From Glossary of Meteorology
A pyranometer developed by M. Robitzsch.
Its design utilizes three bimetallic strips that are exposed horizontally at the center of a hemispherical glass bowl. The outer strips are white reflectors, and the center strip is a blackened absorber. The bimetals are joined in such a manner that the pen of the instrument deflects in proportion to the difference in temperature between the black and white strips and is thus proportional to the intensity of the received radiation. This instrument must be calibrated periodically.