Campbell Scientific NR01 Net Radiometer - Manual (Page 11)

NR01 Four-Component Net Radiation Sensor
To attain the proper directional and spectral characteristics, a pyrgeometer’s
main components are:
1 Thermopile sensor with a black coating—absorbs all LW and SW
radiation, provides a flat spectrum covering the 300 to 50000 nanometer
range, and has a near-perfect cosine response.
2 Silicon window with solar blocking—limits the spectral response from
4500 to 50000 nanometers (cutting off the part below 4500 nm) while
preserving the 150 degrees field of view (not the ideal 180 degrees).
Another function of the window is that it shields the thermopile sensor
from convection.
FIGURE 2.4-1. Spectral Response of the Pyrgeometer Compared to
the Atmospheric LW Spectrum
The black coating on the thermopile sensor absorbs the LW radiation. This
radiation is converted to heat. The heat flows through the sensor to the
housing. The thermopile sensor generates a voltage output signal that is
proportional to the LW radiation that is exchanged between sensor and source.
However, the sensor itself also irradiates LW radiation. This is according to
Plank’s law, so that the pyrgeometer thermopile signal is composed of the
incoming radiation minus the outgoing radiation. In order to estimate the
outgoing component, the pyrgeometer temperature is measured independently,
using a Pt100 or a user-supplied temperature sensor. Equation 2.4-1 calculates
the incoming LW radiation assuming T
pyrgeo
is in Kelvin:
LW
in
= (U
pyrgeo, up
/ E
pyrgeo, up
) + 5.67.10
-8
(T
pyrgeo
)
4
2.4-1
For LW
out
a similar formula is valid. The equations are the same for up- and
down facing instruments.
It is possible to calculate temperatures of the objects within the field of view of
the instrument, assuming these are uniform- temperature blackbodies (emission
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