Before electronic and digital sunshine sensors became available, daily sunshine duration was measured using the Campbell–Stokes sunshine recorder.

The first sunshine recorder was invented by John Francis Campbell in 1853 and later refined by George Gabriel Stokes in 1879. Campbell’s original design used a water-filled glass sphere that focused the Sun’s rays onto a wooden surface. The instrument was carefully aligned for the local latitude and the ecliptic so that the time scale corresponded with solar time. The water-filled sphere was later replaced with a solid glass sphere, but the repeated scorch marks on the wooden surface made the original design impractical.

To improve the instrument, a specially designed photosensitive card was introduced. The glass sphere concentrated the Sun’s rays onto the card, burning a hole when the sunlight was sufficiently intense and leaving only a scorch mark under weaker conditions. As the Sun moved across the sky, the burn or scorch trace progressed across the card, creating a record of the day’s sunshine. At the end of the day, the card was removed and examined. The length of the burn trace was measured to determine the duration of bright sunshine, while the appearance of the marks provided a rough indication of solar intensity.

Although the Campbell–Stokes recorder was a significant advance for its time and served as the international standard for many decades, it had several limitations. The Sun’s rays at sunrise and sunset were often too weak to burn the card, causing sunshine during those periods to go unrecorded. Intermittent sunshine produced broken or faint burn traces that required subjective interpretation, leading to inconsistencies between observers. In addition, rain could damage the recording card, while frost, ice, snow, or condensation on the glass sphere could reduce its ability to focus sunlight accurately, resulting in measurement errors.