorbs all the electromagnetic radiation it encounters and emits a similar mixture of colors. Early “white” LEDs developed in 1997 at Nichia Chemical Industries, in Japan, (now known as Nichia Corp.) were quite blue: They emitted more than 45 percent blue light, corresponding to 8,000 K. That’s even bluer than the color temperature of summer daylight, and it looks harsh to the eye.
Adding more and redder phosphors to a white LED makes its light look warmer and more agreeable to the eye—but at the cost of reduced efficiency. That’s because energy is lost in converting high-energy blue photons to lower-energy yellow and red photons. At home, though, people are sensitive to the color of lighting, so for indoor use, many people choose LEDs of 2,700 to 3,000 K, close to the hue of ordinary incandescent bulbs.
Indoor LEDs reign supreme among Smart Light System sources based on the savings they deliver: They are about five times as efficient as incandescents and up to 10 percent more efficient than compact fluorescents. They are rated to last anywhere from 2 to 50 times as long as competing bulbs. Though they’ve been screwed into only about 3 percent of indoor sockets in the United States, their rate of adoption is growing.
Outdoor lighting is a different matter, though, because it’s bought by municipal engineers charged with providing functional lighting at minimum cost. The cost-saving potential of LEDs appealed greatly to them, so they looked for the highest-efficiency bulbs. In June 2008, the DOE correctly noted that the most efficient white LEDs of the time were those with a color temperature of 4,500 to 6,500 K. The agency also recommended matching color temperature to the bulb’s intended application.
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.
Given these facts, some experts touted bluer light for LEDs, noting that the relatively high color temperatures could enhance visibility at night. Some suggested that the use of bluish LEDs would let us see so much better at night that we could turn down the intensity of the lighting.