11
OPERATION AND MEASUREMENT
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.
