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The impact of LED technology on light pollution in Montréal

Light pollution map observed at night by the Suomi NPP satellite in April 2021.
Credit: Radiance Light Trends
Carte de la pollution lumineuse perçue la nuit par le satellite Suomi NPP en avril 2021.
The impact of LED technology on light pollution in Montréal

With technological breakthroughs and the growth of big cities around the world, we’ve gradually erased the beauty offered by the night sky from our daily lives. It’s a distressing habit of ours to sacrifice our common well-being without realizing it when we make rapid technological advances. Some other recent examples that come to mind are the use of concrete for covering large areas, the removal of most of the trees in certain big cities, the use of fossil fuels without consideration for its environmental impact, or even the use of X-rays for picking out a pair of shoes (yes – that was really done in the 1940s).

And while we were waking up to issues like air quality and noise pollution, only the last fifteen years or so have brought the realization that light pollution also has negative impacts. Not only does light pollution considerably interfere with amateur and professional astronomers in their observation of the sky, it also seems to have adverse effects on wildlife, plant life, and on human health in general. Among other things, think of the consequences of intrusive light in our homes for our circadian clock. Intrusive light can inhibit production of the hormone melatonin late in the day, which in turn can delay the time when we fall asleep and potentially shorten our night or diminish the quality of our sleep. And as the American neuroscientist Matt Walker recently pointed out, there’s been a rapid increase in the number of scientific studies demonstrating that reduced amount or quality of sleep affects nearly all known aspects of human health.

Light pollution before and after the arrival of LED lights

A new chapter in the history of light pollution is being written with the rapid adoption of the technology of light-emitting diodes (LED). Many cities and industries are adopting this new technology, which is more flexible and less expensive to maintain. Montréal is no exception to that rule. The Ville de Montréal’s urban planning and mobility department began adopting LEDs around 2018 rather than carrying on with old technologies like high-pressure sodium bulbs. In the summer of 2020, our team at the Planétarium Rio Tinto Alcan decided to test light pollution in a quantitative way before and after the arrival of these new LED lights.

We thus focused on the Radiance Light Trends database, which hosts images of the Earth taken by the VIIRS camera onboard the Suomi NPP satellite. With the collaboration of the urban planning and mobility department, we targeted 32 regions on Montréal Island where new LED lighting was installed around 2018. We then extracted the VIIRS’s historical data from between 2012 and 2020 for these regions as a whole.

Location of all lighting fixtures controlled by the Ville de Montréal.

Location of all lighting fixtures controlled by the Ville de Montréal.
Source: Ville de Montréal urban planning and mobility department.

Interpretation of these data required a complex and detailed analysis. The VIIRS camera is not sensitive to blue light, whereas the human eye is very sensitive to it at night. As it happens, the sensitivity of the human eye to different colors varies in different lighting situations, which is an evolutionary adaptation to bluer light reflected by the Moon during the night. In addition, the VIIRS camera captures an aerial view image of Montréal Island and detects all the light that is emitted directly towards the sky. This is a very interesting measurement, but we are more interested in measuring the light deflected by our atmosphere (diffused is the technical term) that returns towards the planet’s surface and has the potential to reach an observer on the ground. Fortunately, it was possible to take all these effects into account (in addition to seasonal variations caused by snow on the ground), given that we’re well acquainted with the detailed properties of the bulbs used by the city before, of the new LEDs that have been used to replace them, of the VIIRS camera, and of the average human eye.

Visualization of the visibility offered by LEDs in comparison with sodium lighting.

Visualization of the visibility offered by LEDs in comparison with sodium lighting.
Source: Aubé & Lamphar (2017).

The results of our analysis are both encouraging and very worrisome. They’re encouraging because we deduced that the contribution of new lights to Montréal’s light pollution dome have dropped by about 15 percent with the human eye response of a representative low-light environment, a figure that’s likely to improve even more as and when the City completes the transition to LEDs. Our data also allowed us to predict the impact that this transition would have had if different decisions were made by Montréal especially if LEDs of a different color had been used. If our city had opted for bluer 4000 K LEDs (which had been initially planned), we would instead have worsened light pollution by somewhere between 20 and 50 percent ! Fortunately, Montréal decided to use 3000 K LEDs, which are less blue, and they made further decisions that helped to limit light pollution. For example, fixtures with only downward light projection were selected, and light intensity was slightly reduced under the previous lighting standard given that LEDs afford better visibility.

Our analysis determined that if the City had omitted a single one of these strategies there would have been an unfavorable impact on light pollution. If we were to have simply swapped the bulbs for 4000 K LEDs, with no additional effort, our data show that we might have quadrupled their contribution to Montréal’s dome of light pollution. Happily, we avoided the worst, but the reality is not that simple or reassuring if we compare photos of Montréal taken by astronauts onboard the International Space Station in 2013 and 2021. In doing so, we see that a large quantity of bright white light sources have appeared. These do not correspond to lighting controlled by the city because of their white color indicative of DELs at temperatures 4000 K or higher. Instead, they likely correspond to lighting used by the industrial, private or commercial sector.

Comparison of two photos captured by astronauts onboard the International Space Station: in 2013, then in 2021.

Comparison of two photos captured by astronauts
onboard the International Space Station: in 2013, then in 2021.
Crédit : NASA.

Thees new white DELs do not seem to have been installed with any great concern for preserving the night sky, and thus they could contribute up to four times more light pollution – or even more, if the total amount of lighting fixtures went up as well!

Areas around the Montréal neighborhoods Saint-Sulpice (above) and André-Grasset (below) photographed from the International Space Station in February 2021.

Areas around the Montréal neighborhoods Saint-Sulpice (above) and André-Grasset (below) photographed from the International Space Station in February 2021.
Crédit : NASA.

These photos show the impact of the city’s new LED light fixtures in a qualitative and direct way: we see here a residential neighborhood of Montréal where the new LEDs light up the sky with a slightly bluer light, but also more faintly than the previous sodium bulbs.

Some recommendations to follow

This analysis made it possible to identify three recommendations to follow for limiting light pollution during a transition to LED technology:

  1. Blue-colored LEDs (of over 3000 K) must be avoided at all costs. LEDs that significantly reduce blue light (1800 K and under) are particularly beneficial for reducing light pollution.
  2. Directional light fixtures must be used (“full-cutoff” type).
  3. The lighting intensity must be reduced by 30 percent compared with the lighting standard IES RP-8-14, which was followed with old lighting technologies.

It should be noted that all three recommendations must be followed to avoid a worsening of light pollution. If only one or two of them are adopted, our data indicate that the transition to the LED technology would increase light pollution.

I recently gave a virtual lecture to the Société d’astronomie du Planétarium de Montréal (SAPM) at which I presented these results in greater detail. That lecture is available on the SAPM YouTube channel and slides from the presentation are also available (in french only).

The complete technical study (in french only) will be of interest to anyone wishing to know more, as well as to engineers and scientists who’d like to reproduce our results or apply them in using other data sets.

We’d like to thank and acknowledge the cooperation of the NASA VIIRS Calibration Support Team, who supplied us with valuable information about the VIIRS camera and who helped us make this project a success.

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