Bat Friendly Lighting In Denmark

Red street lighting along Frederiksborgvej is designed to protect biodiversity

Gladsaxe Municipality in Denmark switches to red light to limit behavioural effects on bats and other nocturnal and light-shy species along Frederiksborgvej near Skovbrynet. This design is based on studies on bat-friendly lighting, which show that red light is less disruptive on wildlife while allowing for people to find their way and even maintaining peoples’ dark adaption for nighttime viewing.

“Overall, we hope that everyone welcomes the new lighting and that the red light not only has functional value, but also symbolic value. The red light should make passers-by aware that this is a special natural area that we want to protect,” says Philip Jelvard, Lighting Designer at Light Bureau.

This project is part of Gladsaxe Municipality’s implementation of UN Sustainable Development Goals with an aim to take the lead in ensuring the best conditions for both animals, humans and biodiversity.

As humans we have become accustomed to using light to prolong the day wherever we need it. The illuminated night makes us feel safe and we are inclined to feel that more light is better. However, every time we use outdoor lighting we are intervening in systems, disrupting rhythms, and shifting balances. Spill lights from a stadium will drown out the stars in the sky making it harder for migratory birds to find their way. Cold streetlight shining into a bedroom disturbs the circadian rhythm of its inhabitants. A streetlamp that attracts insects will provide better hunting possibilities for the species of bat that are bold enough to fly high than its more timid fellow species.

As lighting consultants, it is our responsibility to understand how light affects our surroundings, and how we can use current technologies to adapt our lighting solutions to have the lowest possible negative impact. We call for a more balanced approach to when, where and how much light we use.

Light pollution, circadian rhythm, and biodiversity

Light pollution has become such an integral part of our nighttime reality, that we don’t notice it anymore. The spectacle of the milky way spread out across the night sky is an experience that most people in the western hemisphere regards as extraordinary. To humans it is natural heritage lost or rarely experienced. To migratory birds it is confusing, and to sea turtles even deadly when the night sky doesn’t match with what is coded into their genes, and the nearby streetlight is mistaken for the full moon.

We know that sound pollution from a busy road shortens the life span of people living nearby. Even though the effects of lighting pollution are not as obvious or thoroughly researched we need to consider if we should seek to limit light pollution that disturbs our circadian rhythm. Streetlight and other light sources may not be the main culprit, but when all other light sources in our homes are turned off, it is the trespassing outdoor lighting entering our bedrooms that effects our circadian rhythm and sleep pattern.

Any artificial light will affect behavioural patterns of nocturnal animals to a certain extend. Either directly, because they seek or avoid the light, or indirectly because their pray, potential partners or enemies does. Different species reacts differently and as with all other human interference, a change also causes an adaption. We need to attempt to analyse and understand the changes we generate in both the short and the long term to make sure that the light we introduce has the lowest possible impact on the biotope.

For our own sake and for the sake of our environment we need to consider our use of lighting to avoid light pollution, impact our circadian rhythm as little as possible and lower the amount of stress we impose on natural systems.

The colour of light and its influence

Using narrow spectrum red light is a rarely seen approach in outdoor lighting. We are used to preferring as broad a spectrum of light as possible to ensure a good colour rendition and get as close to daylight as possible without considering any negative consequences that this might have.

Nocturnal animals are active in the light from the moon and the stars, which does not contain the red wavelengths and therefore their eyes have become highly sensitive to the “cold” wavelengths, while red wavelengths are invisible or only barely visible to them, in the same way that infrared and ultraviolet are to humans. The use of narrow spectrum red light can therefore be used to lower, though not removing, the environmental impact from a lighting installation at night. As a bonus the human night vision is affected less by red light than by full spectrum light. We can maintain the ability to perceive the night sky and effectively read our surroundings in the narrow spectrum red light.

The red light can, in some if not all situations, be used to accommodate the human needs. We can find our way with minimum impact on our night vision, circadian rhythm, and the natural environment. However, the use of narrow spectrum red light is not a one-size-fits-all solution. No two projects are the same. There are always compromises between the need for human safety, way finding, and the impact on the environment. The important thing as that we analyse and understand the effects light can have on our surroundings and implement these results in our projects to mitigate their negative impact.

When you know better – you do better

When we use light outdoors, we need to consider who we are helping, who we are harming and how we can best help and harm the least. We need an analytical approach to understanding the impact of lighting on the individual project and on the specific biotope. When we have decided what needs are to be satisfied and what negative impacts are to be avoided, we can use the available technology to reach our goals. With proper knowledge of modern fixtures, sensors, light controls, programming, and wavelength specific LEDs we can design lighting solutions with as low impact as possible and still fulfil the needs identified.

The progression towards a more sustainable future is a brilliant opportunity to test out new solutions and new collaborations across disciplines. We need to put our common knowledge to good use and start sharing the night.

Photo Credit Rune Brandt Hermannsson

Written by Rune Brandt Hermannsson, Designer and Visualiser at Light Bureau

AFRY