On Saturday night, April 10, 2021, the public lighting in the whole of Brno was turned off for several hours. The aim of the event was to find out how much Brno suffers from light smog. In addition to aerial photography, Petr Baxant from the Faculty of Electrical Engineering and Communication Technologies at BUT also took part in the observation. The first results show that although public lighting creates light pollution, the regulation of lighting in private buildings and premises could have a much greater result.

"By turning off the streetlights, the brightness was reduced by about 45 %. It is not much, but we must also consider that public lighting is regulated and at night it was no longer at 100 % of the evening maximum. It turned out that the level of the night light is more influenced by entities outside the Brno city administration. We will be interested in how many of them are dominant and which they are. We would then like to focus on these in the next measurement," Petr Baxant summarizes some of the data.

The resulting images are coloured in the so-called pseudo-colours, which is a colour scale of brightness. The human eye is sensitive to brightness and perceives contrasts in particular. The brightness of a computer monitor or telephone display is in the order of hundreds of candelas per square metre (candelas is a unit of the luminosity of the source), while the surface of the full moon has a brightness of about 3500 cd/m2, which is a thousand times greater than the brightness of Spilberk Castle under architectural lighting. The artificial illumination of such a façade is about twenty times greater than the natural illumination of the Moon.

"Round fisheye images show us a view of the zenith, and therefore the entire upper hemisphere. Here we have very interesting numbers from the calculation of illuminance caused by a low clear sky. At the measurement site, there was only 0.008 lx of light intensity when the public lighting was off. This is a value about thirty times smaller than we would measure with a very bright full moon," Baxant points out that even a bright moon can interfere with the measurement. His team's research is aided by a brightness analyser they build, which allows the Moon and its brightness to be read and replaced by background. A conventional lux meter does not allow such a measurement. At the time of Saturday's measurement, however, the Moon was not visible.

"Saturday's attempt was truly unique. We were ready and we managed it. Thanks to this, we will start the next stage of research. The public might think that these are small numbers and we may not be interested in the topic. We will probably look in vain for a truly zero light background, or rather what the stars and natural light from the planet's atmosphere create for us. However, we would like to at least get closer to it and find out how we are doing in the conditions of the Czech Republic and towards other places on the planet," Petr Baxant concludes with a positive outlook.