Research network publishes practical guidelines for exterior lighting

Fairy lights, streetlights and advertisement displays: Especially in December we light up the dark season with all sorts of exterior lighting. It is a known fact that this illumination not only creates a seasonal atmosphere, but also has negative consequences for man and nature. An international research team, which has worked together for four years as part of the European "Loss of the Night" network, is now for the first time publishing practical guidelines for exterior lighting. They are intended to contribute to the sustainable use of light in our cities - for the benefit of humans and the environment.

"Light has effects," says coordinator of the network Sibylle Schroer, "on plants and animals as well as on us humans." Even small quantities of artificial light at the wrong time can disturb the inner clock, the release of hormones or even interfere with whole ecosystems. Predator-prey relations get mixed up, and nocturnal species increasingly lose their habitats. "In the long term, this can affect biodiversity," says Sibylle Schroer who works as a scientist at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin.

Nevertheless, we tend to illuminate our cities intensively - and light at night is increasing rather than decreasing, as scientists have been observing at a global scale for numerous years.

How this situation can be improved is outlined by practical recommendations for action, which the researchers have now published:

Avoid cold white light with wavelengths below 500 nanometres as far as possible: The trend towards cold white light in the form of LEDs and energy saving lamps is critically discussed by the scientists. The high proportion of short-wave blue light disturbs the circadian rhythms of higher vertebrates as well as humans. Chronic shifts can interfere with sleep, metabolism and immune defence, leading to serious health problems. Possible consequences are suppressed fat catabolism, diabetes or depression. Outdoor lighting should have therefore a colour temperature of 3000 Kelvin or lower recommend the researchers.

Use lampshades to direct the light where it is needed: Suitable lampshades do not allow the light to be seen directly. Properly directed light does not shine in the eyes of pedestrians, in windows or into the sky, where it spreads quickly and illuminates entire landscapes. Light - including façade lighting - should always shine downwards. For example, ground-recessed spotlights that radiate the light upwards should be avoided.

Illuminate streets with the lowest possible intensity: Rural roads should only be illuminated if absolutely necessary. Where needed, an average luminance of 0.3 candelas per square meter should not be exceeded, which typically corresponds to an illuminance of 4 lux. This complies with the lowest class of the EU standard for street lighting (EN13201). "EU standards recommend much brighter values than they are currently implemented in most municipalities," reports Sibylle Schroer. Compliance with the standards across Europe could therefore lead to drastically higher energy consumption and CO2 emissions in road lighting.

Adapt exterior lighting to times of use. After 10 o'clock or after midnight, considerably less light is needed, such that street lights can be dimmed. A reduction of illumination levels by 50-80% is possible. "Private lighting should be turned off completely if there is no immediate need" recommends Sibylle Schroer.