LEDs are great for streetlights ... unless you want to see the stars

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    A map of North America’s artificial sky brightness, in twofold increasing steps, as a ratio to the natural sky brightness, from "The new world atlas of artificial night sky brightness" Courtesy—AAAS

Monitor Staff
Published: 6/21/2016 2:46:08 AM


We’re all familiar with the phrase “It’s an ill wind that blows no good,” the optimistic belief that you can find silver linings in all but the worst of events. 

Unfortunately, the less-cheerful antithesis – “It’s a rare wind that blows no ill” – is also true: You can find unintended downsides in all but the very best of events. 

Today’s case in point involves LED streetlights. 

Swapping out conventional street lighting for LEDs is an obvious move. Light-emitting diodes use much less electricity and last much longer than the high-pressure sodium and metal halide bulbs they replace. How much less? In Los Angeles, the change cut power usage so much that the city is installing electric car chargers on some light poles to take advantage of no-longer-necessary heavy wiring leading to street lights.

In New Hampshire, Nashua and Manchester are making the change to LEDs and the state Department of Transportation’s lighting guidelines are cautiously supportive.

So what’s the downside? Light pollution.  

“Blue LEDs, which are the cheapest and more prevalent, are the worst you could use – they’re worse than practically any other lighting solution available,” said David Gilmore, a software engineer in Greenfield and a telescope-toting member of the New Hampshire Astronomical Society.

Like all stargazing fans, Gilmore bemoans the way an increase in outdoor lighting has dimmed the glories of the night sky, even if the situation is still pretty good at his rural home. “I moved to Greenfield in 2006 and took up astronomy because I had nice dark skies right in my front yard,” he said.

Thanks to his travels as a Astronomical Society ambassador giving outdoor skywatch events, he knows that most of us don’t have this luxury. If you’re anywhere near a city, even a small city, the glow from reflected or poorly aimed lights (I’m talking about you, car dealers) will wash out all but the brightest stars in half the sky and will render the Milky Way invisible. 

It’s so bad that even an experienced star-gazer can be flummoxed. “Any city is so dramatically light-polluted I have a hard time finding things,” he admitted.

I talked to Gilmore because I wanted some local context on the World Atlas of Artificial Night-Sky Brightness just released American Association for the Advancement of Science.

I’ve been writing about light pollution for decades and in many ways the situation has improved. Most communities, for example, now have light pollution standards in their planning regulations and building codes, which do things like requiring new outdoor lighting to point downward. Plus, tourism officials are beginning to realize the monetary value of dark skies  – notably Mont Megantic in Quebec, an hour north of the New Hampshire border, which became the world’s first Dark Sky Reserve in 2007. 

Gilmore agrees that light pollution is no longer as obscure an issue as it once was. 

“I hear more about light pollution every year, which I take as being encouraging because at least people are aware of it,” said Gilmore.

Alas, the AAAS report says attention hasn’t translated into improvement.  “The Milky Way is hidden from more than one-third of humanity, including 60 percent of Europeans and nearly 80 percent of North Americans. … Almost half of the United States experience light-polluted nights,” says the report, available at http://advances.sciencemag.org/content/2/6/e1600377. 

New Hampshire is well within the east coast light blob on the report’s satellite maps.

The report is particularly concerned about LEDs, especially blue LEDs (which look white to me) that shine at around 4,000 Kelvin, using the temperature measurement by which LED light is calibrated. The problem isn’t brightness so much as wavelength.

By design, LEDs are tweaked to channel all their energy into form that we can make use of, part of the reason they’re so energy efficient. Our rods, the photoreceptors on the back of our eye that are sensitive to dim light, are maximized for wavelengths right around those released by 4000K LEDs. One theory, incidentally, holds that our rods evolved this way because this are the wavelengths of moonlight.

As a result, a given amount of lumens (that is, light intensity) from white LEDs will affect our ability to see stars more than the same lumens from other lighting.

“Unless blue-light emission is restricted, a transition toward this technology can be expected to more than double the night sky brightness as perceived by our dark-adapted eyes,” notes the AAAS report. 

The report includes an alarming satellite image of Europe, estimating the effect on our night vision if all lights were switched to blue LEDs. It looks like most of the continent  has been dipped in luminescent white paint.

Not even the most ardent astronomer thinks we should abandon LED lighting, of course. Cutting electricity use can cut pollution from electricity plants, which can help astronomy by reducing haze that scatters light around the sky. 

Gilmore thinks the solution is to dim the LEDS to compensate for their increased effectiveness in boosting our night vision.

“It doesn’t mean they can’t be used. They should be used at a fraction of the power level they are,” he argued.

Good luck with that. I’ve covered more than a few streetlight debates in my newspaper career, and they always involved the general public asking for more lights and being unconvinced by technical details. If something seems dimmer and less safe to me, no amount of lumen analysis will convince me otherwise.

So I guess I won’t expect the Milky Way to reappear overhead any time soon.

But there is one thing I can do: I’ve moved Visit Mont Megantic to the top of my bucket list.

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek)

David Brooks bio photo

David Brooks is a reporter and the writer of the sci/tech column Granite Geek and blog granitegeek.org, as well as moderator of Science Cafe Concord events. After obtaining a bachelor’s degree in mathematics he became a newspaperman, working in Virginia and Tennessee before spending 28 years at the Nashua Telegraph . He joined the Monitor in 2015.

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