Thermopile Sensor Physics

Thermopile sensors are based on thermocouples. A thermocouple consists of two dissimilar metals connected in series. To detect radiation, one metal junction is typically blackened to absorb the radiation. The temperature rise of this junction with respect to another non-irradiated junction generates a voltage. This effect is the basis of all thermocouple temperature sensors. The thermocouple materials used in thermopiles are usually bismuth and antimony, which have a relatively high thermoelectric coefficient (a measure of the magnitude of the induced voltage in response to the temperature difference). An individual thermocouple typically produces a low output voltage, which results in a low detectivity and limits its use as a sensing device. So, one way to increase the output voltage is to connect many thermocouple junctions (typically 20 to 120) in series. All the “hot” junctions are placed close together to collect the radiation.