Light emissions and light pollution, worldwide

The term light emission (light pollution, light smog or light dome) describes the artificial brightening of the night sky caused by the scattering of light from light sources in the lower layers of the atmosphere. It thereby impairs the darkness of the night sky, and is therefore also known as light pollution.

The following are the main types of light pollution:

• Light trespass: Unwanted light illuminates the area surrounding one’s property
• Glare: Temporary visual disturbance caused by a direct light beam
• Skyglow: A glow effect over cities caused by excessive use of artificial or natural light.

The leading causes of light pollution are big cities, industrial plants and commercial leisure facilities, which light up the night sky due to street lighting, neon signs, floodlights and Skybeamers. Street lighting is responsible for approx. 50% of skyglow. Light pollution is therefore a consequence of industrialisation, which means it mainly occurs in the densely populated areas of western industrial nations. In Europe, more than half of the population is currently affected by light pollution. The annual growth rate of light pollution averages at 6% worldwide. In Germany it is around 6%, in Italy 10% and in other countries such as Japan up to 12%. The peak growth values are measured in fast-growing metropolitan areas.

Light pollution can have a disruptive effect on flora, fauna and humans, as well as negatively impacting astronomical observations of the night sky. A town with a population of 30,000 lights up the sky in an area with a radius of about 25 kilometers. The number of stars visible to the naked eye in areas with high light pollution is only two to five hundred at most, whereas it is up to three thousand at observation sites with less light pollution. Due to the very high level of light pollution in industrialised countries, the second half of the twentieth century saw several large astronomical observatories built in remote places far from the industrialised world, such as Chile, Hawaii or the Canary Islands. It is not only optical systems that are disturbed, however – many animals also suffer the consequences of nighttime illumination. The widespread presence of white light sources with a high proportion of blue in the spectrum can pose serious problems for the navigation or orientation of nocturnal insects and migratory birds.