A closer look at city lighting

German researchers have deployed citizen scientists to record the sources of night-time lighting in cities — offering detail that satellite images cannot provide.

There are pressing reasons to study the sources of urban night-time lighting. As well as contributing to energy consumption, light pollution affects the rhythms of life of animals, plants and humans.

It’s a growing environmental problem, affecting a quarter of the world’s land area and 88% of Europe, according to Germany’s GFZ Helmholtz Centre for Geosciences. While satellite images provide some insight into where this light is coming from, local authorities really need a more detailed analysis in order to come up with targeted countermeasures.

Enter physicist Dr Christopher Kyba and his team of citizen scientists.

In autumn 2021, Kyba, then a geographer at the GFZ Helmholtz Centre for Geosciences and the Ruhr University Bochum, organised a group of 258 volunteers to count and evaluate light sources in city centres, residential areas and commercial areas.

The citizen scientists used the ‘Nachtlichter-App’ (Nightlight App), specially developed for the project, to register a total of 234,044 lights on 3868 individual measurements. The counting covered a total area of 22 square kilometres in predefined areas of 33 municipalities (nine of which were outside Germany).

Lights were counted and classified along with additional information such as the degree of shielding or the size of the installation. Extrapolated across Germany, the researchers reported that 78 million lights remain on after midnight: roughly one light per person.

The team found that streetlights are only responsible for part of the night-time light in urban areas. In such areas, there are nearly twice as many illuminated advertising signs and shop windows. Private windows were by far the most frequently observed light source, even after midnight, although these are less bright.

A further quarter of the lights consisted of other sources such as floodlights, house number and doorbell signs, and decorative garden lighting. The data also made it possible to examine how the composition of light varied between city centres and less densely built-up areas such as residential neighbourhoods.

Comparing ground data with satellite measurements

The areas where the counting took place were selected so that they corresponded with the measurement zones of a night-time light-observing satellite. This allowed the results of the light detection on the ground to be compared with the satellite data.

The results showed a clear correlation between the number of counted lights and the radiance observed by the satellite; the researchers were able to determine a conversion factor for converting the ‘brightness’ measured by the satellite into the more easily understandable unit of ‘lights per square kilometre’. The number of individual sources in a given community or region could then be easily estimated.

“By scaling our results up to cover all of Germany, we estimate that just over one light per person stays on after midnight,” Kyba said.

Kyba added that the results showed significant potential for future light and energy savings in German municipalities.

“Both energy and lighting policy as well as research on the effects of artificial light on the environment have generally focused on street lighting,” he said.

“Our findings indicate that a broader approach that considers all lighting is necessary in order to understand and reduce the environmental impacts of light in cities.”

The peer-reviewed study has been published in the journal Nature Cities [doi.org/10.1038/s44284-025-00239-5].

Image credit: iStock.com/ollo