In order to detect these radiations, a bolometer is composed of an absorber and a thermometer.
When a radiation with certain energy arrives on the bolometer, it will locally heat the atoms composing the absorber. This heating is measured by an ultrasensitive thermometer, often made of superconductors, that will detect a rise of the temperature as weak as 1 µK (a millionth of a degree). We can then go back to the energy of the incident radiation and to its properties.
To build a thermometer sensitive to the smallest change in temperature, we can use a superconductor that is maintained at a temperature slightly below its critical temperature Tc, i.e. the temperature at which it stops being superconducting. Generally, we choose materials with a Tc very close to absolute zero (only 0.02 to 0.3 kelvin).
When the radiation heats the superconductor, the latter stops being superconducting and becomes a normal metal again. Its resistance goes from zero to a value not equal to zero. It is this sudden change of resistance that is measured by the bolometer and that enables the indirect measurement of the radiation that caused the change.
