Bolometers are thermal detectors that measures the increase in temperature of an absorber beeing heated by incident radiation. In order to be sensitive enough, bolometers have to be cooled to cryogenic temperature (T<350mK).

Schematic of a bolometer (from J.P. Torre)

This detectors are the most sensitive ones for large band detection of wavelength from about 100µm to 3mm. Massive bolometers are also used for X-rays and particles detection.

Performant bolometers are necessary for sensitive astrophysics instruments, but are not sufficient: a proper instrument definition has to be designed in order to reach the ultimate sensitivity of such detectors. HFI bolometers are indeed sensitive to 10^-17W of incident radiation power!

In particular, any temperature fluctuations of the plates supporting the optics and the bolometer can be taken as a signal if the temperature is not monitored to a high sensitivity. While the HFI sensitive thermometers are not radiation detectors, they have the same position in the HFI detection chain as bolometers.