The LI-COR pyranometer (LI-200SA) measures global solar radiation. It is a photodiode covered by a plastic disk.
While thermopile based pyranometers such as an Eppley PSP have fairly uniform spectral response over the wavelength of interest, solar cell based pyranometers, such as the LI-COR have a marked spectral response to incident solar radiation. In the graph above, the LI-COR shows maximum sensitivity to wavelength in the 900 µ wavelengths and does not see an solar radiation with wavelengths over 1100 µ or under 400 µ.
A good study of the spectral responsivity of the LI-COR pyranometer can be found in a paper by David L. King, William E. Boyson, and Barry R. Hansen, "Improved Accuracy for Low-Cost Solar Irradiance Sensors", Sandia National Laboratories, Albuquerque, New Mexico, USA. Clear day responsivity was examined in that paper; responsivity to the diffuse irradiance was studied in one of our papers: Diffuse Responsivity of Solar Cell Based Pyranometers.
Under cloudy conditions, the diffuse responsivity of the LICOR pyranometer is approximately the same as found for the total responsivity under clear skies. However, under clear sky conditions, the diffuse responsivity is about 30% lower than that found in cloudy conditions. The percentage of solar radiation detected by the solar cell base pyranometer varies depending on whether the sky is blue (clear) or gray (cloudy). Since the diffuse component only makes up about 10% of the total irradiance on a clear day, this difference is hard to detect when the uncertainty in the calibration is on the order of ±5%. Of course, this is a systematic error and can present problems if the diffuse component is of importance.
On a daily basis, the total solar radiation measurement of a LI-COR pyranometer is fairly close to that of an Eppley PSP. However on an hourly or shorter time basis, this comparison varies systematically over the day. It also changes depending on the cloud cover. Therefore, it is very difficult to obtain a good calibration number for a LI-COR pyranometer with a thermopile pyranometer.
© 2006, UO Solar Radiation Monitoring Laboratory.
Last revised: September 25, 2006.
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