# Solar radiation spectrum and spectral response of thermopile

Most of the energy that it is emitted from the Sun is in form of electromagnetic radiation with a specific spectrum given by the temperature of his external layer. The little part of this energy that arrive to earth is our source of life and energy.

The radiation that arrives to the external layer of the earth to a normal plane, before been filtered by the atmosphere is called Extraterrestrial Radiation and can be approximately calculated by :

$\mbox{E}_{ext}\; =\; \mbox{E}_{sc}\; \cdot \; \left( 1+0.033412\cdot \cos \; \left( 2\pi \frac{dn-3}{365} \right) \right)$

been

$\mbox{E}_{sc}$ =  Solar constant, 1367 [watt/m2]
dn = day number of year (1 … 365)

Then the radiation is filtered by the gases presents in the atmosfere like H2O, CO2, O3 and O2, and reflected by the clouds. The radiation that finally arrives to the lands and oceans it’s in part absorbed and part reflected.

The absorbed radiation is transformed in heat and emitted back to the space like infrared radiation.

Image from Kipp & Zonen

To measure the solar radiation that arrives on Earth requires an instrument with a thermopile, if this is design to measure infrared wavelength the instruments it’s call Pyrgeometer but if is design to measure the visible spectrum It’s call Pyranometer. A good thermopile must have a flat response to the whole spectrum that is measuring.

Image from Kipp & Zonen