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"exec" writes:

Arthur T Knackerbracket has found the following story:

"I feel like we're protecting the last tree, in a way." That's what Flagstaff, Arizona, city council member Austin Aslan said at a recent meeting. The subject of that earnest statement might surprise you: it was streetlights. To be more specific, he was talking about a careful effort to prevent streetlights from washing out the stars in the night sky.

Flagstaff became the first city to earn a designation from the International Dark Sky Association in 2001. That came as a result of its long history of hosting astronomy research at local Lowell Observatory, as well as facilities operated by the US Navy. The city has an official ordinance governing the use of outdoor lighting—public and private.

A few years ago, though, a problem arose. The type of dark-sky-friendly streetlight that the city had been using was going extinct, largely as a casualty of low demand. In fact, as of this summer, there are none left to buy. Meanwhile, the age of the LED streetlight has arrived with a catch: limited night-sky-friendly LED options.

If the city went out and just swapped lumens for the cheapest LED products out there, the astronomers would have marched on city hall with pitchforks and (night-vision-preserving) torches. And that might have been the least of their concerns, as the Navy informed the city last year that "brightening of skies 10 percent over current conditions is not compatible with the [Naval Observatory's] mission."

The problem with LEDs boils down to blue light. Older streetlights are high-pressure sodium bulbs, which produce a warm yellow glow around a color temperature of 2,000 K. The bulbs Flagstaff relied on for most of its streetlights were low-pressure sodium—a variant that only emits light at a single wavelength (589 nanometers) near that yellow color, producing something resembling candlelight. Many of the LED streetlights on the market have much cooler color temperatures of 3,000 or even 4,000 K.

[...] There are ways to build LED lights that change their natural color and mitigate this blue light problem. One way to do it is to simply throw a filter on the LED that blocks blue wavelengths from passing through. Of course, this significantly reduces the amount of light you produce for each watt of electricity. There are some aesthetic trade-offs, as well—of which not everyone is a fan.

[...] Another way to do it is with phosphor coatings on the LED that absorb light of one wavelength and emit it at another wavelength. Lights known as phosphor-converted amber (PCA) shift all the light out of the blue and into the yellow part of the spectrum at the cost of some efficiency. The result is actually quite close to the ubiquitous high-pressure sodium streetlights we're used to.

Narrow-band amber (NBA) LEDs provide a different option. These lights actually use a type of LED that only emits warmer colors from the start. In this way, they actually compare pretty well to the low-pressure sodium streetlights that recently went extinct. The range of wavelengths emitted is a little broader, but the practical effect is about the same.

[...] Flagstaff's plan is generally to swap in NBA LEDs for all the low-pressure sodium lights, and PCA LEDs for the high-pressure sodium lights that are used along the busier streets (as they're a little brighter). The better directionality of LEDs—combined with resident requests for slightly dimmer lighting on residential streets—actually means that the total output of the city's streetlights is going to drop from about 29 million lumens to about 19 million lumens. That's not unusual.

[...] Of course, this isn't just an engineering optimization problem. There's also public buy-in to contend with. In this case, the city of Flagstaff put up test sections of different fixtures around town so anyone interested could compare and provide feedback. And since public safety is the primary reason streetlights exist in the first place, perceptions (which can vary wildly) about how much or what kind of light qualifies as "safe" can force some compromises.

[...] It comes down to the fact that lighting choices don't just affect the things you're intentionally lighting—there are also the things you can avoid lighting. That means there are always ways to ensure that the pale stars of the night sky don't entirely disappear from your universe.

[...] Flagstaff's hope is basically to do that for cities by producing the first dark-sky ordinance updated to deal with LEDs. That could give other cities an example to follow, even if it's not quite as easy as hitting up a dark-sky aisle at their local store.

-- submitted from IRC

Original Submission