For about 15 years, cities across America and beyond have been upgrading their streetlights from sodium lamps to LEDs, trading the familiar yellowish-orange glow for a stark white light. However, a new and unexpected color has been appearing in the night sky recently: purple. Since early 2021, reports of streetlights emitting a purple hue have surfaced in various locations, from Florida and Utah to Texas and Massachusetts in the US, and even in Canada and Ireland internationally.
While the sight of purple streetlights might seem like a harmless oddity, perhaps even fitting for the Halloween season, the color of roadway illumination can significantly impact how drivers and pedestrians perceive their surroundings at night. This makes the emergence of purple streetlights more than just a quirky phenomenon – it raises potential safety concerns.
The Science Behind White LED Streetlights
Upgrading to LED streetlights is generally seen as a positive change. The U.S. Department of Energy highlights LEDs as one of the most energy-efficient lighting technologies available today. They also boast greater durability and a longer lifespan compared to traditional lighting options, while providing comparable or even superior light quality.
LEDs can generate light across a spectrum of wavelengths, but naturally, they don’t produce white light. The white light we observe from LED streetlights is actually an optical illusion, created by mixing different colors, typically red, green, and blue, or red, yellow, and blue.
There are two main methods to achieve this white light effect. One approach involves combining tiny LEDs that individually emit red, green, and blue light within a single unit. The other, more prevalent method, utilizes blue LEDs coated with a fluorescent material called phosphor. When the blue light passes through this phosphor coating, some of its blue wavelengths are absorbed, and red and yellow wavelengths are emitted in their place. This combination of colors results in what we perceive as white light.
The phosphor method has become popular due to its energy efficiency. According to Jakoah Brgoch, a chemistry professor at the University of Houston, generating red and green light with LEDs requires more energy than producing blue light. Beyond energy savings, the phosphor technique also simplifies the electrical circuitry, making these streetlights more cost-effective, explains John D. Bullough, program director at the Light and Health Research Center at Mount Sinai’s Icahn School of Medicine. While applications requiring multiple colors, like televisions, might use different methods, for simple white street lighting, the blue-LED-and-phosphor combination is often the preferred choice for manufacturers due to its efficiency and lower cost.
The Unexpected Turn to Purple: Delamination of Phosphor
Ram Seshadri, a materials science professor at the University of California, Santa Barbara, confirms the reliability of the phosphor technique. “Outside of my university, we’ve had LED lighting on the street for … years, and there’s [been] no problem,” he states.
This makes the sudden appearance of purple streetlights all the more puzzling. While a definitive diagnosis requires examining a malfunctioning light, scientists have a leading theory: the striking purple color indicates that the phosphor coating around the LEDs has become “delaminated,” or peeled away, exposing the underlying blue LED light. Brgoch explains that while blue LEDs are fundamentally blue, they naturally possess a violet or purple tint.
While lamp malfunctions are a possibility, other failures would manifest differently. A problem with the LED itself would likely cause the light to go dark completely. Chemical degradation of the phosphor would result in a gradual shift from white to off-white, according to Shruti Hariyani, a postdoctoral researcher at Texas A&M University who previously studied phosphor materials.
The delamination of the phosphor coating could stem from several factors. Heat buildup within the light fixture from continuous operation, vibrations from passing traffic, or even the constant pull of gravity on the phosphor layer are all potential culprits, suggest Brgoch and Hariyani.
The Impact of Purple Light on Vision and Safety
Beyond the unsettling aesthetic of purple-lit streets, the more significant concern lies in how this light affects visibility for drivers and pedestrians. Our eyes utilize different photoreceptors depending on light levels.
In daylight, cone-shaped photoreceptors in the retina are active, enabling color vision. However, in low-light conditions, rod-shaped photoreceptors take over. While rods don’t perceive color, they are more sensitive to shorter wavelengths of light – the blue end of the electromagnetic spectrum – than cones are. Another key difference is that rods are predominantly located in the peripheral retina. This means that under bluish-white light, drivers at night might detect movement in their peripheral vision faster compared to yellowish-white light, according to Bullough. (It’s important to note that most studies have used broad-spectrum white lights with blue and yellow undertones, rather than pure blue or yellow lights). However, this improved peripheral vision comes at a cost. When the moving object enters the central field of vision, it becomes harder to see clearly. This is because human eyes have very few blue-sensitive cone photoreceptors, which are dominant in central vision and less effective in dim light.
Bullough expresses concern that purple streetlights are detrimental to both driver and pedestrian safety. Firstly, the loss of the phosphor layer might reduce the overall brightness of the streetlight, potentially falling below the required illumination levels for different road types. Secondly, the blue and violet-heavy light can impair the ability to discern details due to the scarcity of blue-sensitive cones in central vision. Lastly, Bullough points out that blue-violet light makes it very difficult to distinguish between colors, as everything tends to appear in shades of blue or black.
To enhance safety under purple streetlights, or any nighttime lighting, Bullough advises pedestrians and drivers to avoid wearing sunglasses or blue-light-filtering glasses. While the perceived brightness of bluish light might tempt one to wear shades to reduce glare, Bullough emphasizes, “But at nighttime, we really want to have all the light that we can get.”
Reassurance About LED Technology
Despite the purple streetlight issue, Seshadri emphasizes that this incident shouldn’t cast a shadow over all LED lighting. “The materials that are normally used to make the phosphor are extremely stable,” he assures. “There must be some issue with the manufacturing or some particular LEDs, because most LEDs that the world is using don’t face this problem.” The purple streetlights appear to be an isolated issue, likely stemming from manufacturing defects in a specific batch of lights, rather than a fundamental flaw in LED technology itself.
In conclusion, the emergence of purple streetlights is likely due to a manufacturing defect causing the phosphor coating in certain LED lights to delaminate, exposing the blue LEDs beneath. While seemingly unusual, this phenomenon has potential safety implications due to the way blue-violet light affects human vision at night. However, it’s important to remember that this issue appears to be isolated and doesn’t negate the overall benefits of LED street lighting technology.
