  In the last decade numerous Sky Quality Meters (SQMs) were installed
throughout the globe, aiming to assess the temporal change of the night sky
brightness (NSB), and thus the change in light pollution. However, it has
become clear that SQM readings may be affected by aging effects such as
degradation of the sensor sensitivity and/or loss of transmissivity of optical
components (filter, housing window). To date, the magnitude of the darkening
has not been assessed in a systematic way. We report for the first time on the
quantification of the SQM aging effect and describe the applied method. We
combine long-term SQM measurements obtained between 2011 and 2019 in
Potsdam-Babelsberg (23 km to the southwest of the center of Berlin), Vienna and
Stockholm with a readily available empirical twilight model, which serves as
calibrator. Twilight SQM observations, calibrated for changing sun altitudes,
reveal a linear degradation of the measurement systems (SQM + housing window)
with the following slopes: 34$\pm$4, 46$\pm$2 and 53$\pm$2 milli-mag$_{\rm
SQM}$ arcsec$^{-2}$ yr$^{-1}$ for Stockholm, Potsdam-Babelsberg and Vienna.
With the highest slope found in Vienna (latitude $\sim$48$^\circ$) and the
lowest one found in Stockholm (latitude $\sim$59$^\circ$), we find an
indication for the dependence of the trend on solar irradiance (which is a
function of geographic latitude).
