A thermopile is a device made of thermocouples, which are thermoelectric devices that convert temperature differences into electrical voltage. Thermopile detector chips use multiple thermocouples to detect infrared radiation and convert it into an electrical signal. The electrical signal is proportional to the temperature difference between the hot and cold junctions of the thermocouples, which is caused by the absorption of infrared radiation. These detectors are commonly used in temperature measurement and thermal imaging applications.