At night, far fewer stars can be seen over cities than in the countryside – light pollutes the sky. Researchers have now investigated the question of how much street lamps contribute to light pollution. In Tucson, Arizona, USA, they used smart lamps to determine that around a fifth of the light captured by satellite images comes from street lamps.

Night recordings of the Suomi NPP satellite (with the day-night band in the visible infrared Spectral range). They show Tucson, USA, on the nights of 30. March 2019 (as most street lights on 25% of the maximum were dimmed) and on April 7th 2019 (when the street lights were turned on at full power).

( Image: GFZ)

“In a city with well-designed street lighting, most of the light pollution must have other causes, such as bright windows, illuminated signs and facades or sports fields,” explains Dr. John Barentine of the International Dark-Sky Association. He is part of the team of researchers in the USA, Ireland and the German Research Center for Geosciences (GFZ) in Potsdam. You come to the conclusion in the specialist magazine Lighting Research & Technology that local and national governments are therefore Have to think about more than just street lights if they want to reduce light pollution.

The research team let the brightness of street lights in Changing Tucson and watching how this affected the brightness of the city from space. On ten days in March and April 2019 city officials changed the brightness for about 14. 000 of the 19 . 500 Street lights. Usually, most of the street lights in Tucson come on in the evening with 90 percent of theirs possible illuminance switched on and at midnight on 60 percent dimmed. During the experiment, the lighting was instead switched on for a few nights except for 30 percent dimmed, in others to 100 Percentage increased.

Cloud-free nights Tucson’s lights were observed by the Suomi National Polar-orbiting Partnership (NPP) satellite. He captured cloud-free images of Tucson for four nights during the test and two more nights after that. The research team also measured the brightness of the sky over Tucson from the city. By comparing the brightness of the city in the six different nights, the researchers found that in a normal night about 20 Percentage of the light in the satellite images comes from street lamps.

Night sides of the earth (25 Pictures) The earth at night. In order to get a cloud-free image, images from the satellite Suomi NPP were taken from April to October 2012 combined with each other. 2.5 terabytes of data were collected for this.

(Image: NASA) The changes in brightness are barely noticeable for passers-by, as their eyes quickly adapt to the light level, the researchers write. During the test, there were no comments or complaints about the changed lighting. There is also no evidence of negative effects on public safety.

Christopher Kyba from the GFZ, who led the team, suggested to: “Instead of dimming the lighting to the same level every late night, a city could instead switch to 45 percent and on odd days to 55 Dim percent. ” The urban population would not notice a difference, but then it would be measurable how the contribution of the different types of light changes over time.

NASA animation with pictures from the year 2016.

(Source: NASA) (anw)