When satellites take pictures of the earth at night, how much of the light they see comes from street lights?

When satellites take pictures of the earth at night, how much of the light they see comes from street lights? A group of scientists from Germany, the United States and Ireland answered this question for the first time using Tucson, the United States. The result: Only about 20% of the light in the Tucson satellite image comes from street lights. The research was published today in the journal Lighting Research and Technology.

The research team conducted an experiment in Tucson, Arizona, USA, by changing the brightness of street lights to observe how this changes the brightness of the city from space. Dr. Christopher Kyba from the GFZ German Earth Science Research Center led the team that conducted this experiment. He said that this work is important because it shows that related technologies in smart cities can be used to conduct city-scale experiments. "After installing sensors and control systems throughout the city, it is possible to change the way the city operates, and then measure the impact of this change on the environment, and even the impact of outer space," Kyba said.

During the 10 days in March and April 2019, Tucson officials changed the illuminance settings of approximately 14,000 of the city’s 19,500 street lights. Under normal circumstances, most street lights in Tucson start with only 90% of their maximum possible illuminance, and at midnight it will be reduced to 60%. During the experiment, the city dimmed the lights to 30% on some nights and brightened it to 100% on other nights. The Suomi NPP satellite from the United States observed the lights of the city. The satellite is famous for taking a global light map at night. During the test, the satellite captured cloud-free images of Tucson on four nights, and the other two nights after the test under regular lighting. By comparing the brightness of the city over 6 nights, the researchers found that on a normal night, only about 20% of the light in the Tucson satellite image came from street lights.

The co-author of the study, Dr. John Barentine of the International Dark Sky Association, said the results of the study have important implications for sustainability. In the second experiment conducted at the same time, Barentine, Kyba and their co-authors measured the brightness of the sky over Tucson from the ground. They examined how changes in the illumination of street lights affect the brightness of the sky, and the results showed that, like the light seen from space, most of the sky brightness over Tucson was caused by other sources. "Combined together, these studies show that in cities with well-designed street lights, most of the light emissions and light pollution come from other light sources," Barentine explained, including bright display windows, illuminated signs and facades, or stadium lighting. . Therefore, when reducing light pollution, national and local governments need to consider more than road lighting.