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Why Are Streetlights Turning Purple?
Image credit: City of Manhattan, Kansas
1297 words / 5-minute read
Summary: Chromaticity shift is affecting an increasing number of commercial outdoor lighting products. This may affect public perception of solid-state lighting and could change the nature of skyglow over cities. This post explores what it is, why it's happening, and what can be done about it.
It sounds at first like an alien invasion.
"The sky over the city of Vancouver was the color of a television tuned to a Prince concert."
"I've had people call and ask if this was because it's Halloween, or because their football team in that area wears purple."
"Street lights are mysteriously turning purple. Why is this happening?"
In many parts of the United States, residents of cities are watching their new light-emitting diode (LED) street lights behave strangely. While they started out bright white, many are now turning an unsettling shade of purple. They have "spawned theories online about everything from vampires to vaccines". The truth is much more mundane. But as Business Insider notes, "when LED streetlights start changing color for no apparent reason, it's a visual cue that we might need to rethink, just a bit, how we build the future."
Pretty or problematic?
The lighting world has come up with a term for what many people are seeing close to home: 'chromaticity shift'. It's a fancy way of saying that the color of a light source is changing. The usual context of the term is in cases where that change is not part of the design of a lighting product.
To understand why that matters, let's back up for a moment and look at how white LED light sources work. This technology has achieved incredible success, coming to dominate the lighting market in only about a decade. Its high energy efficiency and ability to be carefully controlled make it a lighting workhorse.
But the light it produces isn't really "white". Underneath every white LED is a blue LED. Its blue light shines on a material called a phosphor, which has a particular chemical composition. Depending on the chemical mix, the material gives off light of other colors. Allowing for some of the blue light to leak through, the other colors add with it to give the sensation of "white" light.
Some influences change the relationship between blue LED and the phosphor, or change the nature of the phosphor itself. The balance of colors emitted by the LED changes in turn. This can result in chromaticity shift. The perceived color of the resulting light depends on what has happened to the phosphor. Sometimes it happens when the material binding the phosphor swells or cracks. In other cases, heat changes the chemical characteristics of the phosphor. It's also possible for the capsule of the LED itself to scatter or absorb too much blue light.
An example of outdoor lighting shifting to colors other than purple. These lights beneath the canopy at a filling station have shifted from white toward green, indicating that the phosphors in the LED capsules have oxidized.
Whatever the cause, once chromaticity starts it's impossible to reverse. Correcting it requires replacement of the LED 'light engines'. Because these are now integrated into modern lighting products, it usually means replacing the entire light fixture.
A 2020 U.S. Department of Energy report found that evidence for the shift starts to emerge after only about 8,000 hours of operation. While this time of "emergence" has increased, it is still far less than the expected lifetimes of LED lighting products. An often-quoted figure for the life of an LED chip is about 100,000 hours, or roughly 20 years of service. Chromaticity shift often shows up after about only two years.
Lab measurements of white LEDs show a gradual shift in the color away from white after 6000-8000 hours of operation. Image credit: U.S. Department of Energy.
So why are many white LEDs turning purple? It seems because of a manufacturing defect that causes the phosphor to pull away from the blue LED chip. In 2021, the manufacturer, American Electric Lighting, told the EdisonReport, a lighting trade publication, "The referenced 'blue light' effect occurred in a small percentage of AEL fixtures with components that have not been sold for several years. It is due to a spectral shift caused by phosphor displacement seen years after initial installation." It has since replaced many of the affected lights under warranty.
Blues in the night
At first the problem might seem only one of appearances. AEL stated that light produced by its products suffering chromaticity shift "is in no way harmful or unsafe." There is no reason to think that the purple lights are somehow dangerous to people. But there are real concerns about how their light affects the nighttime environment.
White LED lights that have shifted toward purple are telling us that more of their blue light is escaping. This is especially evident in the images below kindly provided by Bill Kowalik and Cathie Flanigan of the Oregon Chapter of the International Dark-Sky Association. They show images of the visual appearance of both normal street lights in the city of Bend, Oregon, and those affected by chromaticity shift. For each image they show a spectrum of the light pictured.
Comparing the two, it's clear that the purple lights emit much more blue light compared to light of all other colors. Bill and Cathie write, "The measurements show that for the purple lamp, the blue peak is about 2.5x stronger than the peak of green- yellow-red wavelengths. In the normal street lamp, the blue peak is about 1/2x the peak of the green-yellow-red."
There is now a great deal of evidence that blue life is harmful to wildlife. In particular, in many species it disrupts the natural circadian rhythm necessary for wellbeing. We also know that blue light scatters better in the atmosphere than other colors. The strong scattering means that blue light is a main contributor to the phenomenon of skyglow over cities. Bluer light sources mean brighter night skies and fewer stars seen overhead.
The impact of shifting street lights can be reduced if cities replace them promptly. Failed lights are usually covered by manufacturer warranties, and cities are entitled to replacements at no cost. But the saga of chromaticity shift follows reports of other, widespread LED street lighting failures. For example, in 2019 the city of Detroit, Michigan, settled a dispute with the manufacturer of almost 20,000 of its street lights that failed not long after installation. The settlement did not cover the full replacement cost, leaving the city on the hook to the tune of $3 million. Replacing shifting street lights may come with similar costs.
The perils of being an early adopter
There is a bigger story here than just one of purple street lights. There are of course risks attendant to any emerging technology, some of which might not become apparent for years. Business Insider suggests that the problem for American Electric Lighting was sourcing poor-quality parts from overseas manufacturers: "Those vendors are typically building products at scale, trying to squeeze out every efficiency they can without infringing on the patents on the high-quality, higher-priced versions. Sometimes that makes for a less-good LED."
These are dollars-and-cents decisions for lighting manufacturers. In the last decade, market forces exerted a huge downward pressure on LED lighting prices. Converting the world's existing lighting to LED was a tremendous, once-in-a-lifetime opportunity. Responding to customer demand created supply issues, and companies looked toward what was both cheap and available. The quality of some of the electronic components used in their products simply wasn't high. And some companies are now obligated to provide very expensive replacements under warranty.
These cautionary tales should inform the next generation of advanced lighting product development. It calls into question what other elements of the infrastructure of cities, now so deeply dependent on technology, may be next to fail. In our increasingly interconnected world, purple street lights are a metaphor for the ties that bind — and sometimes fail -- us.
The color qualities of light are only one aspect of the decisions that local governments face when they decide to modernize their street lighting. For many tasked with making decisions, it can be a confusing topic to navigate. Our expertise and experience advising cities choosing new lighting can make the difference in the success of retrofit projects. Contact us today to find out how we can help your city.
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Image credit: Mike Knell
959 words / 4-minute read
When you hear the term "light pollution", what comes to mind? While it's actually a broad term encompassing effects both on the ground and in the night sky, most people think of it as the inability to see the stars at night. That effect in particular, called "skyglow", results from light emitted on the ground up toward the sky. That light scatters in the atmosphere, which redirects it back toward the ground. There it competes with starlight and causes the sky to look grayish and washed out.
Thanks to research described in a new paper in the journal Science, we know that skyglow is increasing around the world at a stunning rate. The study, published in January, found the rate of increase is about ten percent per year when averaged around the world.
Counting stars instead of lights
Researchers in Germany analyzed tens of thousands of observations made by citizen-scientists between 2011 and 2022 as part of the "Globe At Night" project. The organizers ask participants to first locate familiar constellations in the night sky. They present maps of those constellations showing differing numbers of stars to show varying degrees of skyglow. The participants decide among the charts which best matches their own local night skies. This provides robust estimates of night sky brightness that scientists can compare against other sources of data about light pollution.
Globe At Night charts for the constellation Orion. To estimate the brightness of the night sky, participants compare these charts to the night sky from their locations and find the best match.
The new work is the first large-scale analysis of the Globe At Night estimates. Until now, Earth-orbiting satellites provided the only global view of light pollution. Earlier studies involving satellite images concluded that light pollution on the ground grew at a much slower pace. A 2017 study concluded that the rate was around two percent per year in the first half of the last decade.
Satellites flying blind
The new estimate in Science looks up from the ground instead of down from space, which may explain some of the difference. The satellites used to gather the night-lights data don't see blue light at all. They were not designed to make light pollution measurements in particular, so their onboard sensors aren't ideal for the job.
Their insensitivity to blue light is a problem given how the color of light at night continues to change. Since around 2010, many cities across the world converted their stock of public lighting to new, white light-emitting diode (LED) technology. They did this in hopes of realizing major reductions in energy use and financial cost with these energy-efficient light sources.
But white LED emits a lot more blue light than earlier lighting technologies. Satellites thus tend to under-count the amount of light at night they receive in space. That same blue light also scatters more strongly in the atmosphere than other colors. All other factors being equal, that effect explains in part why nights are getting brighter.
A global composite view of the Earth at night in the year 2015 made from artificially colored orbital satellite data. The relative 'blindness' of satellites to blue light means images like this underrepresent the true extent of night lights on our planet. Image courtesy of NOAA/NASA.
Besides to the color issue, satellites also miss some of the light directed upward from the ground because it becomes skyglow. Since they are above the atmosphere, satellites only detect light rays that reach them. Some of those rays never make it because they're redirected back down to the ground. So while ground-based observers saw their night skies brighten in the past decade, satellites recorded a much slower rate of change.
Furthermore, some light sources emit in ways that makes it unlikely their light will be seen directly in space. Sources like lit windows of buildings and illuminated signs emit light toward the horizon. Those sources are among the most important contributors to skyglow, but very little light they emit gets to the satellites. And there is evidence that at certain hours of the night, those sources dominate light escaping from cities.
Visual observers on the ground sense at least some of the "missing" light that satellites don't see. That way of estimating the brightness of the night accounts for both the increase in light emissions and a trend toward bluer sources. Citizen-scientists also extend the reach of light pollution studies. It's often easier to recruit them to count stars where they live than place light sensors in the field. That is especially true in very remote locations around the globe.
Citizen science for global change
But even considering that volunteers provided about 50,000 measurements in the preceding decade, the information is incomplete. Vast swathes of land contained no observers at all. That left scientists to make informed guesses in places like Africa, where satellites suggest night lights are increasing the fastest.
Although the study authors note that known methods of reducing light pollution yield reliable results, current initiatives aren't producing results. This concerns light pollution researchers and activists alike. Skyglow is one symptom of a larger problem involving overuse of artificial light at night. We know it has negative impacts on everything from wildlife to energy use, traffic safety, and more. It's about much more than whether we can see the stars at night.
As more people realize the seriousness of the issue, governments are taking note. Countries like Mexico have recently classified light pollution as a form of environmental pollution. This can bring the mechanisms of existing environmental laws to bear on the problem.
The failure to reduce light pollution has real and lasting effects on communities. Wasted outdoor light at night involves measurable social and financial costs. Yet the new study authors admit that these messages about light pollution might heard by governments if the data were better geographically distributed.
More people participating in Globe at Night would improve the reliability of the results. If the number of participants increased by a factor of 10, we would begin to see trends on scales smaller than whole continents. As lead author Christopher Kyba said, "If we could do that, we would surely find places that are doing better than average, and we could try to figure out what they are doing right."
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Image credit: NPS / Will Pedro
753 words / 3-minute read
In January 2022 we wrote here about 'astrotourism', a kind of night-sky themed ecotourism, was "on the rise". The intervening year has seen this industry grow by leaps and bounds around the world. As the COVID-19 pandemic fades, people are hungry to see the world again and experience new adventures. For many who have never seen a starry night sky, an astrotourism experience fits the bill.
There are a few places in the world where this mode of tourism is already well developed. For example, the Mackenzie region of New Zealand has a thriving local astrotourism industry. Many places have the right mix of dark night skies and daytime allure, but few have achieved such success. Those that do stand to gain much in the way of economic development, particularly in rural and economically depressed areas.
The U.S. state of Utah is one of the exceptions. While famous as a winter playground for the "greatest snow on earth", tourists now flock to the Beehive State for other reasons. According to the Utah Office of Tourism, dark skies now have a higher public and media uptake than skiing. The magic of dark night skies above, with the celebrated beauty of the Utah landscapes and its vast public lands below, is a strong visitor enticement. That is especially true for families with children who have never experienced the splendor of the stars.
In a sense, the modern idea of astronomy-themed tourism started in this part of the world. In 1969, formal stargazing programs were first offered to the public at Bryce Canyon National Park. As word of the region's night skies spread, early efforts to commercialize the experience took root. Opportunities available to Utah astrotourists now run the gamut from guided, small-group stargazing to astronomy-themed river running and 'glamping' experiences.
While its full economic value to the state has yet to be quantified, there are reasons to think that Utah is already benefitting. One study, published in 2019, predicted that astrotourism will account for $6 billion worth of economic activity in the region during the 2020s. The surge in post-pandemic tourism means that number is probably on the low side of what's possible.
Utah ranks number one with more accredited International Dark Sky Places than any other state or province in the world. Its dry climate and often favorable weather yield some of the finest night skies in the developed world. Travelers come to Utah from heavily light polluted places such as the East and West U.S. coasts, Europe and Asia to experience this firsthand.
The government of Utah has taken notice, and it has begun to incorporate dark skies into the state's branding. For example, the Utah Governor has declared April as Utah Dark Sky Month for the past two years. And beginning in 2023 the state will offer a themed license plate to motorists. When seen during their travels to other states, it serves as an advertisement attracting more tourism.
An early design concept for the Utah dark-sky specialty license plate, making its debut in 2023.
Officials have come to recognize astrotourism as one of the most profitable sectors in the ecotourism industry. While famous for its "Mighty 5" U.S. National Parks set among the stunning scenery of southern Utah, daytime-only visits have a milder economic impact. Realizing that "half the park is after dark", astrotourism activities in and near the Mighty 5 offer further recreation opportunities to visitors. This not only adds significant value to visitor experiences, but it means more revenue for tourism operators and local governments alike. Even one night spent stargazing adds an overnight stay and two meals to a visitor's tab. This represents a highly profitable proposition.
The state's Office of Tourism developed a well-considered dark sky toolkit, debuting in 2022. One segment of that toolkit, "An Industry Guide To Astrotourism," offers a particularly powerful set of guidance and resources. As the field becomes increasingly professionalized, this level of state support helps bring new operators into the industry. It also grows a local knowledge base that can help with the development of new tourism products.
Utah has become a best-practice model for the economic development possibilities of astrotourism. It provides clear, actionable information to interested communities and their affiliated DMOs (Destination Marketing Organizations). To this it adds branded marketing elements that reinforce a strong connection between Utah outdoor recreation and night skies. The result is a recipe for success to which people in other parts of the world now look as an example to guide their own local efforts. Dark-sky tourism now has a bright future, and Utah leads the way.
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Understanding the Bortle Scale
Image credit: Dark Sky Consulting
854 words / 3-minute read
The brightness of the night sky at a given location is an important indicator of the health of the nighttime environment. Many factors influence what we see when we look up at night, as we recently wrote about here. Light pollution is only one factor to consider, albeit an important one.
Rather than thinking only about brightness, sometimes one hears about night sky "quality". While the ways we characterize the the night sky can be rather technical, we wish for something simple. A convenient shorthand, first described two decades ago, offers such an approach: the Bortle Scale.
Devised by amateur John Bortle, the scale is a simple and intuitive way to classify night sky quality. Everyone is familiar with scales that rank this or that over a range of, say, one to ten. The Bortle Scale ranges from one to nine, and it works backward: a better night sky gets a smaller number.
The value of sky quality estimates
Taking a small step back, why do we care about any of this? Quantitative measurements of sky quality are best, but data are often difficult to get. But it's much easier to get more people out in the field who can make subjective assessments.
In turn, researchers can keep tabs on how conditions are changing in different places due to light pollution. We can also use this information to assess how well lighting retrofits are going, or to identify potential new astrotourism sites.
But the question of "how dark is the night sky" is more complex than it seems on the surface. Besides artificial skyglow, there are many natural sources of light in the night sky. The contributions of these sources span a large range in brightness, and they vary in time; no two nights at the same location are ever the same.
Other means of assessing night sky brightness are often not very consistent, making comparisons difficult. This also complicates quantitative measurements, so we end up relying on long-term statistical distributions that characterize the probability of any particular night having any particular sky quality.
A simple approach
That's where the Bortle Scale comes in. It aims to reduce some of the complexity of quantitative measurements while retaining the value of simple comparisons between sites. The scale divides night sky quality into nine levels ranging from badly light-polluted city skies to pristine rural skies:
For each level, it lists several distinguishing characteristics that users can look for. With some practice, users can tell the different levels apart with reliability.
Because the Bortle Scale levels roughly overlap ranges of other sky quality indicators, it can be predictive. For example, the website lightpollutionmap.info provides an estimate of the Bortle Scale for any location with a click of the mouse. The instance below shows an area on the western outskirts of London, U.K., is a "Bortle 5" site. The Bortle level serve as a first guess of what quantitative measures of sky brightness might show.
Of course, the Bortle Scale approach has its own drawbacks. It involves a subjective assessment specific to a particular observer. In that sense, it is "impressionistic" and depends on the conditions of a particular night. It's also sensitive to an observer's experience and visual acuity. The boundaries between the Bortle levels are often "fuzzy", calling for further shades of difference that the system doesn't permit.
Yet with further impressions by other observers, the picture can become clearer. A study of citizen science data collected as part of the Globe At Night project supports this idea. Researchers compared thousands of visual night sky brightness estimates to satellite measurements and found good correspondence. They found that visual impressions "depend strongly on artificial skyglow and could be used to track lighting changes worldwide."
Power in numbers
Despite its limitations, Bortle Scale estimates have distinct value. Over time, variations among observers will even out and a 'typical' sky quality at a site emerges. The numbers can suggest places worthy of more study, and especially of quantitative measurements. Repeated site visits may suggest slow changes in sky quality over time, for better or worse. In particular, the first notion of degrading night skies may be a slide in the Bortle level.
And the Bortle Scale represents a comfortable language for use in talking to the public and elected officials. It can help support dark-sky conservation efforts by making the subject easier to understand. A straightforward rating can even impart a sense of familiarity to those who otherwise know little about light pollution.
What is the Bortle Scale number for your location? You can look for it on lightpollutionmap.info, or try making your own estimate using John Bortle's original description. What you discover in the process might surprise you!
Need help interpreting your results? Or maybe you want to take the next step and make more rigorous measurements of your site quality? Contact us today for advice.
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Image credit: M. O'Neill / NPS
766 words / 3-minute read
Many people are feeling stressed out these days. In the wake of the COVID-19 pandemic and with major economic and political uncertainties ahead, it's understandable that so many people experience anxiety. The frenetic pace of life enabled by technology disconnects us from our roots in nature. As professionals debate the reality of 'nature deficit disorder', an increasing number of people look for solace in the natural world. Can access to a clear, dark night sky, and the awe it inspires, actually make us feel happier?
The psychology of wonder
As a practicing psychotherapist, Ada Blair found herself asking that very question a few years ago. "I have a longstanding interest in how encounters with nature, including the sky, may positively impact wellbeing and even lead to transformation," she says. Having spent time on Sark, a small island in the English Channel that became an International Dark Sky Community in 2011, she experienced it firsthand. She wondered if the effect would stand up to scientific investigation.
Blair found that the dark nights on Sark, which has almost no electric lighting, led residents to "a widespread belief that observing the night sky results in positive (and sometimes transformative) feelings." Her interviews with islanders revealed an internalized idea that their sense of wellbeing was in part supported by ready access to a dark night sky.
Even before Blair began her investigation, there was evidence that exposure to artificial light at night influenced mental health. Higher levels of light at night correlate with a variety of disorders, [1,2] ranging from insomnia to depression. That in turn suggests that access to nighttime darkness is just as important to maintain good health. But could exposure to cosmic light also influence our sense of wellbeing?
Conquering fear (of the dark)
We don't yet have a lot of scientific evidence to establish a strong relationship between dark skies and happiness. But Dajana Bjelajac, of the University of Novi Sad, Serbia, and her co-authors see signs of a deep connection. "The relationship between humans and natural darkness has always been a dreadful one," they write. "People would admire the heavens, but at the same time, as diurnal beings, we have been and still are relentlessly searching for the best method to tame the night and bring the light into it."  Darkness can be terrifying for some people, although the source of that fear may have to do with a sense of vulnerability.
Fear of darkness may equate to a fear of being unable to escape perceived danger. [4,5] If people sense that they are in safe spaces at night, their anxiety can ease and they are then able to enjoy the night sky. The effect of living in light polluted places like cities might influence this: when people fear for their safety in cities, they can transfer that fear to rural places.
A safe space to be happy
Astrotourism can create safe outdoor spaces at night for participants to shed some of that fear and enjoy the wonder of the night sky. It brings people closer to nature, which can further ease fear and promote feelings of safety. Given the lockdowns of the past few years, people are ready get back out into the world and experience new and varied forms of tourism. Some experiences, like dark night skies, may even help heal some of the trauma many have felt.
In protected places accessible to the public, like certified International Dark Sky Places, people can find the sense of security they need to have such experiences. It can be something they share with friends and family, much as people thousands of years ago did when dark night skies loomed larger in human culture than they do today. Even fleeting glimpses of starry skies may be meaningful. Recent research results suggest that quality of nature experiences, rather than their frequency or duration, is more reliably predictive of mental health and wellbeing outcomes. 
In "The Value of the Night Sky", economist Terrell Gallaway writes of the different ways we can look at natural nighttime darkness as a natural and economic resource. "The night sky is a superlatively wondrous and deeply threatened natural asset," he writes. "As an asset, the night sky is extraordinary in many ways beyond its obvious beauty and grandeur." The senses of wonder and awe that the night sky evoke might be the tonic that a weary world needs in troubled times.
Whether it's conservation planning, field surveys or lighting policy formulation, we can help your achieve your goals in protecting the night. Contact us today to find out more.
 Yu, Z., Hu, N., Du, Y., Wang, H., Pu, L., Zhang, X., Pan, D., He, X., & Li, J. (2022). Association of outdoor artificial light at night with mental health among China adults: a prospective ecology study. Environmental Science and Pollution Research (Vol. 29, Issue 54, pp. 82286–82296). https://doi.org/10.1007/s11356-022-21587-y
 Tancredi, S., Urbano, T., Vinceti, M., & Filippini, T. (2022). Artificial light at night and risk of mental disorders: A systematic review. Science of The Total Environment (Vol. 833, p. 155185). https://doi.org/10.1016/j.scitotenv.2022.155185
 Bjelajac, D., Đerčan, B., & Kovačić, S. (2020). Dark skies and dark screens as a precondition for astronomy tourism and general well-being. Information Technology & Tourism (Vol. 23, Issue 1, pp. 19–43). https://doi.org/10.1007/s40558-020-00189-9
 Boomsma, C., & Steg, L. (2012). Feeling Safe in the Dark: Examining the Effect of Entrapment, Lighting Levels, and Gender on Feelings of Safety and Lighting Policy Acceptability. Environment and Behavior (Vol. 46, Issue 2, pp. 193–212). https://doi.org/10.1177/0013916512453838
 Blöbaum, A., & Hunecke, M. (2005). Perceived Danger in Urban Public Space. Environment and Behavior (Vol. 37, Issue 4, pp. 465–486). https://doi.org/10.1177/0013916504269643
 Richardson, M., Passmore, H.-A., Lumber, R., Thomas, R., & Hunt, A. (2021). Moments, not minutes: The nature-wellbeing relationship. International Journal of Wellbeing (Vol. 11, Issue 1, pp. 8–33). https://doi.org/10.5502/ijw.v11i1.1267