Ascension Technology Rotating Shadowband Pyranometer (RSP)
Ascension Technology's Rotating Shadow Band Pyranometer (RSP) uses a solar cell pyranometer (manufactured by LI-COR to measure global and diffuse irradiance and calculates direct normal beam irradiance. The RSP has a shadow band that rotates once a minute to block the sun from directly shining on the pyranometer. The pyranometer is measuring global irradiance before and after the shadow band starts its rotation to block the sun. The diffuse value is the minimum value that is obtained when the band sweeps in front of the sun. Direct irradiance on a horizontal surface is then calculated by subtracting the diffuse from the global irradiance. The direct normal beam irradiance is then obtained by projecting the direct horizontal irradiance onto the normal in the direction of the angle of incidence.
Yankee Environmental Systems manufactures a Rotating Shadowband Radiometer (RSR) that is similar to an RSP in principle.
The accuracy of the RSP and the RSR are limited by the spectral characteristics of their sensors. Currently solar cell based sensors are required because of their fast time response when the shadowband is swept between the sensor and the sun. While the responsivity of solar cell pyranometers to direct normal beam radiation varies by several percent over the day, the responsivity to diffuse radiation varies by approximately 30% from cloudy to clear periods. The variation in diffuse responsivity leads to systematic errors in the global and/or beam irradiance measurements. Work is proceeding to characterize the systematic errors and it may be possible to correct these in the future (See Diffuse Responsivity of Solar Cell Based Pyranometers.)
© 2000, UO Solar Radiation Monitoring Laboratory.
Last revised: April 2, 2001.
Home page URL: solardat.uoregon.edu