Apogee SP-214 Kullanıcı El Kitabı - Sayfa 13
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Spectral Errors for Measurements with Silicon-cell Pyranometers
Apogee SP series pyranometers are calibrated under electric lamps in a calibration laboratory. The calibration
procedure simulates calibration under clear sky conditions at a solar zenith angle of approximately 45°. However,
due to the limited spectral sensitivity of silicon-cell pyranometers compared to the solar radiation spectrum (see
graph below), spectral errors occur when measurements are made in conditions that differ from conditions the
sensor was calibrated under (e.g., the solar spectrum differs in clear sky and cloudy conditions, thus measurements
in cloudy conditions result in spectral error because sensors are calibrated in clear sky conditions).
Silicon-cell pyranometers can still be used to measure shortwave radiation in conditions other than clear sky or
from radiation sources other than incoming sunlight, but spectral errors occur when measuring radiation with
silicon-cell pyranometers in these conditions. The graphs below show spectral error estimates for Apogee silicon-
cell pyranometers at varying solar zenith angles and varying atmospheric air mass. The diffuser is optimized to
minimize directional errors, thus the cosine response graph in the Specifications section shows the actual
directional errors in practice (which includes contributions from the spectral shift that occurs as solar zenith angle
and atmospheric air mass change with time of day and time of year). The table below provides spectral error
estimates for shortwave radiation measurements from shortwave radiation sources other than clear sky solar
radiation.
Spectral response of Apogee SP series
pyranometers compared to solar
radiation spectrum at Earth's surface.
Silicon-cell pyranometers, such as
Apogee SP series, are only sensitive to
the wavelength range of
approximately 350-1100 nm, and are
not equally sensitive to all
wavelengths within this range. As a
result, when the spectral content of
solar radiation is significantly different
than the spectrum that silicon-cell
pyranometers were calibrated to,
spectral errors result.