Bolometer

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.

It consists of an "absorber" 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&mdash;the higher the energy absorbed, the higher the temperature will be. Temperature change can be measured directly or via an attached thermometer (composite design).

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. However, for sub-millimetre wavelengths (from around 200 µm to 1 mm wavelength), the bolometer is the most sensitive 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).

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, for 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 can be used to test very high radio-purity. 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 having a system at cryogenic temperature.

Langley's bolometer
The first bolometer used for infrared observations 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.

This instrument enabled him to feel his way thermally over the whole spectrum, noting all the chief Fraunhofer lines and bands, which were shown by sharp serrations, or more prolonged depressions of the curve which gave the emissions, and discovered the lines and bands of the invisible infra-red portion.

Microbolometers
A microbolometer is a specific type of bolometer used as a detector in a thermal camera. It is a grid of vanadium oxide or amorphous silicon heat sensors atop a corresponding grid of silicon. Infrared radiation from a specific range of wavelengths strikes the vanadium oxide and changes its electrical resistance. This resistance change is measured and processed into temperatures which can be represented graphically. The microbolometer grid is commonly found in three sizes, a 640x480 array, a 320×240 array or less expensive 160×120 array. Both arrays provide the same resolution with the larger array providing a wider field of view. Larger, 1024x768 arrays were announced in 2008.

Cold-electron Bolometer
A cold-electron bolometer uses a SIN (superconducting, insulator, normal metal) junction. The incoming photon is received from a waveguide into the normal metal. It gives its energy to an electron that becomes excited. The significantly increased energy of that electron causes it to tunnel through the narrow insulator layer into the superconductor where it gives rise to a current, that in turn can be measured. The advantage with this is that the absorber is at the same time cooled due to the energy loss.