![](https://i.imgur.com/CLvY2jA.png")
04-10-2023, 11:29 AM
By Karen Kwon on September 27, 2023
American cities and towns started switching their streetlights from sodium lamps to LEDs about 15 years ago, which changed the color of many nighttime roads from yellowish orange to bright white. But lately an odd new nocturnal color has been spotted across the nation—and the globe. Anecdotal reports of purple-looking streetlights have been popping up since early 2021 in states including Florida, Utah, Texas and Massachusetts, as well as in Canada and Ireland.
......
the hue of the light illuminating a roadway could affect how drivers and pedestrians perceive their surroundings as they make their way through the night. And that makes purple streetlights a potential safety hazard.
......
when you see white LEDs, such as those in streetlights, they are actually emanating some mixture of the colors red, green and blue (or red, yellow and blue), which together produce the appearance of white. There are two popular ways to create this illusion. One is to combine tiny LED lights that each emit red, green or blue into a single big device. The other is to use only blue LEDs but coat them with a type of fluorescent substance called phosphor. When the blue light from the diodes goes through this layer, the phosphor absorbs some of the blue wavelengths and spits out red and yellow ones. This results in a mixture of colors that, once again, appears white.
......
The second method gained popularity because it’s more energy-efficient than the first. LEDs that generate red and green light require more energy than those that emit blue
......
It is hard to determine the exact cause without dissecting one of the defective lights, but scientists have a hypothesis: bright purple light suggests the phosphor layer around the lights has been “delaminated”—peeled off—exposing the blue LED light underneath
......
Cone-shaped photoreceptors in the human retina become active when exposed to daylight, enabling people to discern color. But in low-light conditions, rod-shaped ones become responsible for vision. Despite their inability to distinguish color, rods are more sensitive to low-wavelength light—the blue part of the electromagnetic spectrum—than cones are.
Another difference between these two types of photoreceptors is that rods are mostly located 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 bluish-white light than under yellowish-white light
......
Gaining improved peripheral vision under blue-tinged light comes with a trade-off, however: once the moving object comes into focus, it becomes harder to see. That’s because human eyes have very few blue-sensitive cone photoreceptors that are dominant in their central vision. Plus, cone receptors are less reliable in dim environments.
......
blue- and violet-saturated light can worsen people’s ability to see details because of the lack of blue-sensitive cones in the center of the retina. Finally, the blue-violet light makes it very difficult to distinguish between different colors ...... everything becomes a shade of blue or black.
https://www.scientificamerican.com/artic...heres-why/
American cities and towns started switching their streetlights from sodium lamps to LEDs about 15 years ago, which changed the color of many nighttime roads from yellowish orange to bright white. But lately an odd new nocturnal color has been spotted across the nation—and the globe. Anecdotal reports of purple-looking streetlights have been popping up since early 2021 in states including Florida, Utah, Texas and Massachusetts, as well as in Canada and Ireland.
......
the hue of the light illuminating a roadway could affect how drivers and pedestrians perceive their surroundings as they make their way through the night. And that makes purple streetlights a potential safety hazard.
......
when you see white LEDs, such as those in streetlights, they are actually emanating some mixture of the colors red, green and blue (or red, yellow and blue), which together produce the appearance of white. There are two popular ways to create this illusion. One is to combine tiny LED lights that each emit red, green or blue into a single big device. The other is to use only blue LEDs but coat them with a type of fluorescent substance called phosphor. When the blue light from the diodes goes through this layer, the phosphor absorbs some of the blue wavelengths and spits out red and yellow ones. This results in a mixture of colors that, once again, appears white.
......
The second method gained popularity because it’s more energy-efficient than the first. LEDs that generate red and green light require more energy than those that emit blue
......
It is hard to determine the exact cause without dissecting one of the defective lights, but scientists have a hypothesis: bright purple light suggests the phosphor layer around the lights has been “delaminated”—peeled off—exposing the blue LED light underneath
......
Cone-shaped photoreceptors in the human retina become active when exposed to daylight, enabling people to discern color. But in low-light conditions, rod-shaped ones become responsible for vision. Despite their inability to distinguish color, rods are more sensitive to low-wavelength light—the blue part of the electromagnetic spectrum—than cones are.
Another difference between these two types of photoreceptors is that rods are mostly located 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 bluish-white light than under yellowish-white light
......
Gaining improved peripheral vision under blue-tinged light comes with a trade-off, however: once the moving object comes into focus, it becomes harder to see. That’s because human eyes have very few blue-sensitive cone photoreceptors that are dominant in their central vision. Plus, cone receptors are less reliable in dim environments.
......
blue- and violet-saturated light can worsen people’s ability to see details because of the lack of blue-sensitive cones in the center of the retina. Finally, the blue-violet light makes it very difficult to distinguish between different colors ...... everything becomes a shade of blue or black.
https://www.scientificamerican.com/artic...heres-why/
