Thermopile is a device used to detect thermal radiation. It works on the principle of Seebeck effect.
Since a single thermocouple gives a very small emf, a large number of thermocouples are connected in series. The ends are connected to a galvanometer G (Figure). One set of junctions (1,3,5) is blackened to absorb completely the thermal radiation falling on it. The other set of junctions (2,4) called cold junction is shielded from the radiation.
When thermal radiation falls on one set of junctions (1, 3, 5) a difference in temperature between the junctions is created and a large thermo emf is produced. The deflection in the galvanometer is proportional to the intensity of radiation.