Kipp&Zonen CNR 4 Net Radiometers - Manual (Page 25)

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2.3.4 Spectral properties of the pyrgeometer
The spectral properties of the pyrgeometer are mainly determined by the properties of the absorber paint and the
silicon window. The silicon window is coated on the inside with an interference filter, which blocks the solar
radiation. The spectral characteristics of the pyrgeometer are depicted in figure 2.5
Figure 2.5 The spectral sensitivity of the pyrgeometer window: Theoretically it equals the spectral selectivity of
the total instrument.
2.3.5 Directional / Cosine response of the pyrgeometer
The measurement of the Far Infrared radiation falling on a surface (also called irradiance or radiative flux) requires
that the sensor has to comply with the cosine response.
A perfect cosine response will show a maximum sensitivity of (1) at an angle of incidence of 0E (perpendicular to
the sensor surface) and zero sensitivity at an angle of incidence of 90E (radiation passing over the sensor surface).
Between 90 and 0 degrees, the sensitivity should be proportional to the cosine of the angle of incidence.
0
50
100
1 10 100
Tr
an
sm
itta
n
ce
[%
]
Wavelength [µm]
FIR WINDOW TRANSMITTANCE