# Bolometer

Spiderweb bolometer for measurements of the cosmic microwave background radiation. Image credit: NASA/JPL-Caltech.

A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley.[1]

## Principle of operation

Conceptual schematic of a bolometer. Power, P, from an incident signal is absorbed by the bolometer and heats up a thermal mass with heat capacity, C, and temperature, T. The thermal mass is connected to a reservoir of constant temperature through a link with thermal conductance, G. The temperature increase is ΔT = P/G. The change in temperature is read out with a resistive thermometer. The intrinsic thermal time constant is τ = C/G.

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 of the absorptive element to the thermal conductance between the absorptive element and the reservoir.[2] The temperature change can be measured directly with an attached resistive thermometer, 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 or superconductor absorptive elements rather than metals. These devices can be operated at cryogenic 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 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. 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 strips covered with lampblack. One strip was shielded from 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. By 1880, Langley's bolometer was refined enough to detect thermal radiation from a cow a quarter of a mile away.[3] This radiant-heat detector is sensitive to differences in temperature of one hundred-thousandth of a degree Celsius (0.00001 C).[4] 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 portion of the electromagnetic spectrum. Nikola Tesla personally asked Dr. Langley if he could use his bolometer for his power transmission experiments in 1892. Thanks to that first use, he succeeded in making the first demonstration between West Point and his laboratory on Houston Street.[5]

## Applications in astronomy

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. For sub-millimeter wavelengths (from around 200 µm to 1 mm wavelength, also known as the far-infrared or terahertz), 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 to a fraction of a degree above absolute zero (typically from 50 millikelvins to 300 mK). Notable examples of bolometers employed in submillimeter astronomy include the Herschel Space Observatory, the James Clerk Maxwell Telescope, and the Stratospheric Observatory for Infrared Astronomy (SOFIA).

## Applications in particle physics

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). 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. They can still be considered to be at the developmental stage.

## Microbolometers

Main article: Microbolometer

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 or amorphous silicon, 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 640×480 array, a 320×240 array (384×288 amorphous silicon) or less expensive 160×120 array. Different arrays provide the same resolution with larger array providing a wider field of view. 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. At these very low temperatures, the electron system in a metal is weakly coupled to the phonon system. Power coupled to the electron system drives it out of thermal equilibrium with the phonon system, creating hot electrons.[6] 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 is the electronic heat capacity and the relevant thermal conductance is the electron-phonon thermal conductance.

If the resistance 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 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.[2]

## Microwave measurement

A bolometer can be used to measure power at microwave frequencies. In this application, a resistive element is exposed to microwave power. A dc bias current is applied to the resistor to raise its temperature via Joule heating, such that the resistance is matched to the waveguide characteristic impedance. After applying microwave power, the bias current is reduced to return the bolometer to its resistance in the absence of microwave power. The change in the dc power is then equal to the absorbed microwave power. To reject the effect of ambient temperature changes, the active (measuring) element is in a bridge circuit with an identical element not exposed to microwaves; variations in temperature common to both elements do not affect the accuracy of the reading. The average response time of the bolometer allows convenient measurement of the power of a pulsed source.[7]

## References

1. See, for example, bolometers – Definition from the Merriam-Webster Online Dictionary
2. P. L. Richards, "Bolometers for infrared and millimeter waves," Journal of Applied Physics 76, 1–36 (1994), doi:10.1063/1.357128
3. Samuel P. Langley Biography Archived index at the Wayback Machine. High Altitude Observatory, University Corporation for Atmospheric Research
4. NASA Earth Observatory
5. Tesla, Nikola (1992). "section 4". NIKOLA TESLA ON HIS WORK WITH ALTERNATING CURRENTS and Their Application to Wireless Telegraphy, Telephony and Transmission of Power : An Extended Interview. Leland I. Anderson. ISBN 1-893817-01-6. I suppose I had hundreds of devices, but the first device that I used, and it was very successful, was an improvement on the bolometer. I met Professor Langley in 1892 at the Royal Institution. He said to me, after I had delivered a lecture, that they were all proud of me. I spoke to him of the bolometer, and remarked that it was a beautiful instrument. I then said, "Professor Langley, I have a suggestion for making an improvement in the bolometer, if you will embody it in the principle." I explained to him how the bolometer could be improved. Professor Langley was very much interested and wrote in his notebook what I suggested. I used what I have termed a small-mass resistance, but of much smaller mass than in the bolometer of Langley, and of much smaller mass than that of any of the devices which have been recorded in patents issued since. Those are clumsy things. I used masses that were not a millionth of the smallest mass described in any of the patents, or in the publications. With such an instrument, I operated, for instance, in West Point—I received signals from my laboratory on Houston Street in West Point.
6. F.C. Wellstood, C. Urbina and J. Clarke, "Hot-electron effects in metals," Physical Review B 49, 5942–5955 (1994), doi:10.1103/PhysRevB.49.5942
7. Kai Chang (ed), Encyclopedia of RF and Microwave Engineering, (Wiley 2005) ISBN 0-471-27053-9 pages 2736–2739