[]These pyrheliometers are from the 50's and were used for the observing practical to measure the temperature of the sun. The front side is covered by baking paper in order to diffuse the light and shine it onto a brass element that can be found inside the tube. The brass is painted black in order to absorb all the light, which in turn makes the temperature of the brass go up. This difference can be seen on the thermometer and be used to calculate the flux of the Sun. This can in turn be used to calculate the luminosity of the Sun within 10%. These instruments were made in the early 50's in the instrument workplace of Utrecht.