In order for policies targeting light pollution to be effective, the amount of pollution at each area needs to be quantified, taking into account the geographical size and the population density at the location. This allows a more accurate comparison of light pollution levels across landscapes, negating the effect geographical size and population densities may have on light pollution. It also allows policies to be prioritised at areas most light-polluted – for instance, small cities with high population density and high pollution levels should be prioritised over small cities with low population density and low pollution levels.
To measure the night sky brightness (an indicator of light pollution levels) at different geographical locations, few factors are considered, including the light flux per capita (FpC), light flux per GDP unit (FpD) and the average artificial night sky brightness at Zenith (Zenith Radiance). FpC measures the light pollution adjusted for geographical size and location of area, FpD measures light pollution adjusted for population density and affluence whereas Zenith Radiance accounts for only artificial sources of lighting.
PfC, PfD and the Zenith Radiance are considered in totality, where having a low FpD (ie low pollution level given its population density) and a low FpC (ie low pollution level given size of geographical area) alone does not imply that the country experiences low levels of light pollution. For instance, Paris ranks 5th in terms of FpC and 1st in FpD, but 1363th in average sky brightness – the 3rd worst in Europe). This is due to its high population density that makes it appear virtuous in terms of FpD and FpC, but overall wasteful in light.
An instance of how these factors are used to rank different areas according to their extent of light pollution is seen in figure 1.
Figure 1: ranking of most light-polluted (red), and least light-polluted areas (green) in Europe
References
Falchi, F., Furgoni, R., Gallaway, T.A., Rybnikova, N.A., Portnov, B.A., Baugh, K., Cinzano, P. and Elvidge, C.D. (2019). Light pollution in USA and Europe: The good, the bad and the ugly. Journal of Environmental Management, 248, p.109227.
Globeatnight-network.org. (2015). Globe at Night – Sky Brightness Monitoring Network. [online] Available at: http://globeatnight-network.org/lp-measurement.html
International Dark-Sky Association. (2021). Measuring Light Pollution – International Dark-Sky Association. [online] Available at: https://www.darksky.org/light-pollution/measuring-light-pollution/