Bolometer

# Bolometer

Discussion

Encyclopedia
A bolometer is a device for measuring the power of incident electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

via the heating of a material with a temperature-dependent electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

. It was invented in 1878 by the American astronomer Samuel Pierpont Langley
Samuel Pierpont Langley
Samuel Pierpont Langley was an American astronomer, physicist, inventor of the bolometer and pioneer of aviation...

. The name comes from the Greek word bole (βολή), for something thrown, as with a ray of light.

## Principle of operation

A bolometer consists of an absorptive element, such as a thin layer of metal, connected to a thermal reservoir (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 reservoir — the greater the absorbed power, the higher the temperature. The intrinsic thermal time constant, which sets the speed of the detector, is equal to the ratio of the heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...

of the absorptive element to the thermal conductance
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

between the absorptive element and the reservoir. The temperature change can be measured directly with an attached resistive thermometer
Thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

, or the resistance of the absorptive element itself can be used as a thermometer. Metal bolometers usually work without cooling. They are produced from thin foils or metal films. Today, most bolometers use semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

or superconductor absorptive elements rather than metals. These devices can be operated at cryogenic
Cryogenic particle detectors
Cryogenic particle detectors operate at very low temperature, typically only a few degrees above absolute zero. These sensors interact with an energetic elementary particle and deliver a signal which can be related to the type of particle and the nature of the interaction...

temperatures, enabling significantly greater sensitivity.

Bolometers are directly sensitive to the energy left inside the absorber. For this reason they can be used not only for ionizing particles and photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s, but also for non-ionizing particles, any sort of radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

, and even to search for unknown forms of mass or energy (like dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

); this lack of discrimination can also be a shortcoming. The most sensitive bolometers are very 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.

## Langley's bolometer

The first bolometer used by Langley consisted of two platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

strips covered with lampblack. One strip was shielded from radiation and one exposed to it. The strips formed two branches of a Wheatstone bridge
Wheatstone bridge
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...

which was fitted with a sensitive galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...

and connected to a battery. Electromagnetic radiation falling on the exposed strip would heat it and change its resistance. By 1880, Langley's bolometer was refined enough to detect thermal radiation from a cow a quarter of a mile away. This instrument enabled him to thermally detect across a broad spectrum, noting all the chief Fraunhofer lines. He also discovered new atomic and molecular absorption lines in the invisible infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

portion of the electromagnetic spectrum.

## Applications in astronomy

While bolometers can be used to measure radiation of any frequency, for most wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

ranges there are other methods of detection that are more sensitive. For submillimeter wavelengths (from around 200 µm to 1 mm wavelength, also known as the far-infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

), bolometers are among the most sensitive available detectors, and are therefore used for astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

at these wavelengths. To achieve the best sensitivity, they must be cooled down to a fraction of a degree above absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....

(typically from 50 millikelvins
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

to 300 mK). Notable examples of bolometers employed in submillimeter astronomy include the Herschel Space Observatory
Herschel Space Observatory
The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....

, the Atacama Large Millimeter Array
Atacama Large Millimeter Array
The Atacama Large Millimeter/sub-millimeter Array is an array of radio telescopes in the Atacama desert of northern Chile. Since a high and dry site is crucial to millimeter wavelength operations, the array is being constructed on the Chajnantor plateau at 5000 metres altitude...

(ALMA), the James Clerk Maxwell Telescope
James Clerk Maxwell Telescope
The James Clerk Maxwell Telescope is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. Its primary mirror is 15 metres across: it is the largest astronomical telescope that operates in submillimetre wavelengths of the electromagnetic spectrum...

, and the Stratospheric Observatory for Infrared Astronomy
Stratospheric Observatory for Infrared Astronomy
The Stratospheric Observatory for Infrared Astronomy is a joint project of NASA and the German Aerospace Center to construct and maintain an airborne observatory. NASA awarded the contract for the development of the aircraft, operation of the observatory and management of the American part of the...

(SOFIA).

## Applications in particle physics

The term bolometer is also used in particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

to designate an unconventional particle detector
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...

. 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
Calorimeter
A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal microcalorimeters, titration calorimeters and accelerated rate calorimeters are among the most common...

in thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

. However, the approximations, ultra low temperature
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...

, and the different purpose of the device make the operational use rather different. In the jargon
Jargon
Jargon is terminology which is especially defined in relationship to a specific activity, profession, group, or event. The philosophe Condillac observed in 1782 that "Every science requires a special language because every science has its own ideas." As a rationalist member of the Enlightenment he...

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)
Calorimeter (particle physics)
In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its...

).
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.

## Microbolometers

A microbolometer
Microbolometer
A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Infrared radiation with wavelengths between 7.5-14 μm strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures...

is a specific type of bolometer used as a detector in a thermal camera. It is a grid of vanadium oxide
Vanadium oxide is the chemical compound with the formula V2O5. Commonly known as vanadium pentoxide, this brown/yellow solid is the most stable and common compound of vanadium. Upon heating it reversibly loses oxygen...

or amorphous silicon
Amorphous silicon
Amorphous silicon is the non-crystalline allotropic form of silicon. It can be deposited in thin films at low temperatures onto a variety of substrates, offering some unique capabilities for a variety of electronics.-Description:...

heat sensors atop a corresponding grid of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

. Infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

from a specific range of wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s strikes the vanadium oxide and changes its electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

. This resistance change is measured and processed into temperatures which can be represented graphically. The microbolometer grid is commonly found in three sizes, a 640×480 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
Field of view
The field of view is the extent of the observable world that is seen at any given moment....

. Larger, 1024×768 arrays were announced in 2008.

## Hot electron bolometer

The hot electron bolometer (HEB) operates at cryogenic temperatures, typically within a few degrees of absolute zero
Absolute zero
Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....

. At these very low temperatures, the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

system in a metal is weakly coupled to the phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

system. Power coupled to the electron system drives it out of thermal equilibrium with the phonon system, creating hot electrons. Phonons in the metal are typically well-coupled to substrate phonons and act as a thermal reservoir. In describing the performance of the HEB, the relevant heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...

is the electronic heat capacity and the relevant thermal conductance is the electron-phonon thermal conductance.

If the resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

of the absorbing element depends on the electron temperature, then the resistance can be used as a thermometer of the electron system. This is the case for both semiconducting
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

and superconducting materials at low temperature. If the absorbing element does not have a temperature-dependent resistance, as is typical of normal (non-superconducting) metals at very low temperature, then an attached resistive thermometer can be used to measure the electron temperature.

• Scintillating bolometer
Scintillating bolometer
A scintillating bolometer is a scientific instrument used in the search for dark matter. It works by simultaneously measuring both the light pulse and heat pulse generated by a particle interaction within its internal scintillator crystal....

• Pyrometer
Pyrometer
A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry.This device can be used to determine the temperature of an object's surface....

A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....

• Thermistor
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...

• Transition edge sensor
Transition edge sensor
A transition edge sensor or TES is a type of cryogenic particle detector that exploits the strongly temperature-dependent resistance of the superconducting phase transition.-Principle of operation:...

• Cryogenic particle detectors
Cryogenic particle detectors
Cryogenic particle detectors operate at very low temperature, typically only a few degrees above absolute zero. These sensors interact with an energetic elementary particle and deliver a signal which can be related to the type of particle and the nature of the interaction...