Research was published January 20, 2024, in the journal Lighting Research & Technology, about the potential benefits and risks of using red lights outdoors at night to reduce light pollution and its impacts on wildlife and humans. The paper is titled, Red light to mitigate light pollution: Is it possible to balance functionality and ecological impact?
Some European countries have adopted regulations promoting red outdoor lighting to protect bats (Habitats Directive 92/43/EEC). Recent guidance (GN08/23) by the Bat Conservation Trust and the Institution of Lighting Professionals suggests using red light to reduce adverse impacts on some bat species. A small Dutch town recently became the first town in the world to adopt all red street lights to protect bats and reduce light pollution (story here).
The researchers were Alp Durmus with Penn State University, AK Jägerbrand with University of Gävle (Sweden), and MN Tengelin with RISE Research Institutes of Sweden. The researchers conducted a literature review on the use of red light to reduce light pollution impacts. Their overall finding is that there is not sufficient scientific understanding of the potential ecological consequences of red light, the challenges created for human lighting design, nor the broader impacts of red lights on diverse species.
The recent push for red outdoor lights at night, in Europe, are because red lights are believed to create less sky glow and glare, and less negative impacts on insects and bats. Red light is believed to create less suppression of melatonin, reducing disruption of circadian rhythms for humans and wildlife. Other potential benefits of red outdoor lights include:
- Red light preserves human night vision, potentially reducing glare and preserving visual capabilities in low-light conditions.
- Some insects, such as nocturnal moths, are not spectrally sensitive to longer wavelengths, attracting less bats.
The researchers found that while some bat species are less impacted by red lights, other bat species have a variety of behavioral responses to red lights indicating benefits may be species specific and not generalizable to all bats. Some migratory birds have their magnetic orientation disrupted by red lights, causing disorientation, circling the lights, and potential increased risk of collisions. Some other bird species have also been observed to have their daily routines disrupted by red lights.
It’s also known that narrow band red light has very poor human color rendering, and reduces object identification, including faces. This may create human safety risks. Red light also has much lower luminous efficacy, in large part due to the lower ability of the human eye at seeing red photons vs. blue or green. This is built into the definition of the lumen and the spectral sensitivity function. As a result, the paper suggests that a red street light produces roughly half the luminous flux of a white street light, consuming similar power.
The paper concludes by recommending a very cautious approach before adopting red outdoor lighting at night, for all the reasons above. The full paper can be found here.
Top image: Rya Åsar, Borås, Sweden
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