A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density (in watts per metre square) from a field of view of 180 degrees. The name pyranometer stems from Greek, "pyr - πῦρ" meaning "fire" and "ano - ἄνω" meaning "above, sky".
A typical pyranometer does not require any power to operate.
The solar radiation spectrum extends approximately from 300 to 2,800 nm. Pyranometers usually cover that spectrum with a spectral sensitivity that is as “flat” as possible.
To make a measurement of irradiance, it is required by definition that the response to “beam” radiation varies with the cosine of the angle of incidence, so that there will be a full response when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0 degrees angle of incidence), zero response when the sun is at the horizon (90 degrees angle of incidence, 90 degrees zenith angle), and 0.5 at 60 degrees angle of incidence. It follows that a pyranometer should have a so-called “directional response” or “cosine response” that is close to the ideal cosine characteristic.
Design of pyranometers
In order to attain the proper directional and spectral characteristics, a pyranometer’s main components are:
- A thermopile sensor with a black coating. This sensor absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nanometer range, and has a near-perfect cosine response.
- A glass dome. This dome limits the spectral response from 300 to 2,800 nanometers (cutting off the part above 2,800 nm), while preserving the 180 degrees field of view. Another function of the dome is that it shields the thermopile sensor from convection.
The black coating on the thermopile sensor absorbs the solar radiation. This radiation is converted to heat. The heat flows through the sensor to the pyranometer housing. The thermopile sensor generates a voltage output signal that is proportional to the solar radiation.
Pyranometers are frequently used in meteorology, climatology, solar energy studies and building physics. They can be seen in many meteorological stations - typically installed horizontally and next to solar panels - typically mounted with the sensor surface in the plane of the panel.
Pyranometers are standardized according to the ISO 9060 standard, that is also adopted by the World Meteorological Organization (WMO). This standard discriminates three classes. The best is (confusingly) called "secondary standard," the second best "first class" and the last one "second class."
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