The atmosphere and the pyrgeometer (in effect its sensor surface) exchange long wave IR radiation. This results in a net radiation balance according to: :E_\mathrm{net} = E_\mathrm{in} - E_\mathrm{out} Where (in
SI units): • = net radiation at sensor surface [W/m2] • = long-wave radiation received from the atmosphere [W/m2] • = long-wave radiation emitted by the sensor surface [W/m2] The pyrgeometer's
thermopile detects the net radiation balance between the incoming and outgoing long wave radiation flux and converts it to a voltage according to the equation below. : E_\mathrm{net} = \frac{U_\mathrm{emf}}{S} Where (in SI units): • =net radiation at sensor surface [W/m2] • =
thermopile output voltage [V] • = sensitivity/calibration factor of instrument [V/W/m2] The value for is determined during calibration of the instrument. The calibration is performed at the production factory with a reference instrument traceable to a regional calibration center.{{cite web To derive the absolute downward long wave flux, the temperature of the pyrgeometer has to be taken into account. It is measured using a temperature sensor inside the instrument, near the cold junctions of the
thermopile. The pyrgeometer is considered to approximate a
black body. Due to this it emits long wave radiation according to: :E_\mathrm{out} = \sigma T^4 Where (in SI units): • = long-wave radiation emitted by the earth surface [W/m2] • =
Stefan–Boltzmann constant [W/(m2·K4)] • = Absolute temperature of pyrgeometer detector [K] From the calculations above the incoming long wave radiation can be derived. This is usually done by rearranging the equations above to yield the so-called pyrgeometer equation by Albrecht and Cox. :E_\mathrm{in} = \frac{U_\mathrm{emf}}{S}+ \sigma T^4 Where all the variables have the same meaning as before. As a result, the detected voltage and instrument temperature yield the total global long wave downward radiation. == Usage ==