Pyrgeometer - A device for measuring the near-surface infrared radiation spectrum in the wavelength range from about 4.5 μm to 100 μm.
A pyrgeometer (from the ancient Greek πῦρ /pyr/ "fire" and γῆ /geo/ "earth") is used to measure the atmospheric counter-radiation arriving from half-space; this is thermal radiation in the range from about 4 µm.A pyrgeometer consists in principle of an edge filter, which keeps out the solar component of the radiation, and a black surface, which absorbs the transmitted radiation and thus becomes warm.
The filter consists of a dome-shaped support (dome) with an interference filter vapour-deposited on the inside. Since the radius of the dome is large compared to the measuring surface, rays hitting the absorber surface pass approximately perpendicularly through the layers of the interference filter, which is important for its function. With the opposite filter effect, with a sensitivity of 0.3 to 3 µm, the device would be a pyranometer for measuring solar global radiation.
The carrier material must be transparent to at least 50 µm and consists of silicon or very thin polyethylene held in shape by overpressure. The silicon may be thicker, which makes the device more robust and allows the temperature of the dome to be measured at one or more points - since the material is not completely transparent, its temperature contributes to the radiation flux.
The black absorber surface is one of two surfaces of a thermopile whose terminal voltage is proportional to the heat flow through the column. The other surface of the column is in thermal contact with the chassis of the unit. To avoid irradiation of the column from the side, the absorber surface is fitted into a black plate, which is also in thermal contact with the chassis. The temperature of the chassis is also measured and the temperature of the absorber is deduced from this in order to be able to take into account its radiation towards the sky.
Accurate calibration of the pyrgeometer is done against a black body radiator at various combinations of the radiator, absorber and dome temperatures.
A sunshield reduces the contribution of solar radiation to the measured value in clear weather (2 % of solar radiation is longer wavelength than 3 µm). A slightly heated air flow from a ring nozzle prevents dew from forming on the dome at night.