The road lights in a complete district have been switched to LEDs. The sunshine air pollution bought worse

“The best plans of mice and humans often go wrong” – this famous paraphrase by the Scottish poet Robert Burns sometimes sums up human ingenuity. This is exactly what happened when a county in Washington state decided to replace at least some of all of its county street lights with LEDs to combat light pollution. New research shows that it actually made light pollution worse.

Dr. Li-Wei Hung and her colleagues from the National Park Service recently published a paper available on arXiv describing the work they did to monitor the night sky before and after Chelan County replaced their street lights with LEDs.

Map of Chelan County and where the street lights are.
Credit – Hung et al.

Chelan County is located in the north central portion of the state and serves as the gateway to several nearby outdoor recreation areas, including North Cascades National Park. Given this interest in nature, less light pollution seems to be a benefit for stargazing in hopes of catching a glimpse of the Milky Way.

As a result, the county decided to replace all 3,693 county street lights (60% of all outdoor street lights in Chelan County) with “all-off” light emitting diodes for lightbulbs. About 80% of these new LEDs were “3000K” or “warm white light” while the other 20% were slightly brighter “4000K” lamps that were installed to meet the lighting requirements of the Washington State Department of Transportation.

NG video on light pollution.
Credit – National Geographic YouTube Channel

The retrofit process took place between 2018 and 2019, and Dr. Hung and her team took measurements both before and after the new lights were installed. What they found was … surprising, to say the least. The sky in the county actually got brighter after installing the LEDs.

Such a result is only counter-intuitive as LEDs are generally considered to be much more energy efficient and better at protecting against light pollution. An experiment in Flagstaff, Arizona, attempted to use LEDs in 2018 to create the world’s first “International Dark Sky City,” although the results are not yet known.

Video about some of the best stargazing spots in the United States.
Credit – Check the Facts 360 YouTube Channel

If the results are true in Chelan County, this experiment may not be successful. Dr. Hung and her colleagues studied three different metrics used to calculate the darkness of the sky – how bright the county sky glow was, how high it went, and “upward radiation,” which measures the light directed upwards.

Several tools were required to fully calculate the differences in light pollution. First, the National Park Service has a “night sky camera system” that allowed researchers to take simple before and after pictures of the night sky in the Chelan County area. Additional data was then collected from the Visible Infrared Imaging Radiometer Suite (VIIRS) located on the Suomi National Polar-orbiting Partnership Satellite.

Description of Suomi and its data.
Credit – USGS

Of the three metrics measured by this instrument, the brightness and height of the sky glow increased while the upward radiation actually decreased. The authors’ explanation for the exacerbation of light pollution is that light emissions below 500 nm, closer to the blue end of the visible spectrum, increased. The decrease in the amount of light directed upwards was most likely due to a combination of the alignment of the LEDs and better shielding on the lamppost itself.

While light pollution is actually made somewhat worse, the LED retrofit can still be seen as a success for its other two goals – lowering energy consumption and lowering costs. The LEDs are undeniably more energy efficient than the high pressure sodium lamps they replaced and last much longer, lowering their overall cost to the county. While the energy and cost savings are great, astronomy enthusiasts everywhere can hope that other local authorities considering similar upgrades can consider how new LEDs might affect their night skies. Using all of the best aspects of LEDs could result in happy people and mice all around.

Learn more –
arXiv – Changes in the brightness of the night sky after a nationwide LED retrofit
Chelan County – Sharing the night sky
UT – Most of the light pollution doesn’t come from street lights

Mission statement –
Panoramic images showing light pollution before (top) and after (bottom) installing LEDs in Chelan County’s street lights.
Credit – Hung et al.

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