It quantifies the astronomical observability of celestial objects and the interference caused by light pollution.
Amateur astronomer John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help skywatchers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites.
It gives several criteria for each level beyond naked-eye limiting magnitude (NELM).
[1] The accuracy and utility of the scale have been questioned in 2014 research.[2][why?]
The table summarizes Bortle's descriptions of the classes.