The Why & the How:
Dimming urban light pollution

Excessive artificial light in cities is not only annoying and unpleasant, it’s harmful to human health and natural systems. Fortunately, it's one of the easiest types of pollution to tackle

May 2023

Words by Annukka Larsen

“Ultimately, light pollution is wasted energy, and we need to address it to deliver many of our shared aims, from meeting commitments on climate change, to improving public safety”


The modern city lives and works around the clock, and after dark it depends on good lighting. Street lights provide comfort, safety and a sense of security, and enable us to get around, whether by foot, bike, car or public transport. But as the amount of light in the urban environment has increased, we’re also seeing a number of detrimental effects.

The International Commission on Illumination (CIE) defines obtrusive light as “light directed above the horizontal and to directions not needed”. It can take the form of a dazzling glare from a private yard or construction site, illuminated advertising screens or facades, or over-lit sports grounds or public spaces. This is unpleasant and irritating, and makes it difficult for people to take in essential information about their environment. In other words, light pollution can undermine the sense of comfort and safety that it is expressly intended to convey.

Light pollution can also have more serious consequences for our health. Light is the most important factor in our circadian rhythms, regulating our internal clock. Exposure to too much light at night disrupts production of the sleep hormone melatonin, and has been associated with many adverse affects on our physical and mental wellbeing, from sleep deprivation, fatigue and anxiety to an increased risk of cancer. Nature suffers too: 28% of vertebrates and 64% of invertebrates are nocturnal, and a significant proportion of pollinators are night-active. Light interferes with migration patterns, the growing seasons of plants and the health of natural habitats. Clouds are not supposed to glow at night.

Happily, controlling obtrusive light is a relatively quick win — this is one of the easiest environmental pollutants to eradicate. Street and road lighting are typically assumed to be the greatest contributors, but research has shown that advertising, facades, accent lighting, and parking and sports facilities all provide a significant share. A 2020 experiment in Tucson, Arizona saw scientists dim 14,000 points of light in the city’s street network, and then use satellite images to examine changes in its radiance. They found that streetlights accounted for just 13% of the total visible light after midnight.

Ultimately, light pollution is wasted energy, and we need to address it to deliver many of our shared aims, from meeting commitments on climate change, to improving public safety, to supporting healthier communities and ecosystems. A more harmonious nocturnal landscape also means brighter, starrier skies — and the chance to reconnect city dwellers with a beauty our ancestors once took for granted.

The municipality in Helsinki commissioned WSP to carry out a study of obtrusive light in the city. Photo: Annukka Larsen/City of Helsinki


Direct light only where it is needed

Before installing or replacing a light, ask whether it’s really necessary. Could reflective paints or luminous markers be used for signs, curbs or steps? Where greater illumination is required, direction and distribution are crucial. Light does not respect boundaries, so think about how it could spill into other areas, and the potential affects on neighbours, wildlife and the sky. When choosing a luminaire, specify a model that does not produce light beyond its own horizontal plane — opal-shaped pole and wall luminaires should be avoided, as should horizontal wall projectors.

Photo: Annukka Larsen/City of Helsinki

Manufacturers should be able to provide information on the gamma angle and upward light ratio, as well as disability and discomfort glare. Advertising screens, facades and accent lighting should always be viewed from several directions to ensure that there is no overspill.

Embrace the dark

As part of an obtrusive light study commissioned by Helsinki’s city government, WSP examined whether it would be possible to create dark zones in less populated areas to allow observation of nature, the landscape or the starlit sky. Even where complete darkness was not desirable, we found that light levels could often be drastically reduced. Most LED screens were too bright, with the luminous intensity of some measured to be as high as 3,000 candelas (cd) per m2, compared with a permitted maximum of 300cd/m2. We recommended better monitoring and new, lower limits between 120 and 200cd/m2 in the city centre and regional and business centres.

It is also sensible to use warmer colours where possible. Blue tones have shorter wavelengths, which reflect off air particles and scatter light more widely. With this in mind, Helsinki has decided to use warm 3,000 Kelvin white light for street lighting.

"Light does not respect boundaries, so think about how it could spill into other areas, and the potential affects on neighbours, wildlife and the sky"

Light smarter

Modern technology helps to control lighting better than ever before. Controls such as timers or motion detectors can ensure light is available when it is needed, dimmed when possible, and turned off at all other times. For example, traffic lights can be dimmed at night when less contrast is required. Or in areas close to nature, routes can be illuminated to a suitable level when people are using them, but otherwise left in darkness, which is beneficial to both animals and plants.

Yet many areas still rely on outdated lighting methods. The Helsinki study found that the most problematic areas were private yards and parking areas, some of which still used mercury and sodium lamps, which don’t switch off or fade. In some instances where they had been updated, much brighter LED floodlights had been installed on the existing poles, causing glare into surrounding homes.

Photo: Annukka Larsen/City of Helsinki

Establish guidelines — and make sure they’re followed

Urban lighting suffers from poor design but also from a lack of knowledge or instruction about how to do things better. In Helsinki, we used an aerial masterplan and terrain surveys to map the lighting used in different areas and designate them with one of five environmental, or E-classes, for target light quality — E4 being the brightest and E0 the darkest. These were developed in accordance with CIE guidelines. The city can use these E-classes as a basis for guiding stakeholders towards better lighting practice, and also as a means for developing its own monitoring system. By following the values of the E-classes and accurately mapping the city’s luminosity, this pervasive form of pollution can be largely eliminated, and the night sky can return to the urban realm.

Annukka Larsen is an urban lighting designer and team leader in WSP’s design studio in Finland

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