Buzz
Traditionally, roads and pedestrian areas have been lit by streetlights casting a bright white, orange, or yellow light. The focus is on visibility rather than appearance, and strange hues are rarely seen in street lighting.
However, in recent years, reports have been surfacing about streetlights casting a purple glow—reminiscent of an emo aesthetic better suited for Hot Topic than city streets. What's the reason behind this phenomenon?
According to Scientific American, the purple color is not intentionally designed. Cities have been replacing old sodium lamps with LED lights as part of cost-cutting efforts, similar to how people have switched out incandescent bulbs at home.
LED lights can’t produce white light on their own—they require additional steps. To create white, they either combine red, green, and blue LEDs or use a blue LED coated with yellow phosphor that filters out blue wavelengths, resulting in white light. The thickness of this phosphor layer affects the warmth of the light, ranging from warm to cool tones.
The purple hue is a byproduct of the second method. As the phosphor coating deteriorates or separates, the blue LED light shines through unchecked, creating a purple or violet effect.
The degradation of the coating could have several causes, including wear over time, heat from the LED, or vibrations from passing vehicles. Manufacturing defects might also contribute to this issue. Acuity, one LED supplier, took note of the phenomenon and offered this 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.”
As local governments typically rely on one contractor for their lighting, a single faulty light can lead to many others in the same area. In 2023, The Boston Globe reported multiple purple lights in the Massachusetts capital. Similar occurrences have been noted in Vancouver, Wisconsin, Florida, and California.
Driving through a street bathed in a purple light may seem interesting, but it could pose a danger. Our eyes are less sensitive to blue light than to white, and this sensitivity is even lower at night. Objects or pedestrians may be harder to spot compared to normal streetlights.
This unusual effect is likely to be short-lived. In 2023, Baltimore Gas and Electric reported the replacement of over 14,000 LEDs. However, given the slow pace of municipal upgrades, you may still have the chance to witness this peculiar sight for some time.
