“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.

Lighting issues

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.

As Lowell Observatory Director Jeff Hall told Ars, “As day turns to night and your photopic cone-based vision turns into scotopic, rod-based vision, your sensitivity shifts a little bit blue. And so very blue-rich light at night comes off as really harsh and glaring and creates a lot of visible skyglow. So the less of that spectrum you touch, the better off you are for both visual observations, the night sky, as well as astronomy.”

Hall continued: “I see this wherever I go in my travels. By default, cities just put up, you know, 3,000 degrees CCT white, sometimes 4,000, which is this blue light. Just lumen for lumen [that] will create two-and-a-half to three times the skyglow of a high-pressure sodium system and, like, six times the skyglow of a low-pressure sodium system.”

And the greater the skyglow around you, the harder it is to see the stars.

Dark skies vs. efficiency

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.

“What's left is green,” Hall said. “And so you stand under this and it's like the zombie apocalypse, because everybody's green. They've gone to these in Hilo, Hawaii, and we were standing in a parking lot trying to talk to each other, and it's just like straight out of Night of the Living Dead.”

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.

The downsides of the NBAs are basically cost and efficiency. But both have improved considerably over the last few years. Flagstaff Traffic Engineer Jeff Baumann—who is in charge of the plan for replacing the city’s 3,500 streetlights—told Ars that the available NBA options have recently climbed over 30 lumens per watt (on the ground), with efficiencies over 40 right around the corner. For comparison, the city’s low-pressure sodium streetlights weigh in at about 50 lumens per watt.

Lumen budgets

Separately from all this wavelength wrangling, though, LEDs do have a strong natural advantage—they’re highly directional. That is, LED streetlights do a much better job of only lighting the street (rather than the adjacent homes). That means that fewer lumens coming out of the fixture can give the same result you had before.

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.

“If you absolutely must use white LEDs, you could do what Tucson has done,” Hall said. “They... switched out their whole high-pressure sodium system to 3,000 degree white but reduced their lumen budget for street lighting from 480 million to, like, 170 million [lumens] or something. And you need to do that. For every white LED lumen, you're increasing your skyglow by a factor of about three, but they cut the lumen budget by about a factor of three. So overall, they managed to wash out the skyglow because they’ve got a lot of observatories down there.”

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.

The astronomers in the community may point to studies suggesting that increasing lighting beyond a basic threshold level won’t reduce crime or accidents, but this isn’t always persuasive to those recalling tragic pedestrian collisions.

A single-track emphasis on energy efficiency might also push you toward the most efficient—and therefore bluest—LEDs you can find. But there are more knobs to turn than just color; without careful analysis, there can be a temptation to put in brighter lights than necessary so long as the overall wattage is lower than what you used to have. It’s possible to choose to prioritize lighting color and still bring down energy usage by being careful about brightness levels.

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.

The trick is that it’s yet another thing you have to study up on in order to get it right. When Ars asked Jeff Hall whether he winces while walking through the outdoor lighting aisle at the local big box home improvement store, he had a solution in mind.

“We certainly have had conversations internally about a dark-sky-compliant aisle,” he said. “There are so many choices to choose from. You know, people don't want to put a lot of duty cycles into it, even if they want to be helpful and be dark-sky compliant. They don't want to have to sit down and do four hours of research to figure out what light they should get. What they want is a box that's got an ‘OK’ stamp on it. And there are fixtures that have an IDA (International Dark Sky Association) seal of approval.”

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.