Pyranometers are frequently used in meteorology, climatology, solar energy studies and building physics. They can be seen in many meteorological stations - typically installed horizontally and next to solar panels - typically mounted with the sensor surface in the plane of the panel. Pyranometers are standardised according to the ISO 9060 standard, that is also adopted by WMO, the World Meteorological Organisation. This standard discriminates three classes. The best is (confusingly) called "secondary standard", the second best "first class" and the last one "second class".