American cities and towns started switching their streetlights from sodium lamps to LEDs about 15 years ago, which changed the color of many nighttime roads from yellowish orange to bright white. But lately an odd new nocturnal color has been spotted across the nation—and the globe. Anecdotal reports of purple-looking streetlights have been popping up since early 2021 in states including Florida, Utah, Texas and Massachusetts, as well as in Canada and Ireland.
This phenomenon might seem weird but innocuous. After all, what harm could purple streetlights possibly do other than scare nearby residents as Halloween approaches? But it turns out that the hue of the light illuminating a roadway could affect how drivers and pedestrians perceive their surroundings as they make their way through the night. And that makes purple streetlights a potential safety hazard.
How do white LED streetlights work?
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Updating streetlights to use light-emitting diodes makes sense. LEDs are one of the most energy-efficient lighting technologies that currently exist, according to the U.S. Department of Energy. They also tend to be more durable and longer-lasting than other types of lighting while producing light that is comparable or better in quality.
These diodes can emit light at a variety of wavelengths—but one color they do not produce naturally is white. So when you see white LEDs, such as those in streetlights, they are actually emanating some mixture of the colors red, green and blue (or red, yellow and blue), which together produce the appearance of white. There are two popular ways to create this illusion. One is to combine tiny LED lights that each emit red, green or blue into a single big device. The other is to use only blue LEDs but coat them with a type of fluorescent substance called phosphor. When the blue light from the diodes goes through this layer, the phosphor absorbs some of the blue wavelengths and spits out red and yellow ones. This results in a mixture of colors that, once again, appears white.
The second method gained popularity because it’s more energy-efficient than the first. LEDs that generate red and green light require more energy than those that emit blue, says Jakoah Brgoch, an associate professor of chemistry at the University of Houston. In addition to saving energy, the phosphor method also requires less electric circuitry than LEDs of three different colors do. This makes the streetlights less expensive, says John D. Bullough, a program director at the Light and Health Research Center, part of the Icahn School of Medicine at Mount Sinai. This method does not work for all applications; televisions and color-changing lightbulbs, for instance, need to produce multiple colors of light. But to simply illuminate a street with a white light, manufacturers often opt for the cheaper and more efficient blue-LED-and-phosphor-coating combination.
But why are some streetlights suddenly turning purple?
The phosphor technique is sound, says Ram Seshadri, a materials science professor at the University of California, Santa Barbara. “Outside of my university, we’ve had LED lighting on the street for ... years, and there’s [been] no problem,” he adds.
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