6 May 2026

Light pollution

By Katharina Zeuner|

When light ends up where it doesn’t belong

Living in Stockholm has its perks – one of them is having a ski slope within walking distance. It’s so close that I can see it from our living room. In the evenings, the floodlights illuminating the slopes are glaringly visible from our dinner table.

Those lights don’t only illuminate the slopes. They also spill into the surroundings, to the point where even my colleague on the neighboring island can see them. This is a prime example of light pollution. Light pollution is artificial light that is (intentionally or unintentionally) present in otherwise dark conditions. It can take many forms:

Glare, reducing visual comfort and contrast
Skyglow, the bright haze above cities
Over‑illumination and light clutter, distracting or obscuring what we actually want to see

It is a by‑product of urbanization and has increased rapidly in recent decades. Today, 99% of Europe’s population and biodiversity live under light‑polluted skies [1].

LEDs – part of the solution, part of the problem

Since around 2010, LED lighting has largely replaced incandescent, fluorescent, and halogen lamps. LEDs use less energy and last much longer – which is great. However, there is a rebound effect. Because lighting becomes cheaper to operate, we tend to install more of it or keep it on for longer. The result is more artificial light at night and increased skyglow, both in brightness and geographical reach. Satellite data shows that between 2011 and 2022, the average night sky became about 10% brighter every year – meaning it doubles in brightness roughly every eight years [2]. These numbers might be underestimating the problem as spectral response of the used radiometer doesn’t cover the full LED spectrum [3].

Why does light pollution matter?

Light pollution isn’t just an annoyance for people who enjoy dark evenings. Almost every species studied is affected by artificial light at night.

Wildlife: It disrupts daily and seasonal rhythms such as migration, feeding, hiding, and reproduction, and weakening ecosystems in the long run [4]. Just to name a few examples here:
- Fish and zooplankton as deep in the arctic sea as 200 m can react to lights from ships in the otherwise pitch-black polar night [5].
- Artificial light can interfere with natural cues to start the breeding season of wild mammals [6].
- The leaf morphology in herbaceous plants can change in the presence of LED lights, leading to reduced net photosynthesis [7].
Humans: Artificial light can disturb circadian rhythms, affecting sleep and health. More light does not automatically mean better visibility or safety – Bright lights can cause visual discomfort and harsh contrasts can make objects outside of the brightly lit area harder to see.
Astronomy: For hobby astronomers (and professionals alike), light pollution makes observing the night sky increasingly difficult, even in remote areas.

Light is an environmental pollutant – but unlike many others, it can be reduced immediately by changing how we design and use lighting.

For optics nerds: what can we do about it?

If you design illumination systems, the good news is that many mitigation measures are technically straightforward.
Key principles for responsible outdoor lighting [8]:

Direct the light: Avoid upward emission. Shield luminaires and minimize emission near the horizon (especially 90–100°).
Use adaptive controls: Dim or switch lighting based on traffic or occupancy.
Choose spectra carefully: Short wavelengths scatter more and contribute more strongly to skyglow and circadian disruption.
Use shielding where high intensities are needed: For example, in greenhouses, to protect surrounding ecosystems.

DarkSky.org provides a clear infographic summarizing these principles:

Principles for Responsible Outdoor Lighting, from DarkSky.org.

In essence: Illuminate only what is needed, when it is needed, and no brighter than necessary.

A simple illustrative calculation

Let’s look at a rough calculation for the ski slope example:

30 floodlights
6 lamps each
500 W per lamp
30% of the light misses the target

That results in:
30 × 6 × 500 W × 0.3 ≈ 27 kW of wasted power.

With 6 hours of operation per day over 90 days, this amounts to roughly 15 MWh per season. If we consider that one medium-sized household/apartment consumes on average 3000 kWh per year (according to Vattenfall). So instead of unintentionally illuminating the sky, we could use that energy to supply a few households with electricity for a year.

Light pollution and sustainability

While there are currently no centralized EU regulations specifically addressing light pollution, momentum is building through research projects and policy initiatives [1], [9]. These projects have already produced a policy brief that provides strategic guidance for the European Union on how to acknowledge and address light pollution. As sustainability is a topic that is close to our hearts here at Eclipse, we also want to summarize how light pollution or rather its prevention touches all 3 pillars of sustainability:

Environmental: Protects wildlife, especially in nocturnal ecosystems
Economic: Reduces wasted energy and operating costs
Social: Improves visual comfort and can support better sleep and well‑being

If you enjoyed this article, keep an eye out – more content on optics and sustainability is coming soon.

Light pollution

When light ends up where it doesn’t belong

LEDs – part of the solution, part of the problem

Why does light pollution matter?

For optics nerds: what can we do about it?

A simple illustrative calculation

Light pollution and sustainability

Further reading

Stockholm (HQ)

Social

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