  The Sky Quality Meter (SQM) has become the most common device to track the
evolution of the brightness of the sky from polluted regions to first class
astronomical observatories. A vast database of SQM measurements already exists
for many places in the world. Unfortunately, the SQM operates over a wide
spectral band and its spectral response interacts with the sky's spectrum in a
complex manner. This is why the optical signals are difficult to interpret when
the data are recorded in regions with different sources of artificial light.
The brightness of the night sky is linked in a complex way to ground-based
light emissions while taking into account atmospheric-induced optical
distortion as well as spectral transformation from the underlying ground
surfaces. While the spectral modulation of the sky's radiance has been
recognized, it still remains poorly characterized and quantified. The impact of
the SQM's spectral characteristics on the sky brightness measurements is here
analysed for different light sources, including low and high pressure sodium
lamps, PC-amber and white LEDs, metal halide, and mercury lamps. We show that a
routine conversion of radiance to magnitude is difficult or rather impossible
because the average wavelength depends on actual atmospheric and environment
conditions, the spectrum of the source, and device specific properties. We
correlate SQM readings with both the Johnson astronomical photometry bands and
the human system of visual perception, assuming different lighting
technologies. These findings have direct implications for the processing of SQM
data and for its improvement and/or remediation.
