Self calibrating cavity pyrheliometers are used to define the scale of solar radiation. This type of instrument can be constructed and characterized to yield absolute radiation values in Standard International (SI) units by employing the electrical substitution method. A selected group of these instruments is know as the World Standard Group (WSG) which is maintained at the World Radiation Center (WRC) in Davos, Switzerland. Using this group of instruments, the World Radiation Reference (WRR) is periodically determined. All other cavity instruments are referenced to the WRR by intercomparison.
The Eppley version of the self-calibrating cavity pyrheliometer is the Absolute Cavity Pyrheliometer (Model HF). This device has a cavity receiver configured as an inverted cone within a cylinder. A precision aperture defines the area of the direct solar beam for the measurement. One model HF instrument is included in the WSG.