Cities around the United States have recently been exposed to a curious issue with LED streetlights: They have turned purple. Karen Kwon, a journalist from scientificamerican.com, says that “these purple lights may seem innocuous, but they could be dangerous to road users”.
According to scientists in the US, LED streetlights that suddenly turn purple (from their standard white colour) are suffering from one of many possible failures. “These tones of purple suggest that the phosphor layer around the lights have delaminated, and the blue LED light underneath has now been exposed,” explains lighting expert John D Bullough from the Light & Health Research Center at the Icahn School of Medicine.
The problem comes in with human eyes, adds Bullough. “Rod-shaped photoreceptors in the eye are more sensitive to low-wave-length light – the blue part of the electro-magnetic spectrum. These rods are located physically in the peripheral part of the retina. As a result, people driving at night may notice a moving object in their peripheral vision more quickly under blue-purple light.” But there is a trade-off: Once the eye focuses on the moving object, it becomes harder to see “because human eyes have very few blue-sensitive photoreceptors,” says the expert.
Streetlights are designed to emit a certain level of light, so a change this drastic will have an effect on the quality of light people are driving in. “This particular shade of light also worsen people’s ability to see details because of the lack of blue-sensitive cone photoreceptors in the centre of the retina. Blue-violet light makes it very difficult to distinguish between different colours. So if you have a purple light on your street, try to get it changed immediately to protect people’s lives,” he warns.
