German cities at night appear in satellite images as darker than U.S. cities of similar size, according to a new study.
The study, appearing in a recent publication of the journal Remote Sensing, found German cities emit several times less light per capita than comparably sized cities in the United States.
The research team gathered data for 28,804 U.S. cities and 4,492 German cities using the VIIRS instrument (Visible Infrared Imaging Radiometer Suite Day-Night Band) on the Suomi NPP (National Polar-orbiting Partnership) satellite.
The pictures were taken at a resolution of about 750 metres, starting about two years ago.
"The size of the difference in light emission is surprisingly large," said lead author Dr. Christopher Kyba of Potsdam's German Research Centre for Geosciences.
The study found a typical U.S. city with 100,000 inhabitants is five times brighter than a typical German city with the same population.
Chicago, L.A. brighter than Berlin
Chicago and Los Angeles, as examples, were found to have an SOL (sum of lights) per capita that was nine and 3.5 times larger than Berlin, respectively.
The size of the gap grew with city size, with light per capita increasing with city size in the United States but decreasing with city size in Germany. In other words, larger American cities are brighter per capita than smaller towns, while the reverse is the case for Germany.
Kyba told CBC News that researchers will explore the results in a followup study, where they will compare cities within each country with large differences in light emission in more detail.
Variations in architecture, city planning related to history and climate, vegetation, and the type of lighting used, could explain some of the differences in light emissions between the two countries.
When it comes to urban landscapes, the study noted that younger U.S. cities typically have wider streets than denser, old-world cities, producing a larger observed "sum of lights."
'Only as much light as is sensible'
In addition, newer cities and subdivisions, notably those in the American West, tend to have younger and less abundant tree cover, which could obscure artificial lights.
"A final possibility is that German streets may be simply less brightly lit than American streets," said the study. It did not address what kind of lighting the two countries rely on the most, but said the the findings could be useful in other studies on light pollution, energy consumption and the epidemiology of illness related to light exposure.
"Berlin’s lighting policy document advises careful use of light under the guiding principle 'only as much light as is sensible and necessary.' Berlin’s success in maintaining public safety with conservative light levels may suggest that other cities are consuming more light than is necessary," the study said.
Kyba said the adoption of energy-saving LED lamps, which can reduce the amount of light that shines upwards, isn't widespread enough to pinpoint that technology as the reason German cities emit less artificial light at night.
Within Germany itself, former East Berlin was found to be emitting more light per capita than the former West.
"The imprint of 40 years of divided lighting management remains visible in Berlin, despite nearly a quarter century since the German reunification," the study said.