BORCAL

(Broadband Outdoor Radiometer CALibration)
A method of calibratingpyrheliometers andpyranometers based on the summation technique at theSolar Radiation Research Laboratory (SRRL).Up to three days of clear-skysolar irradiance measurements taken at 30-second intervals from sunrise tosunset are used to compute the individual radiometer responsivities.
For pyrheliometer calibrations, the reference direct normal irradiance ismeasured with an electrically self-calibrating absolute cavity radiometertraceable to theWorld Radiometric Reference (WRR). The individualpyrheliometer responsivity is computed as the mean ratio of the signal fromthe pyrheliometer (microvolts DC) to the reference irradiance (Watts persquare meter) for each of the 30-second data samples.
Pyranometer responsivities are computed from the ratio of the signal fromeach pyranometer (microvolts DC) to the referenceglobal horizontal irradiance determined by thedirect normal irradiance (measured with anabsolute cavity radiometer) and the simultaneousdiffuse horizontal irradiance(measured by a reference pyranometer placed under a solar-trackingshading disk):
&#160 &#160 Global Horizontal = Direct Normal x cos(Z) + Diffuse Horizontal
where
&#160 &#160 Z = Solar Zenith Angle at the time of measurement.
The reference pyranometer is calibrated prior to use in a BORCAL event bymeans of the Shade Calibration Technique.