Bolometer
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A bolometer is a device for measuring incident electromagnetic radiation. It was invented in 1878 by the American astronomer Samuel Pierpont Langley.
It consists of an "absorber", which is connected to a heat sink (area of constant temperature) through an insulating link. The result is that any radiation absorbed by the absorber raises its temperature above that of the heat sink—the higher the power absorbed, the higher the temperature will be.
A thermometer of some kind, attached to the absorber, is used to measure the temperature, from which the absorbed power can be calculated. In some designs the thermometer is also the absorber; in others the absorber and thermometer are separate; this is known as "composite design".
While bolometers can be used to measure radiation of any frequency, for most wavelength ranges there are other methods of detection that are more sensitive. However, for sub-millimetre wavelengths (from around 200 µm to 1 mm wavelength), the bolometer is the most sensitive type of detector for any measurement over more than a very narrow wavelength range.
Bolometers are therefore used for astronomy at these wavelengths. However, 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); this makes their operation technically somewhat challenging.
The term bolometer is also used in high-energy physics (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.
More conventional particle detectors are often sensitive to ionization effect of ionizating particles. Bolometer are almost directly sensitive to the energy left inside the adsorber. For this reason they can be used not only for ionizating particle and photons, but also for non-ionizationg particle, for any sort of radiation and even to search for unknown forms of mass or energy (like dark matter). They are very slow and they have a high dead time. They lack completely of any sort of discrimination. On the other hand, compared to more conventional particle detectors, they are extremely efficient in energy resolution and in sensitivity. They can be used to test very high radio-purity. They are also known as thermal detectors.
In principle the way of operation is similar to that of a calorimeter in thermodynamics. However the non-standing of many approximation usually taken while dealing with thermodynamic, the need of working at ultra low temperature, and the different aim of the device make the operational use rather different. In the jargon of the 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 usage as particle detectors was advice from the beginning of 20th century but the first regular use, even if in a pioneering way, was only in the 1980s because of the difficulty associated with having a system at cryogenic temperature.
Langley's bolometer
The first bolometer used for infrared observatons by Langley had a very basic design: It consisted of two platinum strips, covered with lampblack, one strip was shielded from the radiation and one exposed to it. The strips formed two branches of a wheatstone bridge which was fitted with a sensitive galvanometer and connected to a battery.
Electromagnetic radiation falling on the exposed strip would heat it, and change its resistance, the circuit thus effectively operating as a resistance temperature detector.
References
- {{cite book
| last = Knoll | first = Glenn F. | authorlink = | coauthors = | year = 2000 | title = Radiation Detection and Measurement | publisher = Wiley | edition = 3rd edition | location = New York | id = ISBN 0471073385
}}
- {{cite journal
| author = McCammon, D. | coauthors = et al. | year = 1993 | month = March | title = Thermal calorimeters for high resolution X-ray spectroscopy | journal = Nuclear Instruments & Methods in Physics Research, Section A (Accelerators, Spectrometers, Detectors and Associated) | volume = A326 | issue = 1-2 | pages = 157-165 | id = | url = | accessdate = }}