Bolometric Measurements are based on the dissipation of the RF Power in a small temp.sensitive resistive element called a Bolometer.
This bolometer may be short ultra thin wire having a +ve temp. co. efficient (PTC) of resistance Called Baretter or a bead of Semiconductor having a -ve temp co- efficient (NTC) called Thermistor. Both Baretters andThermistors can measure small powers of the order of a fraction of a micro -watts. They can also indicate or monitor large amounts of power by inserting a directional coupler.
The RF power to be measured heats the bolometer and causes the change in its electrical resistance , which serves as an indication of the magnitude of power.
The Bolometer is generally used in a bridge network so that small changes in resistance can easily be detected and hence power can be mesured.
The temperature sensitive resistor absorbs the measurand power because of which the heat is generated in it. This heat changes the resistance of an element. The bridge circuit measures the change in resistance. The figure below shows the circuit of the bolometer.