Trivia 083: The blue hour

If you have been driving late at night, probably in an area which is not very populated, you may have seen some streetlights casting a strange blue/purple light. I’ve seen this myself in a few different areas. At the time, I thought that there must be some reason behind it – some sort of function these streetlights must be serving. Could it be due to some interaction with wildlife in the area? Maybe the variety helps wake drivers up on long night drives? Or perhaps there’s something in the area which is more noticeable when bathed in blue light? Well, as I always tell people, you shouldn’t ever shrug and forget a question like that. Look it up! You might learn something.

In this case, I learned that the answer is unfortunately a bit more mundane than any of my guesses. However, it is still interesting!

The root of the blue street lights lies in how their LEDs were constructed. Now, how an LED works is a bit of complex physics, which I don’t really have the space to explain here. But the important thing to note is that unlike an incandescent light bulb, which produces a broad spectrum, LEDs emit very specific wavelengths of light, usually red, blue, or green. In order to get white light, then, you must combine multiple LEDs so that their colors combine to give white. However, this is not very cost-effective.

For years, manufacturers searched for the best way to create a white light without having to use multiple LEDs. My roommate in college had one of the first commercially available LED lightbulbs. It used yellow plastic to absorb blue light given off by a blue LED to produce a warm white light. The problem with that, though, was that it needed massive heat sinks because a large portion of the light it was absorbing was being converted to heat.

But eventually, in the last two decades, the industry-wide solution has been to use a phosphor coating on the LED casing. This coating absorbs the higher-energy blue light given off by the LED and re-emits it at lower wavelengths. Unlike the yellow plastic solution, this does not result in the buildup of heat. This is also a very cheap and simple solution. In fact, if you look closely at an LED flashlight, you can actually see the phosphor backing underneath the diode. (I couldn’t find a really good image, but here’s the best I’ve got. The phosphor is the yellow stuff).

The reason some street lights turn purple is because if the phosphor coating is improperly laminated on the surface, it can come off (or delaminate). This allows the blue light to shine through. According to a 2022 article on Business Insider, most of these street lights come from a company called Acuity, and they claim to have solved the problem and are rolling out replacements. But you may still see a blue/purple street light here or there, particularly in areas people don’t go to as often.

LEDs have many advantages over fluorescent or sodium lights. First of all, they are incredibly energy efficient compared to alternatives. This not only saves municipalities on their electric bill, but it also means less carbon dioxide will be put into the atmosphere by power plants. In 2022, when Washington, D.C. replaced their street lights with LEDs, they estimated that it would prevent the emission of 38,000 tons of carbon dioxide each year. Additionally, they last have very long lifespans – usually 15-20 years. And they can be designed to be directional, and avoid spreading light upwards or creating glare.

As manufacturing technology improves, the performance gap between LEDs and their less-efficient alternatives will likely only widen. However, for now, we may still occasionally get a blue street light. So keep an eye out for them, and recognize them for what they are – a sign that, although there may be one or two hiccups along the way, we are moving forward to a better and brighter (pun) future.

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