Why Are Some Streetlights Turning Purple?

December 28, 2022

Purple streetlights in a city
The Mysterious Purple Streetlights

A strange phenomenon is happening around the United States with the color of streetlights. You may or may not have noticed it in your area, but some streetlights are putting off a purple glow, and it’s not intentional.

Since 2020, people have been noticing that some of the streetlights in different areas of the country have been turning purple. The purple streetlights have shown up in Texas, North Carolina, New Mexico, Iowa, Kentucky, Kansas, Florida, California, and Wisconsin, just to name a few. They’ve even turned up in Ireland and Canada. But if purple streetlights weren’t put up on purpose, then what happened?

Streetlights usually give off a white color, usually from LEDs, or an orange color, which comes from low-pressure or high-pressure sodium vapor lights. When electricity runs through these lights and vaporizes the sodium, it gives off an orange glow. Cities began switching from sodium-vapor streetlights to bright-white LEDs, which were cheaper, more energy efficient, and lasted longer, almost up to a decade.

The mystery of the purple streetlights is that they are, in fact, LED streetlights, but they have a significant flaw. Though many of the reports of streetlights inexplicably turning a purple color started in 2020, the defect began during the manufacturing process between 2017 and 2019.

The company making the lights dominates most of the LED light market and is called Acuity Brands. According to reports, the lights used by cities that reported purple lights came from a brand of this company called American Electric Lighting.

The lights are made of a blue LED under a ceramic and glass lens impregnated with a yellow phosphor. Using just a blue LED with the yellow phosphor coating is cheaper than using red, green, and blue LEDs together to make the same white light. The blue LED light combines with the yellow phosphor, and we see white light.

This is where the problem came about, however. The streetlights were turning purple because the phosphor coating was delaminating, and as it degraded, the white light changed to a purple color. The delamination was believed to be caused by heat and temperature changes. It’s also thought that something was amiss with the manufacturing process. Acuity outsources LED production to vendors in Asia because it’s cheaper to produce them there, and they’re able to make them at high volumes but can be of lower quality.

The reason the purple streetlights have shown up in many parts of the country and even different parts of the world is because the source of these LED streetlights was Acuity Brands. Duke Power reported that 1% of the LED streetlights have the color problem, which equates to about 5,000 streetlights. Still, not a small number of streetlights that will eventually have to be replaced again. Acuity states that it will replace all streetlights under warranty that are defective.

So if you happen to be driving around and see one of these purple streetlights, you’ll know that it isn’t some new lighting technology but a major flaw that is going to take a long time to fix.

Sources: WFDD, Business Insider, Motor Biscuit, Larson Electronics, CBC

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Daniel Ganninger - The writer, editor, and chief lackey of Knowledge Stew, the author of the Knowledge Stew line of trivia books, and editor of Fact World and the Knowledge Stew sister site on Medium, our ad-free subscription sites (you can find out how to join below). I hope you find things here to annoy those around you with your new found knowledge.

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