Changes in night sky brightness after a countywide LED retrofit

TL;DR

This study measured the effects of replacing sodium street lamps with LEDs in Chelan County, revealing increased sky brightness at night despite reduced upward radiance, highlighting the need to revise existing skyglow models.

Contribution

It provides the first comprehensive comparison of ground-based and satellite measurements of sky brightness before and after a large-scale LED retrofit.

Findings

Skyglow became brighter and extended higher after retrofit

Upward radiance decreased despite increased sky brightness

Satellite data models need revision to account for LED spectral characteristics

Abstract

The US National Park Service (NPS) Night Skies Program measured changes in sky brightness resulting from a countywide lighting retrofit project. The retrofit took place in Chelan County, a gateway community to North Cascades National Park and Lake Chelan National Recreation Area in Washington State. The county retrofitted all 3,693 county-owned high pressure sodium (HPS) street lamps to full cutoff LEDs. This number is about 60% of the County's total outdoor street and area lights. About 80% of the newly installed lights were 3000K in color temperature and 20% were 4000K. The 4000K LEDs were used to meet Washington State Department of Transportation guidelines. To measure sky brightness, we used the NPS night sky camera system before the retrofit started in 2018 and after its completion in 2019. These images were photometrically calibrated and mosaicked together to provide hemispherical images in V band. For comparison with our ground-based measurement, we obtained the satellite imagery taken by Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership satellite. Our measurements show that the post-retrofit skyglow became brighter and extended higher in the sky, but upward radiance, as measured by the day-night band radiometer, decreased. These divergent results are likely explained by a substantial increase in light emitted at wavelengths shorter than 500 nm, and a relative decrease in zenith light emission due to better shielded luminaires. These results also demonstrate that earlier models relating VIIRS day-night band data to skyglow will, at a minimum, require substantial revision to account for the different characteristics of solid state luminaires.