A few months ago, Tara Benson noticed something weird about the streetlights on the drive to her evening yoga class in south Charlotte.
“There would be a string of like, white lights or orange lights, and there would just be this purple light,” Benson said. “And they’re kind of spooky.”
Streetlights tend to vary in color — at least slightly. The lights with sodium vapor bulbs can look orange, while the newer LED bulbs emit more of a white or yellow light. But purple?
Benson wondered if maybe the city of Charlotte was changing the color of the streetlights on purpose, like to reduce light pollution or avoid disrupting birds’ migration patterns.
She asked FAQ City: “Why are some streetlights purple? Does it serve a special purpose? And are all of the streetlights eventually going to be purple?”
‘Reduced Phosphor Correction’
Purple — and sometimes even blue — streetlights have popped up across the U.S., from Wisconsin to Kansas to Iowa to South Carolina.
“We believe it’s affecting a few thousand lights in western North Carolina and central North Carolina, as well as several hundred in eastern North Carolina,” said Meghan Miles, a spokesperson for Duke Energy. The Charlotte-based utility company maintains 936,000 streetlights in the Carolinas.
Miles said Duke first started hearing from customers about the color switch early this spring .
“I think it started initially like in March,” she said. “So people thought maybe it was connected to Easter or some sort of event.”
Residents have been tweeting, calling and emailing Duke to share pictures of the streetlights. Miles said most of them seem to like the purple. But these kooky-colored streetlights aren’t in the majority. According to Miles, they make up a fraction of 1% of all of the streetlights that Duke Energy maintains in the Carolinas.
“A lot of people have indicated they actually like them and how unique they are,” Miles said. “But we are not going to implement a purple streetlight program.”
Miles said the color change is caused by a small defect in some LED lightbulbs made in 2018 that Duke purchased from a company called Acuity Brands Lighting. Acuity declined WFAE’s request for an interview.
“(The change) is due to a reduced phosphor correction and a change in output that manifests itself sometime after initial fixture installation,” an Acuity spokesperson said in an emailed statement.
‘Think Of A Cupcake’
What exactly is a “reduced phosphor correction?”
Nan Jokerst, a professor of electrical and computer engineering at Duke University, agreed to explain. She does a lot of work with LED lights. LED is short for “light-emitting diode,” essentially a device that gives off light when an electric current passes through it.
“White light … is made up of the colors of the rainbow,” Jokerst said. “If you mix all of those colors together, you get white.”
To make an LED lightbulb that gives off white light, companies need to mix different colors together. One way they do that, according to Jokerst, is by picking an LED device of one color and adding other colors on top using little particles called phosphors.
“Think of a cupcake with dots of frosting,” Jokerst said. “The cupcake is blue and our dots of frosting are red and yellow.”
In this analogy, the blue cupcake represents a blue LED device and the dots of frosting are the phosphors. When the phosphors absorb the blue light from the LED, they begin emitting red or yellow light. Once the red, blue and yellow light mix together, they create white light. This is what happens in white LED bulbs, including the ones Duke Energy uses in some of its streetlights.
But sometimes, the colors don’t mix together properly.
“If the frosting falls off (of the cupcake)—we lose the red, we lose the yellow—all we have left is sort of the purplish blue color,” Jokerst said.
Comparably, if the red and yellow phosphors fall off of the blue LED device, the lightbulb only gives off purple or blue light, hence a purple (or blue) streetlight.
Don’t Get Too Attached
Jokerst admitted this is not the most exciting explanation for the eerie purple glow of certain Charlotte-area streetlights.
“At the beach, they tell you to use a red flashlight so you don’t disturb turtles,” she said. “That (answer) would have been wonderful — if it was like, ‘It’s better for owls and bats.’ No, it’s just a phosphor that’s come off.”
It’s not clear why the phosphors…