Sky & Telescope - January 2024 - 40

Relative Sensitivity
BEATRIZ INGLESSIS / S&T, SOURCE: THE AUTHOR
Light Pollution: Wildlife and Health
1.0
Human photopic
(daytime)
0.8
0.6
Human scotopic
(nighttime)
Insecta
Arachnida
Aves
0.4
Amphibia
Mammalia
Reptilia
0.2
300
400
500
Wavelength (nm)
SEEING THE LIGHT Each class in the animal kingdom (six shown here) reacts in its own way to different wavelengths, although there are a lot of
similarities. These average sensitivities have a similar peak to those of humans' day and night vision, but we are far less sensitive to short wavelengths
than other animals are.
attracted to shorter wavelengths, and horses and related species
shouldn't be disturbed by red light at night because it's at
the edge of what they can see. This would also apply to most
insect groups and some species of rodents, as well as bats.
The prospect of using color to reduce lights' impacts comes
with some conspicuous exceptions and challenges. Bioluminescent
species such as fireflies are often sensitive to the very
yellow to amber wavelengths that are less harmful to other
species. For these groups, which communicate using light, it
is far better to avoid exposing them to stray light altogether.
The other challenge is that some species are sensitive to light
no matter where it is on the spectrum, so spectrum is not a
" get out of jail free " card that allows indiscriminate use of
light without adverse effects.
My work looked at color vision, which uses photoreceptors
called cones; we also have rods, for dim light. But our eyes, as
well as those of other species, also have daylight detectors that
set our circadian rhythms yet don't contribute to vision. These
photoreceptors are most sensitive to blue light, very close to
the color of the blue sky, because before artificial light we only
encountered bright bluish light during the day. The deleterious
effect on our internal clocks - which govern our sleep patterns
- gives another reason to avoid blue light at night.
Takeaways
We can conclude a few things from the research covered
here. To minimize the impact on wildlife, plants, and even
ourselves, we need to control both light levels and light colors
in nighttime environments. We should avoid shorter wave40
JANUARY 2024 * SKY & TELESCOPE
lengths (ultraviolet through blue) in favor of yellow and red,
to enable us to see at night while reducing adverse effects on
other species.
But the best solution is to reduce light overall and direct it
only where it is needed, when it is needed. From a technical
perspective, this means shielding lights to make them " full
cut-off, " so that no light escapes upward, and directed, so
that light falls on its target and not in nearby habitats.
For color, I recommend lights with a CCT less than 2200K,
which looks like warm candlelight. Such lights are increasingly
available from manufacturers who recognize the growing
market potential, as awareness of the impacts of light
pollution grows.
Once the harms of disrupting darkness are explained,
people tend to intuitively understand them and are open to
supporting better lighting. When I give presentations, I hear
stories about sleep lost from a neighbor's light and hear wonder
at the nuanced patterns of activity in response to light at
night. Some places we should not light at all, but great strides
could be made by simply lighting better - in a way that is less
wasteful and aware of the natural rhythms of life on Earth.
¢ TRAVIS LONGCORE is an adjunct professor at the UCLA
Institute of the Environment and Sustainability and co-chair of
the Environmental Science and Engineering (D.Env.) Program.
FURTHER READING: Catherine Rich and Travis Longcore, eds.
Ecological Consequences of Artificial Night Lighting. Island Press.
2006.
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Sky & Telescope - January 2024

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