  Light pollution can be rigorously described in terms of the volume
concentration of anthropogenic photons (light quanta) in the terrestrial
atmosphere. This formulation, consistent with the basic physics of the
emission, scattering and absorption of light, allows one to express light
pollution levels in terms of particle volume concentrations, in a completely
analogous way as it is currently done with other classical pollutants, like
particulate matter or molecular contaminants. In this work we provide the
explicit conversion equations between the photon volume concentration and the
traditional light photometry quantities. This equivalent description of the
light pollution levels provides some relevant insights that help to identify
artificial light at night as a standard pollutant. It also enables a
complementary way of expressing artificial light exposures for environmental
and public health research and regulatory purposes.
