FLAGSTAFF, Ariz. -- Since the passage in the 1980s of Flagstaff's comprehensive lighting ordinance, the golden hues of low-pressure sodium lights have come to prevail in Flagstaff skies.
They are a response to one of the three key components of the lighting law: minimizing the light spectrum.
Some find the glow romantic, others think it dingy.
The low-pressure sodium lights are about as energy efficient as the modern technology of LEDs, but they have just a tiny fraction of the impact on night skies. Whereas a white LED emits light across almost the entire light spectrum, from red to blue, low-pressure sodium lights throw out yellow light in just a tiny bandwidth.
But as the city looks to celebrate its dark skies, new challenges continue to arise.
Until recently, the city had a policy of replacing its old high-pressure sodium lights with the more efficient and dark-sky friendly low-pressure sodium. About 60 percent of the city's street lights are now low-pressure sodium, with the remainder being high-pressure sodium.
But in 2012, the city converted streetlights to LEDs on a one-mile stretch of Highway 89 just past the Flagstaff Mall.
That caught astronomers' attention.
Flagstaff had agreed to take possession of the stretch of highway from the Arizona Department of Transportation under the premise of providing dark-sky friendly road lights.
"That lighting was designed right from the get-go using low-pressure sodium," said Chris Luginbuhl, an astronomer at the U.S. Naval Observatory and leading dark skies expert. "The city changed about half in October 2012 to LED. That got our attention and we started a conversation with the city."
The light poles often fail because they were designed for the smaller, high-pressure sodium lights and no engineering went into adding much larger fixtures.
Steven Hill, of Flagstaff's Public Works section, told the Flagstaff City Council on Tuesday night that 12 of the light poles had failed on that stretch of Highway 89 the previous winter. Eight more of the poles have failed in the last year, plummeting 40 feet to the ground under wind stress.
"To date, none of these assemblies have fallen onto pedestrians or vehicles," Hill said. "However, the failure of streetlight mast arms creates the potential for injury."
The mast arms are about the length of a car.
Out of the city's 3,500 streetlights, he estimated that about 3,000 poles might be at risk of failure. Because of that, the city expects a steep rise in maintenance costs in coming years.
Hill said the city had the following options:
— Modify or replace 3,000 lightpoles at a cost of $1.5 million to $15 million;
— Move to more expensive, less-efficient amber LEDs at a cost of $28 million and higher annual power bills;
— Move to filtered LEDs at a cost of $2.8 million. Astronomers say filtered LEDS are no alternative because their reduction of the light spectrum is only minimal.
Amber LEDs are a new technology, but one that could be a solution as the technology advances. It emits light in a narrow spectrum very similar to low-pressure sodium bulbs, but Hill said it was still costly and less efficient.
"Are we in danger of these masts randomly falling off poles and perhaps striking someone?" Flagstaff Mayor Jerry Nabours asked at the end of Hill's presentation Tuesday.
Hill started to answer by telling the mayor it was impossible to predict whether the poles would fail, but he was cut off by the city attorney urging the city council to enter executive session to discuss the matter in private.
Another reason for the turn toward LEDs is that skepticism about longevity has waned in recent years. Many cities are starting to change over in order to save money.
But Luginbuhl said cost savings aren't necessarily as significant as advertised. New technology always experiences growing pains.
Low-pressure sodium bulbs need to be changed about once every four years and some think LEDs could now last upward of a decade. However, other components could also fail over such a long span, negating the potential savings.
Lowell Observatory Director Jeff Hall also made a presentation at Flagstaff City Hall on Tuesday, suggesting that amber lights could be an option as the technology advances.
"I was trying to remind them as we come up on the celebration that LEDs by themselves are not the enemy. Amber LEDs are a potential solution," Hall said.
He also said that dark skies are worth preserving for a variety of reasons. The astronomy industry is growing in Flagstaff with a massive potential project near Meteor Crater promising more than $100 million in construction if the site is selected. The Naval Observatory is also looking at a major upgrade of its facilities on Anderson Mesa.
"LED technology is coming and why shouldn't Flagstaff be out in the lead helping define good dark-sky usage," Hall said.
The city of Flagstaff and Lowell Observatory have organized a conference this summer aimed at addressing the problems with LEDs and low-pressure sodium bulbs. The National Park Service, astronomers and lighting industry representatives will also be in attendance.
"Flagstaff and (Coconino) County have been astonishingly supportive in the grand scheme of things," Luginbuhl said. "They pay attention. They're serious. That said, it does require constant vigilance."
Added Luginbuhl: "I don't think the challenges we face today are any less solvable than what we've faced in the past."