In 2014, the International Dark-Sky Association (IDA) amended its Seal of Approval Program to accept only luminaires in 3000K and below. The lighting community seeks to understand this pushback against efficacious, scotopically rich outdoor lighting. But we see – and measure – light pollution through human eyes. Exploring the fundamental consequences of outdoor lighting on wildlife and ecosystems reveals the LED lighting revolution in powerful ways.
Dark-sky initiatives gained momentum in the eighties, supported by professional and amateur astronomers battling to see through astronomical light pollution: the haze, or skyglow, created by electric lighting at night. Along the way, the astronomers allied with the burgeoning ecology movement, calling attention to the consequences of light pollution on nocturnal animal activities, such as foraging bats, frogs and rodents or migrating birds. Later studies explored nonvisual impacts on diurnal species, such as reproduction in songbirds and human sleep patterns.
The LED revolution has brought the issue to a head. Due to incredible long-term energy savings – and readily available funding from the US government – vast swaths of cities, towns and highways have converted to LED lighting. Whereas the International Dark-Sky Association once focused primarily on luminaire cutoff, since 2010 the focus has been shifting toward spectra. IDA cites the case against bluish-white light in terms of “discomfort glare, circadian rhythm disruption, light scattering, skyglow and biological system disruption in wildlife.”
Both direct upward light (considered inefficient by today’s standards) and reflected, indirect light contribute to skyglow. Ecological light pollution includes both skyglow and more-problematic nearby lighting in tremendous ranges of habitats across the globe. Try to look at it with compound eyes.
An invisible world
The sea turtle is definitely the poster child for the eco light pollution cause. But you have only to see a moth enthralled by a compact fluorescent porch light to understand that light generated by human activity shapes the environment at a fundamental level. Due to the structure of individual visual systems and unique foraging and reproductive behaviors, different species are affected in different ways and to different degrees. That moth, caught in a “light trap,” will likely not be pollinating a neighborhood garden the following evening.
“The thing to remember when we start talking about other species is that they can and do have completely different visual response curves – we call them action curves – than do people,” said Travis Longcore, assistant professor of Architecture, Spatial Sciences, and Biological Sciences at the University of Southern California. “The world as perceived by other species differs greatly both in the colors that are seen and also the relative intensity of those colors and the acuity of the eye to perceive the colors. So things that we might think of as being dark, to other species are shades of gray.”
Humans are unusual in the animal kingdom in that, due to filtering by the lens, we do not see ultraviolet light. Where our red-green-blue color vision might overlook a pale flower, a moth with green-blue-ultraviolet photoreceptors might see striking variegations and the promise of a sugary meal.
Configurations of photoreceptors and peak sensitivities are more varied than species themselves. (Some humans, called tetrachromats, may discern colors with four sets of cones.) There are birds, fish, amphibians and reptiles that are tetrachromats. The superpowered mantis shrimp may hold the record for the most complex visual system with up to 16 photoreceptors that can see UV and circularly polarized light.
The trouble with lux
Different types of photoreceptors have different peak sensitivities, but the response is, in fact, a peaked absorption curve. Remember, a lot of light in an off-peak wavelength can stimulate a photoreceptor just as much as less power at the peak.
Over the course of a year, “retired” lighting researcher and innovator Ian Ashdown reviewed more than 100 papers on light pollution. His July 2015 blog, “Color Temperature and Outdoor Lighting,” attempts to bridge the astronomy and lighting communities. Ashdown looked closely at models that show higher CCTs and their increased contribution to skyglow. “Light pollution is due to both Rayleigh scattering from air molecules and Mie scattering from aerosols such as dust, smoke, and haze,” he wrote. Rayleigh scattering is wavelength-dependent: it more readily scatters shorter wavelengths, which is why the sky is blue. Mie scattering is wavelength-independent.
Ashdown’s review of up-to-date atmospheric modeling, requiring massive computing power, confirms that overall, under both photopic (light-adapted) and scotopic (dark-adapted) viewing conditions, higher CCT sources do contribute to increased sky brightness. In an interview he pointed out that dropping from 4000K to 3000K would lead to a small decrease in sky brightness: “The light is scattered more, but our eyes don’t notice the difference. But in high-CCT LEDs there’s a huge blue spike at 450 nm.” Ashdown’s important analysis supports the case of astronomers, particularly in the observation of stars, which show emission lines in this region of the spectrum.
Insects, birds and other animals with blue and UV photoreceptors would likely be even more sensitive to this spike. Ashdown corrected, or multiplied, the raw spectral data of the LEDs he examined by the human photopic and scotopic luminous efficiency curves; thus his results are anthropocentric. Light sensitivity falls to zero at the UV threshold, in both the photopic and scotopic luminous efficiency curves. “Unless you know the wavelengths insects or animals are sensitive to, you can’t calculate what the sky brightness would be,” he explained.
Candela, lumens, lux – they’re all measuring light based on daylight-adapted human visual sensitivity.
The human visual system, predictably, is far more sensitive to light overall under scotopic adaptation; in addition, the visual sensitivity curve shifts about 50 nm towards the blue and UV. Ten years ago this discussion emerged as a pull for bluer outdoor lighting, which was identified as higher efficacy under mesopic conditions at night (between scotopic and photopic).
This “scotopically rich” lighting trend prefaced the widespread conversion to LED outdoor lighting still ongoing. And the acceptance of switching from HPS to 4000K, or even 5000K, LED retrofits is common. “The new LEDs you see everywhere, people like them because they look like daylight,” said James Karl Fischer, executive director of the Zoological Lighting Institute.
Fischer explained that there are whole layers of the night that humans just can’t appreciate. “Typically, there are 10 orders of magnitude of light, just in terms of intensity, from noon at the tropics to a dark and stormy night. And eight of them occur from twilight on down. So basically, when the streetlights go on, you’re eliminating orders of magnitude of the luminous habitat. When you do that it degrades the habitat. It actually takes away all of that diversity and makes it very uniform.”
Electric light affects animals, including us, in three general paradigms:
- Circadian regulation, including diurnal patterns but also seasonal cycles. From zooplankton to humans, photoperiods synchronize reproduction, hibernation, migration, predator avoidance, foraging, etc. This can be a combination of hormonal regulation and response to light cues to sing, hide, rest, forage, spawn, mate, molt or feed the kids.
- Visual perception, including navigation within their environment and recognition of predators, prey, signals from potential mates, food sources, etc. Many animals are intrinsically dark adapted and may be blinded to important visual cues, depending on light intensity and spectral composition. The time it takes for a species to recover from light exposure and regain darkness adaptation varies widely.
- Spatial orientation. Light traps draw the moth to the porchlight, the turtle to the highway, the bird to the beacon, and so on. In contrast, light avoidance can crowd some species into small patches of habitat or block them from traveling to new ranges.
Some lucky diurnal species can extend their activities thanks to a local “night-light.” Their prey are not as lucky. Similarly, we think of light as a resource, enabling us to extend our activities into the night. But it can be destructive, according to Fischer. “Animals are a product of the habitat. If you lose habitat, you lose animals. If the habitat isn’t as diverse as it should be, you lose biodiversity. So with the onslaught of lighting at night, very complicated systems are essentially reduced down into monotones.”
Lack of biodiversity can create pests and spread disease, among crops, animals and humans. In addition to “vacuuming” vital species out of the food web, light can attract pests to human habitation. “These are people that are going to die if you have an outbreak of a disease because you put up a new light that attracts a particular species – the kissing bug or whatever,” said Longcore. “There are many reasons why insects matter, and we ignore them at our peril.”
Options with LED
Advances in LED technology can provide some solutions – or at least an opportunity to stop and look at ecological impacts. “Unlike other light delivery tools, not as much is ‘baked in the cake’ with an LED. With HPS you’ve pretty much got the spectral output that you’ve got, unless you put a filter on it. Whereas LEDs, we can configure them,” Longcore said.
Most LED luminaires emit no UV and can be configured to limit shorter wavelengths. Warmer color temperatures are higher in price and somewhat less efficacious; but continue to approach parity with high-CCT LEDs. Multi-channel LED luminaires can be tuned to lower CCTs, dimmed and/or turned off according to daily and seasonal schedules, or cued by sensors. The compact nature of the technology is far better able to restrict lighting to the areas where it’s needed, through careful cutoff design and implementation.
The City of Davis, CA, was half way through an LED street relighting initiative, when forceful complaints from residents halted the project. The public then reviewed several alternatives and ultimately selected streetlights that are lower brightness and much warmer, at 2700K. Many of the high-CCT luminaires had already been installed, so the additional cost to the city was $350,000. From San Diego to New York City, city halls are fielding complaints from citizenry.
The news about human health hazards of melatonin suppression, both cancer risk and sleep disruption, is now widely disseminated. And over the past 5 years, wildlife studies in the field are proving the disruptive influence of electric light on wildlife, e.g., melatonin suppression in wallabies, supporting evidence from previous laboratory studies. Calls for legislation of outdoor lighting spectra can be heard from both researchers and the IDA.
Longcore sees increasing references to ecological light pollution by environmental consultants for planning and construction projects. (He occasionally consults on remediation measures.) “Almost immediately after our conference in 2002 we had people citing it in environmental impact statements and reports talking about impacts on wildlife,” he said.
“It’s going to take more time and education to understand that this is a consistent message that’s now coming from three, and I would argue four sectors. The astronomical community is very clear that blue light is a problem. The human health community is clear on the melatonin action spectrum: it’s not going away. And the wildlife advocates…. I would argue that the fourth is the idea of public aesthetics. We should be doing outdoor lighting in way that doesn’t try to replicate the daytime. A cozy environment is not one that’s lit up with 4100K lights. It’s shocking to the eye, hard to deal with. It’s worse for wildlife, so why do we keep doing it?”
Research on individual species is coalescing to reveal impacts on entire ecosystems; which could then lead to overall recommendations for outdoor lighting. “The wildlife concerns are different in different contexts. But if I had to have rules, then the first rule is no ultraviolet. There’s no reason for it ever. The second rule would be to keep away from the short wavelengths in almost every circumstance,” Longcore said. He also recommends total shielding of uplight and good horizontal cutoff; always shielding outdoor lighting away from wetlands and bodies of water. All of these recommendations are echoed in the Zoological Lighting Institute’s model lighting ordinance for “wildlife-sensitive” areas. This proposed ordinance goes beyond CCT, specifically eliminating all wavelengths below 550 nm.
Low CCT is not a silver bullet, said Longcore, “but we do now have, I think, a very consistent set of findings that can lead us to some rules of thumb. We can do much better than we’re doing now.”
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