Thermal Transducers:

These are the ones whose response depends, as the name implies, on the thermal effects. There are two kinds of thermal transducers-the thermocouples and bolometers.

The thermocouples consist of a pair of junctions of various metals; for instance, two pieces of bismuth fused to either end of a piece of antimony. The infrared radiations are made to fall at one junction known as hot junction, the other junction called cold junction is kept at a constant temperature. The difference in temperature causes an electrical potential difference among the two joints and its value would depend on

the difference in temperature among the junctions. The potential difference, therefore, would depend on the amount of radiation falling on the hot junction and thus is a measure of the radiation.

The bolometers instead are a type of resistance thermometer. These are prepared using thin strips of platinum or nickel. You know which in common the resistance of a metallic conductor modifies along with temperature; these materials displays quite large changes in the resistance on changing the temperature. While infrared radiations fall on a bolometer, its temperature changes and causes a corresponding modifies in its resistance. The resistance so measured denotes the amount of the radiation falling on the bolometer.