For a temperature difference occurring between two ends of a thermocouple, it develops an electrical voltage difference. A series of such thermocouples form a thermopile sensor, with each element being a thin wire made from two materials differing in the thermal activity. All the hot junctions of the thermocouples are placed on a thin common absorbing area that forms the sensor face, while the cold junctions are placed on a heat sink with a high thermal mass.
As an instrument, the thermopile sensor can remotely measure the temperature of objects and people. The operation of the thermopile is better understood in terms of heat flow rather than temperature rise. Any object with a temperature higher than the ambient is actually sending out heat in specific spectral characteristics and density. According to primary thermodynamics, heat flows from an object at a higher temperature to another at a lower temperature, causing a change in energy levels of both the objects in the process.
The amount of heat absorbed depends on the field of view and surface area the Thermopile sensor presents to the heat source. Heat reaching the thermocouples inside flows through the membrane structure of the thermopile, finally reaching the heat sink or the housing bottom. This heat flow causes a difference of temperature on the ends of the thermocouples located on the absorber and those on the heat sink, ultimately resulting in a voltage difference between the ends of the thermopile sensor.
Compared to the traditional contact-based temperature sensors, thermopile temperature sensors are of the non-contact type, and hence, they are more popular industrially. Rather than use conduction for heat transfer, thermopile temperature sensors use infrared radiation, allowing better reliability and performance in several constrained applications.
The voltage difference on the ends of the thermopile sensor is analyzed by a Thermopile sensor IC, which provides temperature readout in a convenient digital format. Continuous improvements in this field are resulting in devices that consume reduced power, are smaller and more affordable. This translates into more application opportunities for thermopile temperature sensors in home appliances, office equipment, medical instruments, and consumer devices.
Thermopiles with single-element infrared sensors are popular in the low-end market, as they are good for detecting the presence of stationary warm bodies in a room. However, these simple sensors are unable to provide the direction of movement of a moving object in their field of view. For this additional functionality, engineers use thermopile arrays.
Rather than use a single sensing element as in a thermopile temperature sensor, thermopile arrays use multiple IR sensing elements working together. Integrated signal processing capabilities and coordinated sensing elements of the modern thermopile arrays allow the devices to measure not only absolute temperatures but also temperature gradients. This allows thermopile arrays to sense the direction of movement of the heat source, such as up, down, left, right, and diagonally. Thermopile arrays can detect the presence of multiple objects or people even as they move about in different directions. This allows them to sense the proximity of the heat source and handle control tasks with simple gestures.