Net radiometer

4-component net-radiometer clearly showing the instrument main components: 2 pyranometers (with domes, one visible at right facing up and the second at right facing down obscured by the white radiation shield above it) and 2 pyrgeometers (flat windows, again one visible (facing up) and one obscured (facing down)). Dimensions: diameter of the pyranometer dome is 20 mm. Photo shows model NR01.

A net radiometer is a type of actinometer used to measure net radiation (NB) at the Earth's surface in meteorological applications. The name net radiometer reflects the fact that it is supposed to measure incoming minus upwelling radiation. It is most commonly used in the field of eco-physiology.

Terminology

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 the 4 separate components of the surface radiation balance. SW_in or global solar radiation, SW_out or reflected solar radiation, LW_in or infrared emitted by the sky and LW_out or infrared emitted by the ground surface. From these also parameter like SW “albedo”, “sky temperature”, “(ground) surface temperature” and off course “net-radiation” (net value of all SW and LW fluxes) can be calculated. A typical net radiometer does not require any power to operate.

The SW solar radiation spectrum extends approximately from 300 to 2800 nm. Pyranometers usually cover that spectrum with a spectral sensitivity that is as “flat” as possible. The longwave (LW) or far infrared (FIR) extends from 4500 to 50000 nm. This part of the spectrum is covered by a pyrgeometer.

cross section of a 4-component net-radiometer clearly showing the instrument main components: (1) SW_in solar radiation sensor or pyranometer, (2) LW_in far infrared radiation sensor or pyrgeometer (3) radiation shield, (4) leveling assembly for x and y axis, block plus bolts for x-axis adjustment (5) leveling assembly for x and y axis, horizontal rod (6) connection body, containing Pt100 temperature sensor, heater and hole for users own temperature sensor (add cable gland M8) (7) LW_out far infrared radiation sensor or pyrgeometer, (8) leveling assembly for x and y axis, bolts for y-axis adjustment (9) SW_out solar radiation sensor or pyranometer

Calculations

NOTE: the following formulas have T in kelvins. In order to have T in degrees Celsius, please add 273. With U voltage output of a sensor, 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.

SW_in = Upyrano, up / Epyrano, up

SW_out = Upyrano, down / Epyrano, down

LW_in = (Upyrgeo, up / Epyrgeo, up) + 5.67.10-8 (Tpyrgeo)4

LW_out = (Upyrgeo, down / Epyrgeo, down) + 5.67.10-8 (Tpyrgeo)4

NOTE: in the LW_net the instrument temperature is cancelled:

LW_net = Upyrgeo, up / Epyrgeo, up - Upyrgeo, down / Epyrgeo, down

SW_net = Upyrano, up / Epyrano, up - Upyrano, down / Epyrano, down

NR = SW_net + LW_net

Special parameters that can be deducted:

SW_albedo = SW_in/SW_out NOTE: the following formulas have T in kelvins. In order to have T in degrees Celsius, please add 273.

T_surface = (LW_out /5.67.10-8)1/4

T_sky = (LW_in/5.67.10-8)1/4

The SW albedo and the T_surface must be estimated from other sources, and the NR can be calculated using these plus the SW_in and LW_in measurements. SW albedo typically is assumed to be a constant, typically taken from local satellite observations; T_surface can often be calculated from air temperature of ground temperature measurements.

Usage

Net radiometers are frequently used in meteorology, climatology, solar energy studies and building physics. They can be seen in many meteorological stations—typically installed horizontally.

Standardisation

Net-radiometers are not standardised.

Example of a domeless net nadiometer. The sensor contains two black-surface sensors (second one not visible) and has a single output signal representing the total net radiation. This instrument is typically used for lower accuracy net radiation measurement.

References

Specifications, drawings and pictures courtesy of Hukseflux Thermal Sensors, www.Hukseflux.com

See also

• radiometer
• Pyranometer
• irradiance
• Meteo-Technology instrumentation website