A pyrheliometer is used to measure direct solar radiation from the sun and its marginal periphery. To
measure direct solar radiation correctly, its receiving surface must be arranged to be normal to the solar
direction. For this reason, the instrument is usually mounted on a sun-tracking device called an equatorial
mount.
The structure of an Angstrom electrical compensation pyrheliometer is shown in Figure 7.6 (a).
This is a reliable instrument used to observe direct solar radiation, and has long been accepted as a working
standard. However, its manual operation requires experience.