From Glossary of Meteorology
A sunshine recorder of the type in which the timescale is supplied by the motion of the sun.
It consists essentially of a spherical lens that burns an image of the sun upon a specially prepared card. The instrument must be oriented carefully so that the timescale on the card agrees with the sun time. The depth and breadth of the trace may be interpreted in terms of the approximate intensity of the sun.
Middleton, W. E. K., and A. F. Spilhaus 1953. Meteorological Instruments. 3d ed., rev., . p. 218.