In 2007, the city of Ann Arbor, Michigan, announced its plans to become the first American city to convert all of its streetlights from sodium vapor lamps to LEDs.

The city replaced 1,400 of its 7,000 streetlights in just four years, saving roughly $200,000 in maintenance costs and lighting the way (literally) for an LED revolution that would sweep cities and streetlights across the nation.

Sixteen years later, LED lights have become something of a gold standard for street lamps across the U.S., and for good reason.

The Benefits of LED Streetlights

LEDs, also known as light-emitting diodes, come with a number of benefits that make them an enticing option for street lighting. For starters, their lifespans are massive, lasting up to 100,000 hours before needing a replacement. That's over 11 years if left on all the time and nearly 50 times longer than your typical incandescent fixture!

Their long-lasting design also means fewer replacements and lower maintenance costs, a considerable bonus when considering the labor and inconvenience of changing bulbs up to 100 feet in the air.

Plus, they're *way* more energy-efficient, too, sometimes consuming up to 75 percent less energy on average than a traditional light source and generating 48 percent more light per watt along the way!

They're also better at illuminating specific areas because they provide directional rather than diffused light, and because they don't generate much heat or emit ultraviolet rays, they're less of a draw for insects, too — win!

But, with all the benefits of LED streetlights, they haven't been without their roadblocks (no pun intended). And one of the most recent? Well, some lamps have suddenly begun to glow a mysterious shade of purple.

But why?

A Background on LED Street Lights

First things first, there are a few things that are important to understand about LED lighting before we go exploring the reason behind their unexpected shift in color.

To start, the one color that LED lights cannot naturally produce? White.

But aren’t LED streetlights often white? Well, yes. And, no.

Instead of having bulbs that emit pure white light, LEDs emit a combination of either red, green, and blue or red, yellow, and blue hues that appear as white to the human eye.

Achieving this illusion means LED lights are typically constructed in one of two ways.

The first method is to group several smaller LED lights, each emitting one of the three colors (red, blue, and yellow/green), into one bigger LED fixture.

The second method is to use only blue LED bulbs but coat their surface in a substance called yellow phosphor, which absorbs a bit of the blue light as it passes through, changing the appearance of some of the tiny bulbs from blue to yellow. The combination's result is the same: the appearance of white light as we drive along the road.

So why choose one method over the other?

Well, method number two is more energy-efficient as it takes less energy, on average, to power a blue LED bulb, and powering only one color of LED means less circuitry is needed, which helps in reducing manufacturing costs as well!

It's a business model that a company called Acuity Brands discovered early on, and quickly, they've become a dominant force in the LED market.

But, more efficient solutions can often come with a few technical challenges, and every city reporting their streetlights turning purple? You guessed it. They bought their lights from Acuity.

So, Why Are The Streetlights Turning Purple?

According to Neil Egan, a representative from Acuity, the root cause behind the mystery of the purple streetlights is "phosphor displacement seen years after initial installation."

In other words, that yellow phosphor that we were talking about earlier? Well, some of that is likely peeling off in a process called delamination, exposing the blue light underneath and changing the light that we see from a bright white to a shade of deep blue or purple.

The culprit behind this delamination process is most likely heat building up inside the lamp, potentially from improper storage in hot warehouses or container ships, although some experts also suggest it could be triggered by repeated vibrations brought on by passing vehicles or even simply gravity pulling down on the light and causing the phosphor to loosen and peel away from the bulb.

The exact cause is hard to discern without actually dissecting the light itself, and unfortunately for curious minds, Acuity has remained relatively tight-lipped on the matter (although they are replacing all of the streetlights within warranty).

The effect, though, is clear: an eerily spooky ambiance washing over our streets.

What To Do When You Stumble Across A Purple Streetlight

So, what do you do if you encounter a purple streetlight in your neighborhood?

Well, most of the perpetrating streetlights are already in the process of being replaced, but if you do run into a purple streetlight on your evening dog walk or late-night commute home, reporting your sighting is a huge help to providers in ensuring no streetlight is missed. You can usually find the operator's contact information directly on the streetlamp pole!

Mystery of The Purple Streetlights: Solved

No technology's journey into widespread use is without a few bumps in the road, and LED lights are no exception, but for now, this is case closed on the mystery of the purple streetlights.

It's an eerie occurrence with a relatively straightforward explanation, and while a nuisance for utility providers, Acuity, and streetgoers who don't enjoy the Halloweenish ambiance, they're also relatively harmless and simple enough to fix.

On the journey towards more efficient, energy-friendly, and affordable street lighting, we predict this will be just a hiccup in LED history and one that, in a few years, will be nothing but a distant and slightly humorous memory. Until then, keep your eyes out and drive safely!