# Bolometer

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A bolometer is a device for measuring the energy of incident electromagnetic radiation. It was invented in 1878 by the American astronomer Samuel Pierpont Langley.

A bolometer consists of an absorptive element, such as a thin layer of metal, connected to a heat sink (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 heat sink—the higher the energy absorbed, the higher the temperature will be. The temperature change can be measured directly with a thermopile or via an attached 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.

While bolometers can be used to measure radiation energy of any frequency, for most wavelength ranges there are other methods of detection that are more sensitive. For sub-millimeter wavelengths (from around 200 µm to 1 mm wavelength), bolometers are among the most sensitive available detectors, and are therefore used for astronomy at these wavelengths. To achieve the best sensitivity, they must be cooled down to a fraction of a degree above absolute zero (typically from 50 millikelvins to 300 mK).

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. They are very slow to respond and 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.

The term bolometer is also used in particle physics to designate an unconventional particle detector. They use the same principle described above. The bolometers are sensitive not only to light but to every form of energy. The operating principle is similar to that of a calorimeter in thermodynamics. However, the approximations, ultra low temperature, and the different purpose of the device make the operational use rather different. In the jargon of high energy physics, these devices are not called calorimeters since this term is already used for a different type of detector (see Calorimeter (particle physics)). Their use as particle detectors is still at the developmental stage. Their use as particle detectors was proposed from the beginning of the 20th century, but the first regular, though pioneering, use was only in the 1980s because of the difficulty associated with cooling and operating a system at cryogenic temperature.