Granite Geek: LEDs Efficient, But Cause More Light Pollution

Jun 22, 2016

When a city replaces its old-fashioned streetlights with efficient LED lights, it can save a bundle of money on its electricity bills. But it can also dramatically increase light pollution, which is really unpleasant for astronomers and those among of us who like to do a little star-gazing on cloudless nights. The kind of light these LEDs emit can cloud our view of constellations. Granite Geek David Brooks throws some light—so to speak—on the unintended consequences of this efficiency measure in his column this week in The Concord Monitor, and he joined NHPR’s Peter Biello to discuss his findings.

What is it about these LED lights that make them a bigger source of light pollution than other light sources?

The problem with them is the thing that makes them good. LEDs are a digital light source, as compared to heating up a wire or gas, so the wavelength that comes out of an LED is much more focused than in other types of lights. But that wavelength matches very closely with the wavelength that we are attuned to see at night. The rods in the eye, which handle low light situations, are most effective at a certain wavelength (something like 490 nanometers), and blue LEDs can be very close to that number. What light they do put out our rods see really well, which makes it harder to see other things, like stars, the moon, and the Milky Way.

Especially because stars and the moon give off the same kind of light as these LEDs.

That’s the theory, at least. Our rods have evolved to see wavelengths around moonlight because that was the source of light when it was dim. It makes sense that those of us who could see it well would survive, and those of us who didn’t would get eaten by a saber tooth tiger. So if the LEDs are at that wavelength as well, it really overwhelms our rods.

Cities have a compelling interest in using these, though, because they can be such an enormous cost savings. But it’s not great for astronomers, or for people who like to look at the sky; is there a middle ground before these two parties?

There are LEDs that hopefully are just at good at lighting and saving energy, but which don’t blind us as much at night. The LEDs that are the problem are those that are around 4000 kelvin, which is a temperature used as a measurement for the color of LEDs. If the lights are at 3000 kelvin it’s less of a problem, and they can still be good light. Some cities, like Concord, are putting in those types of lights.

But it’s still light pollution. To a certain extent light it light, and we all know you can’t see the stars or the Milky Way in downtown Manchester, no matter how hard you try.

Is it likely that cities that do end up replacing old-fashioned streetlights with LED lights would be able to, or willing to, turn the levels down on these lights so that stargazers could appreciate the night sky?

That’s a discussion that will take place at the city council level. My guess would be that it will be very hard. In my career I’ve covered a lot of streetlight issues, and invariably the public wants more light. Except for a few backyard astronomers and other people who are worried about the effect on nocturnal animals, people want more light. They think it’s safer and better. It can be very hard for cities to ramp down on streetlights once they exist.

Which means astronomers may have to retreat to rural areas as LED lights become more popular.

Yes. As I noted in my column, there’s an effort by the International Dark Sky Association to designate certain places as “dark sky locations.” Personally I’ve always thought that north of the Notches should pursue this as a tourist opportunity. You’re trying to draw people up with zip-lines and skiing, but why not to see the Milky Way for the first time in your life?

“Come to New Hampshire and experience the darkness of our night sky!”

Exactly. You’d have to get the ski areas to turn off their lights for that, but anyway. It’s an idea.