A net radiometer is a type of actinometer used to measure net radiation (NR) at the Earth's surface for meteorological applications. The name net radiometer reflects the fact that it measures the difference between downward/incoming and upward/outgoing radiation from Earth. It is most commonly used in the field of ecophysiology.
Although there are many types of net radiometers, the 4-component design at present is most popular for scientific applications.
A 4-component net radiometer serves to measure 4 separate components of surface radiation balance: SWin direct incoming short wave radiation, SWout or reflected short wave radiation, LWin diffused long-wave radiation from the sky and LWout long-wave radiation emitted by the ground surface. In net radiometers, shortwave radiation is measured with pyranometers which measure incoming shortwave radiation and reflected shortwave radiation (albedo), and longwave radiation is measured with pyrgeometers. The working range of pyranometers is 300 to 2800 nm wavelength and that of pyrgeometers is 4500 to 100000 nm wavelength.
NOTE: the following formulas have T in kelvins. Add 273.16 to convert to temperature in degrees Celsius.
U is the voltage output of a sensor, E is radiation at the sensor surface, up = upfacing instrument, down = downfacing instrument, SW = shortwave or solar radiation, LW = longwave or far infrared (FIR) radiation, in = incoming, out = outgoing, T = temperature, NR = net radiation.
SWin = Upyrano,up / Epyrano,up
SWout = Upyrano,down / Epyrano,down
LWin = (Upyrgeo,up / Epyrgeo,up) + 5.67×10−8 Tpyrgeo4
LWout = (Upyrgeo,down / Epyrgeo,down) + 5.67×10−8 Tpyrgeo4
NOTE: in the LWnet the instrument temperature is cancelled:
LWnet = (Upyrgeo,up / Epyrgeo,up) - (Upyrgeo,down / Epyrgeo,down)
SWnet = (Upyrano,up / Epyrano,up) - (Upyrano,down / Epyrano,down)
NR = SWnet + LWnet
Special parameters that can be deduced:
SWalbedo = SWin / SWout
Tsurface = (LWout / 5.67×10−8)1/4
Tsky = (LWin / 5.67×10−8)1/4
The SWalbedo and the Tsurface must be estimated from other sources, and the NR can be calculated using these plus the SWin and LWin measurements.
SWalbedo typically is assumed to be a constant, typically taken from local satellite observations; Tsurface can often be calculated from air temperature or ground temperature measurements.
Net-radiometers are not standardised.
Specifications, drawings and pictures courtesy of Hukseflux Thermal Sensors, www.Hukseflux.com