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Light pollution a key concern

LED streetlight conversion makes Tucson skies slightly darker, says Dark Sky Association

  • 5 min to read

Two LED streetlights illuminate a griffin sculpture at Ochoa Street and Scott Avenue in downtown Tucson. The city swapped about 23,000 streetlights and connected most to a wireless network that allows for dimming and data collection.

Tucson’s skies are slightly darker thanks to the completed streetlight conversion project, according to a study by the Tucson-based International Dark-Sky Association.

The study, however, was limited in scope, and the association said more studies should be conducted to bolster its claims, but it does highlight what the city has done to help preserve the night sky. Southern Arizona is an astronomy hub, with several world-class observatories in the region.

Cities across the country have been making the switch from older street lighting, such as high- and low-pressure sodium, to more efficient LEDs, or light-emitting diodes, in an effort to save energy and money. In many cases, however, the conversions are taking place in ways that concern some dark-sky advocates.

White LEDs are a combination of red, green and blue LEDs in one module. LEDs with too much blue light have the potential to be harmful to the health of humans and wildlife, according to a report by the American Medical Association. It can also be aesthetically jarring and exacerbate light pollution at night, compromising the dark desert skies that astronomers rely on.

In an effort to protect dark skies, Tucson officials worked closely with lighting experts and astronomers to implement responsible installation of the technology.

The efforts paid off. Dark-sky officials have gone as far as to suggest Tucson be a model for cities that are considering making the conversion by using white lights containing fewer blue wavelengths and dimming the lights upon implementation.

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Gone are the days, like in 1978, when most streetlights in Tucson were using incandescent lamps.

Shifting streetlights to LEDs

The city swapped about 23,000 streetlights and connected most to a wireless network that allows for dimming and data collection on energy usage, said Jessie Sanders, projects manager in the Tucson Department of Transportation.

When all lights were installed, they were dimmed to 90 percent brightness, Sanders said. As gunk builds up on the fixtures, the city will increase the brightness proportionately.

The 16,250 LEDs installed in areas with low nighttime foot traffic are dimmed to 60 percent brightness at midnight. The more than 360 installed in high pedestrian areas around the university, downtown and the streetcar route are dimmed to 60 percent at 3 a.m.

The rest that were installed at high-traffic intersections, historic buildings and bridges are not dimmed other than the initial 90 percent, Sanders said.

The lighting chosen is whiter than the golden high- and low-pressure sodium that used to line the streets, but it does not contain as much blue light as the most efficient LEDs that have been installed in other municipalities.

Moreover, the lights remained fully shielded, meaning light is directed downward, per Tucson’s lighting ordinance.

In fact, because the city dims the lights upon installation, the amount of blue light projected into the environment is less than the high- and low-pressure sodium lights they replace, said John Barentine, lead author on the study and director of policy for the International Dark-Sky Association.

“Many cities have put in the most efficient LED lighting systems just to save as much energy as possible and resulting in aesthetically very poor, garish lighting,” lighting expert Christian Monrad, of Monrad Engineering, told the Star before the conversion.

The city estimated the conversion would cost about $16.5 million and would be paid back in savings over about 10 years, Sanders said.

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An LED streetlight hangs over Congress Street near Fifth Avenue, underneath the towering Hotel Congress sign.

Consequences

White-light LEDs are so efficient because they emit more light than heat, unlike the high- and low-pressure sodium lights.

White light, however, is generated when all colors of light are used together. But the addition of too much blue light in streetlights is what gives the LEDs their harsh daylight glare.

Blue light scatters in the atmosphere more easily than other colors for the same reason the sky is blue. LED street lighting does spread farther and could be more disruptive to observatories that flank the city.

Additionally, the dark-sky association worries that as LEDs become cheaper and more efficient, they could be installed at a higher rate and with less care because of the money cities can save through energy costs, compounding the light pollution problem.

Tucson was aware of poorly implemented LED lights in other cities across the country and of the consequences they could have on the nighttime environment and the sky for astronomy.

To mitigate this, the city consulted with Buell Jannuzi, the director of the UA’s Steward Observatory, and Monrad, who designed the lighting systems for Arizona Stadium and Tucson International Airport and was on the board of directors of the dark-sky association.

The conversion was “right in line with our recommendations,” Barentine said. “The city tried to do the right thing and chose a variety of LED light that has, relatively speaking, not a lot of blue.”

The dark-skies study

The city projected light emission over Tucson would be reduced by 20 percent with the conversion. The dark-sky association models of Tucson estimated an 18 percent decrease in sky glow.

In reality, the sky glow reduction was less than expected with the LED conversion. The association measured a modest 7 percent decrease in sky glow.

Barentine chalks this discrepancy up to the fact that not all lights in the metro area are streetlights. He also pointed to light pollution from surrounding communities and unincorporated areas.

“There’s no way of estimating all the other kinds of lights in the city and how they changed,” Barentine said. “Our prediction was based on best possible scenario. The overall decrease was related to changed streetlights.”

Barentine compared three sites in and outside the city limits of Tucson in 2014 before the conversion and in mid-2017 when about 95 percent of the city’s lights were swapped.

He measured sky brightness in three ways. His core method was measuring sky brightness at the zenith, the point in the center of the sky directly overhead. He also utilized all-sky images and satellite data.

The study did have some shortcomings, however, he admits.

The data Barentine collected in 2014 was for a completely different purpose. It wasn’t until the conversion that Barentine saw the potential for the data he had collected with students years earlier.

Additionally, while only three sites were used in the study, Barentine is satisfied with the distribution of the sites.

However, he added that follow-up studies are key.

“The takeaway message,” he said, “is we did detect a real change and that change traces back to modifications to streetlights as part of the conversion.

“We can provide all the lights that are needed to light streets safely and to save energy and reduce light pollution. And we can do so with white LEDs, and the key is dimming,” he said.

View from the skies

“My only cautionary comment on this (study) would be that the atmosphere is quite variable,” said Steve Larson, co-principal investigator on the Catalina Sky Survey, which relies on dark skies to track asteroids that could potentially collide with Earth.

When asked if the lighting change has affected operations on the mountain at all, Larson said he doesn’t know. “I’m open to the possibility that there has been some change, but it’s hard to quantify. The study is a good attempt to do a before and after.”

Catalina Sky Survey observatories are atop Mount Lemmon, only 12 miles from town.

“It’s going to take a lot more continuous measurements and time to get a better sample to really nail that (number) down,” he said.

For astronomers, the biggest priority before everything else is making sure lights are shielded and that lighting ordinances are enforced, Larson said.

Then he said, it’s important to control the amount of light coming out and plan accordingly for population growth, especially in areas closer to telescopes.

The quality of light is not as pressing a priority, Larson said. “The bottom line,” Larson said, is, “you want to get the light where you need it and no more than you need.”

Tucson's Neon Culture – updated with Part 9

What better way to light up the night than a safe, controlled ignition of a gas! Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a handful of longtime restaurants, hotels and bars served a smaller city.

Neon is the second lightest gas, after helium. It's more expensive than gases because it's extracted from liquid air. It was introduced in the U.S. in 1923 with two signs at Packard dealership in Los Angeles. Its reddish-orange glow warms the darkness surrounding the signs. 

Thanks to the efforts of many businesses, residents, sign shops, the City of Tucson and the Tucson Historic Preservation Foundation, neon is once again igniting the night with a cursive glow.

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Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a ha…

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Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a ha…

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Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a ha…

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Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a ha…

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Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a ha…

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Arizona Daily Star photographer Kelly Presnell revisited neon signs that dot the landscape in Tucson. Many are a throwback to a time when a ha…

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The signs that yielded the letters for "Tucson" on Thursday's Caliente cover image in the Arizona Daily Star:

Contact Mikayla Mace at mmace@tucson.com or (520) 573-4158.