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, but they are not necessarily the most sensitive instruments for some frequency ranges. They can be made competitive in the submillimeter to 1 mm range (far infrared). To achieve sensitivity, such bolometers are cooled to near absolute zero. Micro-bolometers laid out in grid arrays are used for imaging, with on the order of 100 × 100 pixels, and more recently approaching 1000 × 1000.
Note that the adjective bolometric which has the obvious meaning "involving bolometers", is also very often used to indicate the total of all EMR, e.g., from a source. This is the intended meaning 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 complete in that regard, and also, an Earth-bound bolometer will necessarily miss radiation not within atmospheric windows.