we will use the unit “” throughout this paper in order to make this difference clear.

The spectral sensitivity of the SQM was measured by Cinzano [@Cinzano2005]. A comparison between the SQM response function and the Johnson V band shows that the SQM is much more sensitive to light below $\approx$520nm. At a first glance the difference seems to make a calibration between the Johnson V band and the SQM response function difficult, but considering the spectral distribution of the night sky over Vienna which is dominated by emission lines above $\approx$525nm as seen in Fig. \[fig:skyspectrum\], we presume that a constant offset can be found for a comparison to the Johnson V band.

Measurements by other groups indicate that the SQM actually underestimates the brightness of the night sky compared to V band measurements – precisely due to its enhanced sensitivity in the blue (which is less prominent in the night sky than e.g. in the twilight sky). This is supported also by our own preliminary comparative measurements with instruments equipped with a Johnson V filter. The difference may amount up to 0.6mag.for clouded skies, while it is smaller for clear skies (H. Spoelstra, priv. comm.).

![SQM Response function 