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 German Earth Science Research Center of GFZ 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 10 days in March and April 2019, Tucson officials changed the illumination settings of approximately 14,000 of the city’s 19,500 street lights. Under normal circumstances, most street lights in Tucson have only 90% of their maximum possible illuminance at the beginning, 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 six 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 research report, Dr. John Barentine of the International Dark Sky Association, said the results of the research 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 checked how changes in the illuminance 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 showcases, illuminated signs and facades, or stadium lighting. . Therefore, when reducing light pollution, national and local governments need to consider more than road lighting.
According to the researchers, because our eyes quickly adapt to the light level, the difference in the brightness of the street lights at different nights is almost impossible to perceive the people on the street. They reported that the city received no comments or complaints about lighting changes during the test, and there was no evidence or evidence that as part of the experiment, lowering the lighting level had any adverse effect on public safety.
Therefore, Kyba is excited about the idea of conducting such experiments more often in other cities. Kyba suggested: “A city can even be dimmed to 45% on even-numbered days and 55% on odd-numbered days, instead of dimming the lights to the same level in the middle of the night.” “City residents will not notice the difference, but This way we can measure how the contribution of different types of light changes over time."
