Irradiance, formally Ascension Technology Rotating Shadowband Radiometer Pyranometer (RSP)
Irradiance's Rotating Shadow Band Radiometer (RSR), based on Ascension Technology's Rotating Shadowband Pyranometer (RSP), uses a solar cell based pyranometer (manufactured by LI-COR to measure global and diffuse irradiance and calculates direct normal beam irradiance. The RSR has a shadow band that rotates once a minute, the frequency of rotation can vary, 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. An adjustment is made to the minimum value to account for the portion of the sky shaded by the ratating band. 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.
The accuracy of the RSR is limited by the spectral characteristics of their sensors. Solar cell based sensors are used 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 biases in the global and/or beam irradiance measurements. Work has developed algorithms that adjust the measurements to partially remove these systematic biases (See Diffuse Responsivity of Solar Cell Based Pyranometers.)
© 2022, UO Solar Radiation Monitoring Laboratory.
Last revised: March 4, 2022.
Home page URL: solardata.uoregon.edu