Towards an absolute light pollution indicator.

The growing concern about the negative impact of artificial light at night on biodiversity and human health increases the need of defining a general indicator that could be used for characterizing light pollution as well as performing both spatial and temporal comparisons. In this paper, we show that the traditional indicators based on direct numerical measurements of sky brightness suffer from significant limitation due to calibration bias and lack of reproducibility. Furthermore, these measures are most often performed in periods of clear sky. They do not reflect the wide variety of meteorological conditions that can produce highly inhomogeneous levels of light pollution on a given site. To overcome these issues, we propose a statistical indicator called NSB Dispersion Ratio. This indicator is derived from a statistically significant number of individual night sky brightness measurements, under various meteorological conditions. It is independent of any absolute photometer calibration. It only requires on-time precise corrections of the contribution of natural light sources such as the Galactic plane.