A bolometer consists of an absorptive element, such as a
thin layer of metal, connected to a thermal reservoir (a body of constant
temperature) through a thermal link. The result is that any radiation impinging
on the absorptive element raises its temperature above that of the reservoir —
the greater the absorbed power, the higher the temperature. The intrinsic
thermal time constant, which sets the speed of the detector, is equal to the
ratio of the heat capacity of the absorptive element to the thermal conductance
between the absorptive element and the reservoir. The temperature change can be measured
directly with an attached resistive thermometer , or the resistance of the
absorptive element itself can be used as a thermometer. Metal bolometers
usually work without cooling. They are produced from thin foils or metal films.
Today, most bolometers use semiconductor or superconductor absorptive elements
rather than metals. These devices can be operated at cryogenic temperatures,
enabling significantly greater sensitivity.
Bolometers are directly sensitive to the energy left inside
the absorber. For this reason they can be used not only for ionizing particles
and photons, but also for non-ionizing particles, any sort of radiation, and
even to search for unknown forms of mass or energy (like dark matter ); this
lack of discrimination can also be a shortcoming. The most sensitive bolometers
are very slow to reset (i.e., return to thermal equilibrium with the
environment). On the other hand, compared to more conventional particle
detectors, they are extremely efficient in energy resolution and in
sensitivity. They are also known as thermal detectors.