Uncertainty and Accuracy Issues with Broadband Solar Shortwave and Infrared Radiometers.

Radiometric measurements of broadband terrestrial solar shortwave and infrared radiation have long been one of the least accurate fields of metrology, the science of measurement. Over the last half of the 20^th century, typical uncertainties for radiation on a horizontal surface received from the hemisphere of the sky were on the order of 5% to 10% of full scale. For clear sky, typical full scale irradiance is about 1000 Watts per square meter (W/sm). Thus absolute uncertainties approached 50 W/sm to 100 W/sm. Recent research into the radiation budget of the Earth (climate change) and solar energy resources has resulted in methods to reduce uncertainties in solar radiation measurements and instrumentation by factors of two to five, or 10 W/sm to 20 W/sm for hemispherical sky radiation. These improvements arise from the identification and characterization of inherent instrument systematic errors, in new instrument designs, and new and improved calibration references.  Characterization of detector geometrical response corrections functions and infrared thermal offsets in thermopile sensors can improve the accuracy of broadband solar radiation measurements. Calibration of pyrgeometers for measuring Infrared radiation have been improved by development of an interim world infrared standard group (IWISG) by the World Meteorological Organization (WMO), and the development of  a new absolute infrared standard radiometer. The most intractable issues in solar radiometry are inherent in widely deployed and relatively inexpensive silicon photodiode detector radiometers that respond only over a limited wavelength range.. These sensors are popular for measuring total hemispherical sky radiation, photosynthetically active radiation (PAR), and photopic (human eye response) radiation intensities. The variability in solar spectral conditions, sensor spectral response, and spectral matching filters can severely impact the accuracy of these radiometers.  Only very extensive long term on-site comparison with much higher quality measurements can improve the accuracy of silicon photodiode detectors.