A pyrgeometer consists of the following major components:
- A thermopile sensor which is sensitive to radiation in a broad range from 200 nm to 100 µm
- A silicon dome or window with a solar blind filter coating. It has a transmittance between 4.5 µm and 50 µm that eliminates solar shortwave radiation.
- A temperature sensor to measure the body temperature of the instrument.
- A sun shield to minimize heating of the instrument due to solar radiation.
Measurement of long wave downward radiation
The atmosphere and the pyrgeometer (in effect its sensor surface) exchange long wave IR radiation. This results in a net radiation balance according to:
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.
- 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.
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:
- Long-wave radiation emitted by the earth surface [W/m²]
- Stefan-Boltzmann constant [W/(m²·K4)]
- Absolute temperature of pyrgeometer detector [kelvins]
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.
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.
The signals are generally detected using a data logging system, capable of taking high resolution samples in the millivolt range.