Streetlights contribute less to nighttime light emissions in cities than expected

Date:
October 29, 2020
Source:
GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
Summary:
When satellites take pictures of Earth at night, how much of the
light that they see comes from streetlights? A team of scientists
have answered this question for the first time using the example of
the U.S. city of Tucson, thanks to 'smart city' lighting technology
that allows dimming.

The result: only around 20 percent of the light in the Tucson
satellite images comes from streetlights.



FULL STORY ==========================================================================
When satellites take pictures of Earth at night, how much of the light
that they see comes from streetlights? A team of scientists from Germany,
the USA, and Ireland have answered this question for the first time
using the example of the U.S. city of Tucson, thanks to "smart city"
lighting technology that allows cities to dim their lights. The result:
only around 20 percent of the light in the satellite images of Tucson
comes from streetlights. The study is published today in the journal
Lighting Research & Technology.


==========================================================================
The team conducted an experiment by changing the brightness of
streetlights in the city of Tucson, Arizona, USA, and observing how
this changed how bright the city appeared from space. Dr. Christopher
Kyba from the GFZ German Research Centre for Geosciences led the team
that conducted the experiment, and said the work is important because
it shows that smart city technologies can be used to perform city-scale experiments. "When sensors and control systems are installed throughout
an entire city, it is possible to make a change in how the city works,
and then measure the impact that change has on the environment, even
from outer space," Kyba said.

Over a period of 10 days in March and April of 2019, Tucson officials
changed the brightness settings for about 14,000 of the city's 19,500 streetlights.

Usually, most streetlights in Tucson start out at 90 per cent of their
maximum possible illumination, and dim to 60 per cent at midnight. During
the experiment, the city instead dimmed lights all the way down to 30 per
cent on some nights, and brightened them up to 100 per cent on others. The
city lights were observed by the US-operated Suomi National Polar-orbiting Partnership (NPP) satellite, which is famous for its global maps of light
at night. The satellite took cloud-free images of Tucson on four nights
during the test, and on two other nights with regular lighting after
the test. By comparing the city brightness on the 6 different nights,
the researchers found that on a normal night, only about 20 per cent of
the light in satellite images of Tucson comes from streetlights.

The results have important implications for sustainability, according
to study co-author Dr. John Barentine from the International Dark-Sky Association. In a second experiment conducted at the same time, Barentine,
Kyba and their co- authors measured the sky brightness over Tucson from
the ground. They examined how varying the illuminance of street lamps
affected the sky brightness, and showed that as with light emissions
seen from space, most of the sky brightness over Tucson is also due
to other sources. "Taken together, these studies show that in a city
with well-designed streetlights, most of the light emissions and light pollution come from other lights," Barentine explaines, including
light sources such as bright shop windows, lit signs and facades, or
sport fields.

The authors say that local and national governments therefore need to
think about more than just street lighting when trying to reduce light pollution.

According to the researchers, the difference in the streetlighting
brightness on the different nights is barely perceptible to the people on
the street, as our eyes quickly adapt to the light levels. They report
that the city received no comments or complaints about the changed
lighting during the test. There is also no evidence or suggestion that
reducing lighting levels as part of the experiment had any adverse effect
on public safety.

Kyba is therefore excited by the idea of performing such experiments more regularly, and in other municipalities. "Instead of dimming lights to the
same level late each night, a city could instead dim to 45% on even days
and 55% on odd days," Kyba suggested. "City residents wouldn't notice
any difference, but that way we could measure how the contribution of
different light types is changing over time."

========================================================================== Story Source: Materials provided by GFZ_GeoForschungsZentrum_Potsdam,_Helmholtz_Centre. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. CCM Kyba, A Ruby, HU Kuechly, B Kinzey, N Miller, J Sanders,
J Barentine,
R Kleinodt, B Espey. Direct measurement of the contribution of
street lighting to satellite observations of nighttime light
emissions from urban areas. Lighting Research & Technology, 2020;
147715352095846 DOI: 10.1177/1477153520958463 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201029135503.htm

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