A bolometer is a type of thermal detector where the electrical resistance of the material is the property that is measured in response to incident electromagnetic radiation; our particular application is the far infrared / mm wave region. The active element may be a metal with a positive linear temperature coefficient of resistance (TCR), a semiconductor with a negative TCR, a superconductor with negative TCR on the transition between the superconducting and normal states, or some other exotic device.
More generally, thermal detectors convert incident radiation to heat and the active element responds to the heat input resulting in a change in some measureable physical property of the device.
Composite bolometers consist of a number of components, each performing a particular function. Absorption of input radiation occurs in a thin impedance-matched film. The thermistor, which is in direct thermal contact with the film, can therefore be much smaller and have reduced thermal capacitance. This in turn leads to a shorter thermal time constant.