During the measurement phase, the disk is heated by solar radiation and its temperature rises. The
intensity of this radiation is ascertained by measuring the temperature change of the disk between the
measurement phase and the shading phase with the mercury-in-glass thermometer.
The structure of a thermoelectric pyrheliometer is shown in Figure 7.8. This instrument uses a
thermopile at its sensor, and continuously delivers a thermoelectromotive force in proportion to the direct
solar irradiance. While Angstrom electrical compensation pyrheliometers and silver-disk pyrheliometers
Figure 7.7 Silver-disk pyrheliometer