The heliograph

Insolation is when the Sun shines above the horizon on a given point of the Earth. This is measured by means of a helio-phano-graph, which consists of a spherical lens that concentrates the Sun’s rays on to a strip of special thermal paper that blackens when hit by the Sun’s rays that have been concentrated by the lens.
The energy received from a given surface, instead, is called global radiation, and is measured with an instrument called pyranometer or solarimeter, and is expressed in calories per unit of time per surface unit.

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