Abolometer is a device for measuringradiant heat by means of a material having a temperature-dependentelectrical resistance.[1][2] It was invented in 1878 by the American astronomerSamuel Pierpont Langley.
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 theheat capacity of the absorptive element to thethermal conductance between the absorptive element and the reservoir.[3] The temperature change can be measured directly with an attached resistivethermometer, 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 usesemiconductor orsuperconductor absorptive elements rather than metals. These devices can be operated atcryogenic 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 andphotons, but also for non-ionizing particles, any sort ofradiation, and even to search for unknown forms of mass or energy (likedark 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.
The first bolometers made by Langley consisted of twosteel,platinum, orpalladium foil strips covered withlampblack.[4][5] One strip was shielded from radiation and one exposed to it. The strips formed two branches of aWheatstone bridge which was fitted with a sensitivegalvanometer 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 (400 m) away.[6] This radiant-heat detector is sensitive to differences in temperature of one hundred-thousandth of a degree Celsius (0.00001 °C).[7] This instrument enabled him to thermally detect across a broad spectrum, noting all the chiefFraunhofer lines. He also discovered new atomic and molecular absorption lines in the invisibleinfrared portion of the electromagnetic spectrum.Nikola Tesla personally asked Dr. Langley whether 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.[8]
While bolometers can be used to measure radiation of any frequency, for mostwavelength ranges there are other methods of detection that are more sensitive. Forsub-millimeter wavelengths throughmillimeter wavelengths (from around 200 μm to a few mm wavelength, also known as the far-infrared,terahertz) bolometers are among the most sensitive available detectors, and are therefore used forastronomy at these wavelengths. To achieve the best sensitivity, they must be cooled to a fraction of a degree aboveabsolute zero (typically from 50 mK to 300 mK[9]). Notable examples of bolometers employed in submillimeter astronomy include theHerschel Space Observatory, theJames Clerk Maxwell Telescope, and theStratospheric Observatory for Infrared Astronomy (SOFIA). Recent examples of bolometers employed in millimeter-wavelength astronomy areAdvACT,BICEP array,SPT-3G and the HFI camera on thePlanck satellite, as well as the plannedSimons Observatory, CMB-S4 experiment,[10] andLiteBIRD satellite.
The term bolometer is also used inparticle physics to designate an unconventionalparticle 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 acalorimeter inthermodynamics. However, the approximations,ultra low temperature, and the different purpose of the device make the operational use rather different. In thejargon of high energy physics, these devices are not called "calorimeters", since this term is already used for a different type of detector (seeCalorimeter). 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 atcryogenic temperature. They can still be considered to be at the developmental stage.
Bolometers play a pivotal role in monitoring radiation in fusion plasmas. TheWendelstein 7-X (W7-X)stellarator employs a two-camera bolometer system to capture plasma radiation. This setup is optimized to identify 2D radiation distributions within a symmetrical triangular plasma cross-section. Recent progress includes the refinement of a tomographic reconstruction algorithm, which leans on the principle of relative gradient smoothing (RGS) of emission profiles. This has been effectively applied to the W7-X hydrogen discharges powered byelectron cyclotron resonance heating (ECRH). In terms of hardware, the W7-X bolometers are equipped with metal-resistive detectors. These are distinguished by a 5 μm thick gold absorber, sized 1.3 mm in the poloidal direction and 3.8 mm toroidally, mounted on a ceramic (silicon nitride Si3N4) substrate. The inclusion of a 50 nm carbon layer is strategic, enhancing the detection efficiency for low-energy photons. These detectors are notably attuned to impurity line radiation, covering a spectrum from the very ultraviolet (VUV) to soft x-rays (SXR). Given their resilience and innovative design, they are being considered as prototypes for the upcomingITER bolometer detectors.[11][12]
Amicrobolometer is a specific type of bolometer used as a detector in athermal camera. It is a grid ofvanadium oxide oramorphous silicon heat sensors atop a corresponding grid ofsilicon.Infraredradiation from a specific range ofwavelengths strikes the vanadium oxide or amorphous silicon, and changes itselectrical 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. 640x512 VOx arrays are commonly used in static security camera applications with low shock resistance requirements. Different arrays provide the same resolution with larger array providing a widerfield of view.[citation needed] Larger, 1024×768 arrays were announced in 2008.
The hot electron bolometer (HEB) operates atcryogenic temperatures, typically within a few degrees ofabsolute zero. At these very low temperatures, theelectron system in a metal is weakly coupled to thephonon system. Power coupled to the electron system drives it out of thermal equilibrium with the phonon system, creating hot electrons.[13] 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 relevantheat capacity is the electronic heat capacity and the relevantthermal conductance is the electron-phonon thermal conductance.
If theresistance 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 bothsemiconducting andsuperconducting 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.[3]
A bolometer can be used to measure power atmicrowave 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 viaJoule heating, such that the resistance ismatched 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 abridge 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.[14]
In 2020, two groups reported microwave bolometers based on graphene-based materials capable of microwave detection at the single-photon level.[15][16][17]
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.
