In recent years, nighttime revelers in the US have noticed odd streetlights starting to emit an otherworldly purple haze. It isn’t a psychological warfare operation, nor a viral marketing ploy for a posthumous Prince album, but the result of LED silicone phosphor degradation.
It’s estimated that at least 30 states have reported streetlights switching from a beaming white to a purple glow since 2021, according to USA Today. Similar accounts have popped up in Canada and Ireland, too.
The lavender lighting seems to spring up sporadically, with cases reported in cities such as Milwaukee and Eau Claire in Wisconsin; Schaumburg, Illinois; Palm Beach, Florida; and Los Angeles, California.
In most of these instances, it’s believed the cause is degradation of the yellow silicone phosphor component of LEDs (light-emitting diodes). LEDs are semiconductor devices that emit light when an electrical current is passed through them. They’re a great option for things like streetlights as they’re relatively efficient, lasting longer and requiring less energy than traditional sodium-vapor lamps.
Blue LEDs are especially useful because the light has a shorter wavelength (and therefore carries more energy) than colors like red. However, blue light isn’t always desirable.
To address this, scientists developed an ingenious method to make the light from blue LEDs appear white. They coat the LED with a layer of yellow silicone phosphor. When the blue light excites the phosphor, it emits yellow light. The combination of the original blue light and the additional yellow light creates an emission that appears white to the human eye.
Over time, the phosphor coating in certain LED devices can degrade. Without this yellowish filter, the LEDs appear to emit more blue, endowing them with a purple gleam instead of white.
The reason why the purple lights only appeared for a few years after 2021 has something to do with a batch of recently installed streetlights that were dotted around the country.
In a (since-deleted) statement, the largest lighting manufacturer in North America, called Acuity, said in 2023: “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. This light output is in no way harmful or unsafe and we no longer use the light-emitting diodes (LEDs) that caused this spectral shift.”
Along with many of the affected lights being replaced, it seems lighting manufacturers recognized the issue and phased out the problematic components. As a result, you’re unlikely to see this problem reappear in the coming years.
As much as you might enjoy the curious sight of damson-lit streets, that’s a good thing. Despite manufacturers’ assurance that the purpling was harmless – there’s no evidence anyone has been harmed – it’s evident that blue light scatters more easily than most other visible light and can make it more difficult for the eyes to focus, especially in low-light conditions.
Source Link: Why Did Street Lights In The US Turn Purple?
