When solar irradiance is measured with this type of pyrheliometer, the small shutter on the front face of
the cylinder shields one sensor strip from sunlight, allowing it to reach only the other sensor. A
temperature difference is therefore produced between the two sensor strips because one absorbs solar
radiation and the other does not, and a thermoelectromotive force proportional to this difference induces
current flow through the galvanometer. Then, a current is supplied to the cooler sensor strip (the one
shaded from solar radiation) until the pointer in the galvanometer indicates zero, at which point the
temperature raised by solar radiation is compensated by Joule heat. A value for direct solar irradiance is
obtained by converting the compensated current at this time. If S is the intensity of direct solar irradiance
and i is the current, then