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EL Highlights August 2015

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EL Develops Software for High Precision Utilization of Solar Irradiance Sensors

TheEngineering Laboratory has released the Pyranometer Responsivity INTERpolator(PRINTER) software that provides users of solar irradiance sensors a simple wayto more accurately use them over a range of environmental conditions. Solar irradiance measuring pyranometers aresensitive to the angle at which light strikes the sensor, but traditionallyonly a single calibration factor, or responsivity (in units of µV/W/m2), isused to convert the output, neglecting this angle and other environmentalconditions. Calibration services that measure these angular effects haveexisted for ten years, but their practical use has been extremely limited inboth research and meteorological observations partly due to the complexity inutilizing the irregular calibration curves.

Thesoftware developed by EL, which has been described in an article in the journalSolar Energy1, interpolates the pyranometer incidence angle dependentresponsivities provided in a standard format by calibration laboratories intoan accurate and easy to use single polynomial function. The polynomials can beof very low or high degree, are extrapolated outside the measurement range to0° and 90°, and are calculated using robust techniques to avoid oscillationsand overshoots, which can occur with other regression methods.

Thesoftware program imports the calibration file and generates plots of theresponsivities and associated uncertainties. It then calculates coefficientsfor the polynomial regression fits of both the responsivities as well as theupper prediction interval of the associated uncertainties. These coefficientscan be easily copied and implemented in spreadsheet software or a programmingenvironment. PRINTER is available free of charge along with its source code at

1Methodologyand calculator for high precision regression fits of pyranometer angularresponsivities and the associated uncertainties, M. Boyd, Solar Energy 119,233-242 (2015).

Contact:Matthew Boyd (EL), 301-975-6444

Created February 12, 2016, Updated September 21, 2016