File:ESO - Asiago spectra.jpg

Summary
Comparison between a night sky spectrum obtained in a light polluted site (Asiago Astrophysical Observatory – Italy, upper panel) and a dark site (ESO-Paranal – Chile, lower panel). Spectral line identifications for the main features are traced in red for artificial sources and in blue for the natural ones. The emissions generated by street lighting are clearly visible, mainly in the form of strong lines of Mercury and Sodium, which fall not only in the visible range (500-600nm), but also in the in blue and violet parts of the spectrum.

Licensing
The image is non-free fair use in Violet astronomy.

Fair use for Violet astronomy
To illustrate the nature of light pollution emission lines by comparing a light polluted site (Asiago Astrophysical Observatory – Italy, upper panel) with a dark site (ESO-Paranal – Chile, lower panel), since no picture can be obtained of any emission spectrum in visual light itself for the Asiago Astrophysical Observatory.

The image linked here is claimed to be used under fair use as:
 * 1) It is a scientifically significant image of the emission spectra of an especially light polluted site for comparison with a dark site.
 * 2) It is of much lower resolution than the original (copies made from it will be of very inferior quality).
 * 3) The image is only being used for informational/educational purposes.
 * 4) Its inclusion in the article adds significantly to the article because it shows the phenomenon depicted in the article and how it has a significant impact on violet astronomy to the general public.
 * 5) Not replaceable with a free image since this phenomenon so imaged is rare.