In general, street lights emit bright white, orange, or yellow light, with the purpose of providing visibility rather than focusing on aesthetics. However, in recent years, there have been reports of some street lights emitting purple light, which is not suitable for use on public roads and may even pose potential safety concerns. What exactly is going on?
According to a report by Scientific American, the presence of purple light on the streets is not intentional. Purple lights on the streets may seem harmless, but they can potentially affect the safety of drivers and pedestrians.
Over a decade ago, American towns began transitioning from sodium lights to LEDs (Light Emitting Diodes) to save costs, resulting in the color of many nighttime roads changing from yellow-orange to bright white. However, in recent years, a strange new nighttime color has been discovered across the United States and globally. Since early 2021, reports of purple street lights have been appearing in states like Florida, Utah, Texas, Massachusetts, as well as in countries like Canada and Ireland.
LEDs can emit light of various wavelengths, but white light is not naturally produced. So, when you see LEDs emitting white light (such as street lights containing LEDs), they are actually emitting a mixture of red, green, and blue (or red, yellow, and blue) colors that together appear as white. There are two common methods to create this visual effect. One is combining miniature LEDs that emit red, green, or blue light into a larger device. The other method involves using only blue LEDs covered with a fluorescent substance called phosphor. When the blue light emitted by the LED passes through this layer, the phosphor absorbs some blue wavelength and emits red and yellow wavelengths. This creates a blend of multiple colors, appearing as white to the eye. The second method is preferred because it is more energy-efficient.
So why have some street lights turned purple? Jakoah Brgoch, an associate professor of chemistry at the University of Houston, explained that scientists hypothesize that the bright purple light indicates that the layer of phosphor surrounding the light has “delaminated” or peeled off, exposing the blue LED underneath. While blue LEDs are essentially deep blue, they naturally have some hues of purple and blue-violet.
According to Mental Floss, reasons for the delamination of phosphor may include normal wear and tear, heat generated by the LEDs, or even vibrations from passing vehicles. It could also be due to manufacturing defects leading to an unstable coating. LED supplier Acuity noticed this change and explained that the so-called “purple light” effect results from the spectral shift caused by phosphor displacement after several years.
“While only a small portion of our AEL brand luminaires are affected by this effect, we are not the only lighting manufacturer to have experienced this issue (…). We no longer use LEDs that cause spectral shifts.”
Although driving through streets illuminated by purple light may seem intriguing, it can also pose potential dangers. Our eyes’ light receptors are less sensitive to blue light than they are to white light, and this ability is further weakened at night. Objects or people may not be as easily seen under purple light compared to regular street lights, potentially creating safety concerns.
Since local governments often select the same contractor to provide street lights for a particular area, if one light exhibits a purple hue, there may be many lights with similar issues within the same area.