A combined electrical substitution and cavity radiometer where the electrical substitution inequivalence, efficiency of the cavity, the area of the entrance aperture, radiative and conductive losses, and other energy exchanges are accounted for such that the electrically substituted heating can be absolutely equated to the radiant heating of the detector. Most currently existing absolute cavity radiometers are designed for the measurement of direct solar irradiance. The World Radiation Reference (WRR) scale for solar irradiance observations used in many meteorological and climatological applications is defined by a group of well-characterized absolute cavity radiometers maintained by the World Radiation Center (WRC) in Davos, Switzerland.

An instrument used for very accurate measurements of solar irradiance. Absolute cavity radiometers absorb radiation on a blackened conical receiver and are electrically self-calibrating. Absolute cavity radiometers determine the solar constant and provide the reference from which other radiometers are calibrated. Here is a picture of an absolute cavity radiometer used to transfer the World Radiometric Reference ( WRR) to all radiometers calibrated at the Solar Radiation Research Laboratory.