The thermopile sensor generates a voltage output signal that is proportional to the heat flux. Power generation using multiple thermocouples, as in a thermopile, is common. A thermopile is an electronic device that converts thermal energy into electrical energy. The corresponding thermopile of high thermal conductivity surrounds the experimental space within the calorimetric block. HP3-RAD uses thermopile detectors to measure three spectral bands&colon; , and . He developed differential absorption spectroscopy using the electro-magnetic thermopile devised by Melloni. They used a thermopile to determine that sunspots were cooler than the surrounding regions.