Humans are making the night sky brighter. A new study quantifies to what degree blue-rich LED sources exacerbate this problem.
We have discussed at some length the issues raised by artificial lighting at night (ALAN), and the indications that the light produced by LED sources may be worsening them. (See related posts below for some links.) Mostly we have looked at the human melatonin system, and to a lesser extent the corresponding systems in other forms of life, and how they react to the enhanced blue content of light from LED sources.
Figure 1. Spectral power distributions of six modeled light sources. (FLED is 3,000K LED light with blue wavelengths filtered out.)
Here is an in-depth look at another aspect of ALAN, with special attention to the way LED light sources affect the picture. Researchers at the Naval Observatory and Dark Sky Partners, in Arizona, have published peer-reviewed research on the impact of light-source spectral power distribution on sky glow (PDF here). The work appeared in the Journal of Quantitative Spectroscopy & Radiative Transfer, and is freely available online.
Figure 2. Ratio of overhead scotopic brightness levels compared to an LPS source.
Six light sources were considered. Spectral power distributions are shown in Figure 1. The researchers calculated the sky glow apparent to human photopic and scotopic vision at the zenith and near the horizon, at varying distances up to 300 km from a modeled light source. The calculation summed up scattering from atmospheric molecules and aerosols, taking into account the power spectrum of the source.
Figure 2 shows the essence of the results. The researchers sum it up this way:
When the light sources are matched for equal photopic output, scotopic visual sky glow from sources with increased short wavelength emission such as white LED and metal halide appears dramatically brighter. The higher CCT LED and metal halide sources produce 6–8x the scotopic luminance of low-pressure sodium (LPS), and 3x that from high-pressure sodium (HPS), when observed from nearby. At 300 km the ratios decrease due to the greater extinction of the bluer sources, but remain 3-4x that from LPS and 1.6-2.4x that from HPS.
One can quibble with the methodology, which is described in fair detail in the paper. In a discussion on LinkedIn's Innovations in Light group (note: group membership may be required to follow the link), some experts do quibble with various points. But the conclusion seems solid that as the world moves increasingly to the use of LED sources for outdoor lighting at night, most of it at 4,000K or higher, we will be losing easy access to the night sky.
The paper is C.B. Luginbuhl et al., Journal of Quantitative Spectroscopy & Radiative Transfer, 139 (2014) 21-26.
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— Keith Dawson, Editor-in-Chief, All LED Lighting 
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