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Jul 16, 2023

LED streetlights are increasing blue

The shift to LED street lighting is producing more blue-light pollution – an

The shift to LED street lighting is producing more blue-light pollution – an important trend that has not been noticed by the specialized satellites that monitor nighttime lighting. That is the conclusion of researchers in the UK, who have analysed digital photographs of Earth taken by astronauts on board the International Space Station (ISS). The scientists say that the shift to bluer light is having negative consequences for human health, animal behaviour and astronomy.

LEDs have been around for 60 years, but older devices operated towards the red end of the visual spectrum. In the 1990s, however, bright blue LEDs became available – winning their inventors the 2014 Nobel Prize for Physics. The ability to create bright blue LEDs quickly led to the development of white LEDs, which are becoming ubiquitous in many lighting applications.

Indeed, LED streetlights have begun to replace sodium lamps – which produce yellow light – in many European countries. As well as offering lower cost and higher energy efficiency than sodium, LEDs provide better colour rendering, which improves an observer's recognition of illuminated objects.

However, researchers point out that this rollout has a darker side. Previous studies have shown that the amount of light pollution is increasing with the introduction of LEDs. Furthermore, this LED light is much bluer than sodium light and previous research shows that nighttime exposure to blue light can have negative effects on people's circadian rhythm and sleep. There is evidence that blue light can change the behaviour of some insects and it also exacerbates the problem of light pollution on the night sky – making stars more difficult to see for both the public and astronomers.

Now, a team of scientists at the University of Exeter in the UK, led by Alejandro Sánchez de Miguel of both Exeter and Universidad Complutense in Madrid, Spain, has for the first time quantified the increase in blue light pollution in Europe.

"There's no reason why we use LEDs that are so blue-rich," Sánchez de Miguel tells Physics World. "Their energy efficiency is a bit better, but when dimming and directionality, as well as good lighting design, are considered, then that point is irrelevant."

Typically, satellite data on nighttime lighting considers all wavelengths of visible light together and does not differentiate between red, green and blue. Sánchez de Miguel's team instead used images captured by ISS astronauts using everyday digital single-lens reflex (DSLR) cameras and combined the images with data from satellite sensors. This builds on previous work by Sánchez de Miguel's team, which showed that camera images from space capture colour and radiometric data better than Earth-monitoring satellites.

They compared camera images of Europe taken from the ISS in 2012 and 2013 with images taken between 2014 and 2020, when the LED revolution began to take hold. They found a clear increase in blue light coming from the continent. Of the European nations, the increase was most prominent in Italy, Romania and the UK, while the effects of blue-emitting white LEDs were least prominent in Austria and Germany.

Regarding the differences between nations, part of the reason is that some countries have older lighting that needed replacing more urgently, while for others Sánchez de Miguel sees it as a cultural difference.

"Italy is changing too fast," he says. "I do not know well enough the reasons for Romania, but maybe it is because their street lighting was old, as happened with the UK."

As a result of the study, Sánchez de Miguel believes that studies of nighttime light pollution have underestimated the potential harmful effects of LEDs, simply because up until now, all monitoring of light pollution with satellites has not been colour specific.

Reclaiming the stars

Ruskin Hartley, spokesperson for the International Dark-Sky Association, agrees. "Over the past 25 years, it is clear that Europe has got brighter and bluer in a rush to transition to energy-efficient LED nighttime lights," he tells Physics World. "Unfortunately, the quality of the nighttime environment has suffered, and we continue to waste massive amounts of energy through wasted nighttime lights."

Sánchez de Miguel points out that artificial nighttime light is considered a pollutant by the United Nations and says, "The best way of limiting it is by controlling its use. There are simple ways to do this in the [urban] planning phase, and also we have simple ways to measure it with DSLR cameras."

Hartley agrees that some of the problems can be mediated by better planning and better lighting design. "When used with care and attention, a well-designed LED night light system has been shown to reduce light pollution and save energy," he says. "We recommend that anyone considering an outdoor lighting project follow the joint IDA–IES Five Principles for Responsible Outdoor Lighting. A holistic approach to outdoor lighting will ultimately result in improved visibility, a healthy and undisrupted nocturnal habitat, and darker skies."

The research is described in Science Advances.