The measurement for sunshine is the amount of sunshine captured by an electronic sensor during a single day. Before 2011, the duration was typically measured using a device called Campbell-Stokes sunshine recorder, which focused the sunrays through its lens made of spherical glass onto a card located inside the sphere.
Every time the Sun shines on the device, its rays leave a trace on the card in the form of a burn. By gauging the length of the burn, it’s possible to determine the amount of sunshine. When compared to the readings provided by modern electronic sensors, the burn marks on the cards can provide information regarding the solar radiation and cloud thickness in addition to the sunshine duration.