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.
And yet some streetlights have suddenly turned a jarring shade of purple. It is hard to determine the exact cause without dissecting one of the defective lights, but scientists have a hypothesis: bright purple light suggests the phosphor layer around the lights has been “delaminated”—peeled off—exposing the blue LED light underneath, Brgoch says. Although blue LED lights are, in principle, deep blue in color, they do naturally have a bit of a purple and violet tint, he explains.
It’s possible that the lamps themselves could be malfunctioning in various ways, but other malfunctions would produce different results. If the LED part were flawed, the light would go dark. And if the problem were the chemical degradation of the phosphor layer, the color would gradually turn from white to off-white, says postdoctoral researcher Shruti Hariyani at Texas A&M University, who studied phosphor materials with Brgoch for her doctoral degree.
As for the possible delamination of the lamps’ coating, it could be caused by anything: heat buildup inside the light fixture because of constant operation, vibrations from passing cars or even gravity tugging downward on the phosphor layer, according to Brgoch and Hariyani.
How does the purple light affect vision?
Suddenly finding yourself in a street bathed in garish purple light can be unsettling. But a bigger concern is that this light can change how drivers and pedestrians perceive their surroundings. That’s because different types of photoreceptors in the eyes are responsible for vision under different light conditions.
Cone-shaped photoreceptors in the human retina become active when exposed to daylight, enabling people to discern color. But in low-light conditions, rod-shaped ones become responsible for vision. Despite their inability to distinguish color, rods are more sensitive to low-wavelength light—the blue part of the electromagnetic spectrum—than cones are. Another difference between these two types of photoreceptors is that rods are mostly located in the peripheral part of the retina. As a result, people driving at night may notice a moving object in their peripheral vision more quickly under bluish-white light than under yellowish-white light, Bullough says. (He notes that most studies of this kind were done using relatively broad-spectrum lights, white lights with blue and yellow undertones, instead of completely blue or yellow lights.) Gaining improved peripheral vision under blue-tinged light comes with a trade-off, however: once the moving object comes into focus, it becomes harder to see. That’s because human eyes have very few blue-sensitive cone photoreceptors that are dominant in their central vision. Plus, cone receptors are less reliable in dim environments.
Bullough says he believes purple streetlights aren’t good for driver or pedestrian safety. First, it’s possible that losing the phosphor layer dims the lamps’ brightness—and streetlights are supposed to emit a certain level of light depending on the type of the road that they’re used on. Second, blue- and violet-saturated light can worsen people’s ability to see details because of the lack of blue-sensitive cones in the center of the retina. Finally, the blue-violet light makes it very difficult to distinguish between different colors, Bullough says; everything becomes a shade of blue or black.
One thing that Bullough suggests pedestrians and drivers do to stay safe under purple streetlights—or any lights, for that matter—is to remove sunglasses and blue-light-filtering glasses when walking or driving at night. Because blueish light tends to appear brighter to humans, one might be tempted to put on shades to prevent glare. “But at nighttime,” Bullough says, “we really want to have all the light that we can get.”
Most importantly, mishaps with purple streetlamps shouldn’t be used to vilify all LED lights, Seshadri says. “The materials that are normally used to make the phosphor are extremely stable,” he says. “There must be some issue with the manufacturing or some particular LEDs, because most LEDs that the world is using don’t face this problem.”