A sunshine recorder is a meteorological instrument used for recording the amount of sunlight that a particular location receives throughout a day.
Inside the recorder's adjustable frame are two important pieces: a paper strip, and a glass sphere that can focus the sunlight strongly enough to singe the paper. If the sky is clear, the paper is slowly burned as the sun moves across the sky. If there are clouds, the focused light will not be strong enough to burn the paper. It can be difficult to interpret the output of a sunshine recorder - rain can interfere with the paper's burning, and cloud cover is not an all-or-nothing affair. Furthermore, the paper strip must be manually changed every day.
Campbell-Stokes recorder. Sunshine recorders are sometimes called "heliographs," but this can lead to confusion with the signaling devices of the same name, which are unrelated.
The most widespread variety was invented in 1853 by the John Francis Campbell (Iain Òg Ìle). Later improved in 1879 by Sir George Gabriel Stokes In modern times, light intensity is more often measured with photodiodes and computer recording.
- More detail on the Campbell-Stokes at Kuriositas, including many beautiful pictures.
- Science Friday's article about the sunshine recorder at Blue Hill Observatory in Milton, Massachusetts.
- Sunshine Recorders at Wikipedia.