Measuring global radiation

This instrument is based on the internal photoelectric effect in a semi-conductor (photodiode). A light particle (photon) can accordingly transfer its energy to an electron in an atom. If the energy of the light particle is sufficiently high, during the external photoelectric effect, the electron can leave the atom. In the internal photoelectric effect, however, it remains in the material, but the electron can contribute to the flow of the current as a free conduction electron. The more intense the radiation is, the more electrons are released and the greater the photoelectric current is. Photodiodes are thus very sensitive light sensors.
In this instrument, three photodiodes have been mounted in such a way that only sunlight direct from the Sun can be calculated. If more than 120W/ mÇ are measured in this way for a certain period in time, the instrument registers it as “sunlight”. The World Meteorological Organization (WMO) has set this value. (During the summer, values of 500 to 1000 W/ mÇ of direct solar radiation can actually occur in middle latitudes). The instrument thus measures the time at which this threshold value is exceeded.