The [Campbell-Stokes Pattern] Sunshine Recorder is used to measure the duration of sunshine, that is, the number of hours of sunshine per day.
The sun’s rays are focused by a glass sphere to an intense spot which will char a mark on a curved, specially designed card mounted concentrically with the sphere. As the earth rotates, the position of the spot moves across the card leaving a measurable trace. Due to changing cloud cover or the occurrence of precipitation,
the trace may not necessarily be continuous. At the end of the day, the total length of the trace is proportional to the duration of sunshine.