Transducers or Detectors
There are three categories of transducers employed for detecting IR radiation. These
are as follows:

• Thermal transducers : Thermocouples and Bolometers
• Pyroelectric transducers
• Photoconducting transducers

Thermal Transducers: These are the ones whose response depends, as the name
suggests, on the thermal effects. There are two types of thermal transducers-the thermocouples and bolometers.
The thermocouples consist of a pair of junctions of different metals; for example, two pieces of bismuth fused to either end of a piece of antimony. The infrared radiations
are made to fall at one junction called hot junction, the other junction called cold junction is kept at a constant temperature. The difference in temperature causes an electrical potential difference between the two joints and its value would depend on the difference in temperature between the junctions. The potential difference, thus, would depend on the amount of radiation falling on the hot junction and therefore is a measure of the radiation.
The bolometers on the other hand are a kind of resistance thermometer. These are prepared using thin strips of platinum or nickel. You know that in general the resistance of a metallic conductor changes with temperature; these materials show quite a large changes in the resistance on changing the temperature. When infrared radiations falls on a bolometer, its temperature changes and causes a corresponding change in its resistance. The resistance so measured indicates the amount of the radiation falling on the bolometer.
Pyroelectric Detectors: these are temperature dependant capacitors made from a single crystalline wafer of a pyroelectric material, such as triglycerine sulphate. The heating effect of incident IR radiation causes a change in the capacitance of the capacitor which in turn can be measured in the external circuit as current. Pyroelectric detectors have a quick response time to be able to respond to the signal from the interferometer. Therefore they are used in most of the commercially available FT-IR instruments.
Photoelectric Transducers: these are like the mercury cadmium telluride transducers and comprise of a film of semiconducting material deposited on a glass surface which is sealed in an evacuated envelope. This is similar to the photomultiplier tube provided with dynodes in UV-VIS spectrophotometry. Absorption of IR radiation by them causes the electrical resistance of the semiconductor to decrease which can be measured. These  transducers have better response characteristics than pyroelectric transducers, and also find use in FT-IR hyphenated instruments especially GC - FT-IR etc.