Heliographs and pyranometers

The heliograph is an instrument that measures the daily hours of sun (solar radiation) on paper.
It is based on a very simple mechanism, a glass sphere always facing south, through which the sun's rays pass, concentrating on a beam of light, which burns a cardboard or graduated paper located at the bottom. The path or trace of the burned paper marks the hours of sunshine. The most common and homologated heliograph model is the Cambell-Stokes model.

The pyranomer measures the solar radiation that hits the earth in numerical format, in kW / m2, in a field of 180º. The physical principle generally used in the measurement is a thermocouple on which the radiation falls through two hemispherical glass domes. An example of a pyranometer is that of Kipp and Zonen, which consists of a thermoelectric cell contained in a housing with two crystal hemispheres. The thermoelectric cell is made up of a series of horizontally placed thermocouples, the ends of which are welded with vertical copper bars attached to a solid brass plate.
The black bands of the sensor (thermopile) absorb the solar radiation that is transformed into heat. This heat flows through the sensors into the body of the pyranometer, providing an electrical signal proportional to the incident radiation.