Sunshine autograph, heliograph, a device for measuring the duration of direct sunlight. In the sunshine autograph, developed by Campbell and Stokes, the direct radiation is concentrated using a solid glass sphere with a diameter of 9–12 cm, so that the solar radiation leaves a burning mark on a cardboard strip (Fig.). The length of the burning mark corresponds to the duration of direct sunlight. The cardboard strips must be changed daily, and in the tropics and during the polar summer, even twice daily. This effort and the manual evaluation are leading to sunshine autographs increasingly being replaced by pyranometers, whose continuous recording of the energy flux density allows data on sunshine duration to be generated synthetically.
