Why Denmark Is Turning Its Streetlights Red
In a suburb of Copenhagen, city planners replaced ordinary white streetlights with a deep red glow - not for decoration, but to stop a local bat colony from disappearing. It's a small, strange-looking intervention with a serious point: light color, not just brightness, determines how much an artificial night disrupts the natural one.
In a suburb of Copenhagen, city planners replaced ordinary white streetlights with a deep red glow - not for decoration, but to stop a local bat colony from disappearing. It's a small, strange-looking intervention with a serious point: light color, not just brightness, determines how much an artificial night disrupts the natural one.
Overview
On a February evening in 2026, drivers entering Gladsaxe, a municipality just outside Copenhagen, found a stretch of Frederiksborgvej bathed in a deep red glow where ordinary white LED streetlights used to be. It wasn't an art installation or a malfunction. It was a deliberate, carefully targeted piece of lighting engineering, aimed at solving a problem most cities never think to address: the streetlights themselves were quietly wiping out the local bat population.
The problem with white light and bats
Bats hunt at night specifically because there's little competition from other predators and because darkness gives them cover. Artificial white light, rich in the shorter, blue-leaning wavelengths that scatter furthest and travel brightest, disrupts that cover badly for certain species. Research led by ecologist Kamiel Spoelstra at the Netherlands Institute of Ecology found that species like the common pipistrelle and the brown long-eared bat - both present along the Frederiksborgvej corridor - become significantly less active under white and green artificial light, likely because they feel exposed to predators like owls under bright conditions. Not every species reacts the same way: more agile flyers actually exploit streetlights, since the light concentrates insects into an easy meal, and larger species that fly higher than street level are largely unaffected. The disruption is specific to certain bats, in certain lighting conditions, which is exactly why a blanket "just make it darker" policy wasn't the right fix.
Why red light solves it
Red light sits at the long-wavelength end of the visible spectrum, and Spoelstra's field research found that the sensitive bat species remained just as active under red light as they were in true darkness, while white and green light suppressed their activity substantially. Red light also scatters far less than blue-rich white light, meaning it lights the road surface for cyclists and pedestrians without spilling as far into the surrounding tree canopy where the bats roost and hunt. Gladsaxe's installation goes further than a simple color swap: the fixtures create alternating pools of red light and darker gaps along the corridor, rather than flooding it uniformly, so wildlife can move through the space without ever being fully exposed.
A signal as much as a solution
Lighting designer Philip Jelvard, who worked on the project for Light Bureau, has described the color choice as functioning as a form of public signage as well as an ecological fix - the red glow tells residents and commuters that they're passing through a protected natural corridor, not just a badly-lit street. That framing matters for public acceptance: reactions in Gladsaxe and in Odense, where a similar orange-red installation now runs along a 1.3-kilometer riverside path, have been mixed. Some residents find the dimmer, colored light unsettling after decades of bright white streetlighting; others report enjoying being able to spot wildlife on their evening commute for the first time.
The bigger picture
The project sits inside a larger EU-funded initiative called Lighting Metropolis – Green Mobility, spanning Danish and Swedish cities and aiming to upgrade more than 50,000 streetlights with ecologically-informed lighting strategies. It's also a straightforward cost story: LED red-spectrum fixtures use less power and require less maintenance than the sodium and halogen lamps they typically replace, which means the ecological win doesn't come at the expense of the municipal budget. Conventional lighting is estimated to account for roughly 15 percent of global electricity consumption and around 5 percent of global carbon emissions, so even a small shift in how streetlights are specified - not brighter or dimmer, but a different wavelength, applied only where it's needed - adds up. Gladsaxe's mayor has described the ambition plainly: to make the project a model other municipalities can copy, proof that protecting nocturnal ecosystems and lighting a street safely aren't actually in conflict.
Want this kind of execution support around your project?
GridLux helps teams move from fragmented requests into cleaner product, testing, operational, and go-to-market workflows.