In this pyrheliometer, a temperature difference is produced between the sensor surface (called the hot
junction) and the reference temperature point, i.e., the metallic block of the inner cylinder (called the cold
junction). As the temperature difference is proportional to the intensity of the radiation absorbed, the level
of solar radiation can be derived by measuring the thermoelectromotive force from the thermopile. Since
this type of pyrheliometer is a relative instrument, calibration should be performed to determine the
instrumental factor through comparison with a standard instrument. As the thermoelectromotive force
output depends on the unit’s temperature, the temperature inside the cylinder should be monitored to enable
correction.