This chapter discusses instruments that used to measure shortwave radiation in total. Photodiode-based pyranometers are used for measuring global irradiance for clear and cloudy skies and are calibrated under clear-sky conditions. Traditionally, the study of non-concentrating thermal collectors also has used the term global irradiance available to tilted flat plate collectors. Single black-disk thermopile pyranometers measure the heat flow between a black-disk receiver exposed to the incident radiation and the heat sink in the body of the pyranometer. The thermal offset of a pyranometer is a subtle thermal loss mechanism that affects the performance of single black thermopile detector pyranometers. Nonlinearity is the deviation of the radiometer’s responsivity as the solar irradiance level increases or decreases. The spectral response characteristics of thermopile-based pyranometers are largely determined by the spectral transmission of the material used for the protective and insulating domes and the spectral absorption of the detector coating. All pyranometers exhibit some change in responsivity as ambient temperature changes.