Why Are Some Streetlights Purple?

Traditionally, paved roads and pedestrian walkways have been illuminated by streetlights emitting a bright white, orange, or yellow glow. The aim is visibility, not aesthetics, and rarely will streetlamps cast any strange colors.

Yet there’s been a rise in recent years of reports that some streetlights are giving off a purple haze—a very emo teenager vibe that’s better suited for Hot Topic than public thruways. So what’s going on?

According to Scientific American, the purple hue isn’t exactly intentional. For years, cities have been upgrading public lighting infrastructure, going from sodium lamps to LED (light emitting diode) lighting for the cost savings in much the same way consumers have been unscrewing old incandescent bulbs in their homes.

But LED lights can’t produce white without some extra steps. They need to either combine different LED colors (red, green, blue) or use a blue LED and cover it with yellow phosphor that filters blue wavelengths and ultimately produces a bright white light. The thickness of that layer determines the warmth of the light, with tones ranging from warm to cool.

The purple is a consequence of this latter method. When that phosphor coating begins to degrade or delaminate, the blue LED can shine through unencumbered and appear as purple or violet.

There could be several causes for the delamination, including normal wear, heat from the LED, or even vibrations from passing cars. It’s also possible that the coating could be unreliable due to manufacturing defects. One LED supplier, Acuity, took note of the change and offered an explanation:

“The referenced ‘purple light’ effect is due to a spectral shift caused by phosphor displacement seen years after initial installation. The effect occurred in a small percentage of our total population of AEL branded fixtures, and we are not the only lighting manufacturer that has experienced this issue (...) we no longer use the light-emitting diodes (LEDs) that caused this spectral shift.”

Since local governments typically use one contractor as a source, one faulty light may mean there are many in the same vicinity. In 2023, The Boston Globe reported several of the purple lights in the Massachusetts capital. They’ve also popped up in Vancouver and in parts of Wisconsin, Florida, and California.

While driving down a purple-hued street might seem fun, it could potentially be hazardous. Photoreceptors in our eyes are less attuned to blue than white, a capacity that becomes even more diminished at night. Objects—or people—might not be as easily seen as they would be under a normal streetlight.

The phenomenon is likely to be temporary. In 2023, Baltimore Gas and Electric reported replacement of over 14,000 LEDs. Given how slowly some municipal improvements are implemented, however, you might be able to enjoy this unusual spectacle for a while.

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