The most common instrument for measuring sunshine duration until around 2000 was the Campbell-Stokes sunshine autograph. In this device, the sun's rays are focused through a spherical glass lens, burning a trace into a dark, scaled strip of paper. From the length of this burn, the sunshine duration can be read to within a tenth of an hour. The strips are changed daily.
Nowadays, photoelectric sensors are also used to measure sunshine duration. These sensors measure sunshine duration indirectly. They measure irradiance; if this exceeds the internationally established limit of 120 W/m2, sunshine is assumed and a signal is issued.
In total, around 270 stations of the German Weather Service carry out sunshine measurements in Germany.
