Thermocouples and Pyroelectric Detectors
The basic scientific principle behind thermal detectors:
IR radiation striking a material will cause a heating effect which will then cause a change in the physical properties of the material. The requirements for the material are that for a very small change in the amount of incident radiation there is a maximum temperature change for the detector (so, most detectors are rather small). The materials that can be used vary widely and so do the properties that are changed (e.g. expansion of a gas, voltage between 2 metals).

can be used over a wide range of wavelengths and can operate at room temperature
slow response time (in the milliseconds) and low sensitivity

A thermocouple is a type of thermal detector that places two different metals (e.g. bismuth and antimony together). When the metals are heated by IR radiation, a small voltage, proportional to the temperature at the junction between the 2 metals, is sent out (the Peltier effect). Several thermocouples connected in series make up a thermopile.

Advantages of a thermopile:
-adequate sensitivity
-flat spectral response
Disadvantages of a thermopile:
-suffer from temperature drift (the reference portion of the detector is constantly absorbing heat)

Pyroelectric Detector
A pyroelectric detector is composed of a noncentrosymmetrical crystal that has an internal electrical field along its polar axis. As IR radiation is applied there is a change in the polarization caused by an alteration of the crystal lattice of the crystal. By connecting  2 electrodes to the crystal, the pyroelectric detector can act as a capacitator. Note: the effects depend on the rate of temperature change, not the temperature change itself. The detector will also ignore the effects of background radiation.
Pyroelectric detectors are commonly used in FTIR spectrometers.


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Peltier effect.
 Encyclopedia of Industrial Chemistry: