We urge the reader to remember, as he or she studies the rest of this book, that there are no perfect measurements. However, the field of solar radiometry has critically progressed in the last few years, resulting in significantly improved measurement quality at research-class sites. Much more work yet needs to be done to obtain better instruments, reference scales, calibrations, characterizations, and corrections for measuring solar radiation accurately in the field, and improve the data quality at the vast majority of sites that still rely on suboptimal experimental techniques.
The basic uncertainties in the best practical solar radiation data available today are still on the order of 3% in direct beam, 5% in total global horizontal, 3% ±2Wm~2 in diffuse horizontal irradiance (measured with a black-and-white or corrected all-black pyranometer), 15% to 20% in diffuse radiation measured with uncorrected all-black pyranometers behind a shadow band, and perhaps 5 to 20% in sunshine duration, for digital (including pyrheliometer) and analog (burning) sunshine recorders, respectively. For the future, we can only hope for better models through better instrumentation and improved measurement techniques.