A Bolometer is an instrument for measuring Electromagnetic Radiation by allowing the radiation to heat a Metal whose electrical resistance is dependent upon its Temperature. As the material heats, the change in resistance is noted.
Bolometers measure a relatively wide range of Frequencies, thus are more suited to measuring energy over a wide range or estimating the total incident energy from a source.
Bolometers are not necessarily the most sensitive instruments at some frequencies. They can be made competitive in the submillimeter to 1 mm range (Far Infrared). To achieve sensitivity, such bolometers are run near absolute zero.
Micro-bolometers are used for imaging, with on the order of 100 x 100 pixels, and more recently approaching 1000 x 1000.
Note that the adjective Bolometric which has the obvious meaning "having to do with bolometers" is very often used to imply the sum of all EMR, e.g., from a source. This is the use in terms like Bolometric Correction, Bolometric Luminosity, and Bolometric Magnitude. This came about because Bolometers do indeed measure the power of the EMR striking it across a wide band, wider than many other instruments, though they are not actually perfect in that regard, and an Earth-bound bolometer will necessarily miss radiation not in Atmospheric Windows.
Atacama Cosmology Telescope (ACT)
Atacama Pathfinder Experiment (APEX)
Atacama Submillimeter Telescope Experiment (ASTE)
Focal Plane Array (FPA)
Green Bank Telescope (GBT)
James Clerk Maxwell Telescope (JCMT)
South Pole Telescope (SPT)
Transition Edge Sensor (TES)