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The emergence of thermal imagers on the security systems market has the same origins as video cameras. Both are products of the defense industry, which in the 1990s, as a result of conversion, flooded the newly opened market of publicly available security systems. Only if video surveillance cameras, in terms of quality and especially price characteristics, are quite well adapted to the needs and capabilities of a wide range of consumers, then thermal imagers still remain to a large extent products for special units, especially protected facilities, etc., for the equipment and protection of which significant funds can be allocated. And this is not a matter of the vagaries of the market. The issue is both in technical and political aspects.
It is well known that thermal imagers record thermal radiation, which makes it possible to visualize an image not only in complete darkness, but also in thick fog, snow, rain, and behind tree foliage (Fig. 1a and 1b).
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Radiation recorded by thermal imagers
Thermal imagers record infrared radiation in the far spectrum, or so-called thermal radiation. Thermal radiation is emitted by any objects whose temperature differs from absolute zero. All objects emit thermal (infrared) radiation with slightly different wavelengths, i.e. with different energies. This allows objects to be identified and, consequently, visualized. Therefore, a thermal imager records objects even in absolute darkness; no background illumination is required for the thermal imager to operate. Infrared (invisible to the human eye) or thermal radiation is divided into short-wave with a wavelength of λ = 0.76–2.5 μm, medium-wave λ = 2.5–50 μm, and long-wave λ = 50–2000 μm. Recall that visible radiation,
often called light, is electromagnetic radiation perceived by the human eye. It is recorded by CCTV cameras. This radiation is characterized by wavelengths in the range from 380 nm with an energy of 3.1 eV to 760 nm with an energy of 1.6 eV. That is, the longer the wavelength of the radiation, the lower its energy. The maximum of the continuous spectrum of solar radiation is located in the «green» region of 550 nm, which is the maximum sensitivity of the eye.
Thermal imager sensing element
Thermal imaging surveillance, like surveillance in the visible spectrum of radiation using widely used video cameras, is based on the use of sensitive matrices. Both of these areas originate in the defense industry. In the 80s and 90s of the 20th century, thanks to the intensive development of microelectronics, radiation-sensitive matrices of elements were created that allow visualization of images of objects from which radiation falls on these matrices without the use of electromechanical scanning devices.
As is known, all radiation-sensitive receivers can be divided into two large groups according to their operating principle: thermal and photonic.
Photonic receivers are divided into detectors based on:
a) external photoelectric effect (photomultipliers and vacuum photocells, image intensifier tubes);
b) internal photoelectric effect (photoresistors, photodiodes, phototransistors, etc.).
Types of thermal imagers
Thermal imagers are classified by the type of sensitive element.
In thermal imagers with a so-called thermal sensitive element, bolometers are used as the sensitive element.
In thermal imagers with a sensitive element based on an internal photosensitive element, semiconductor sensitive matrices made of materials sensitive to the far infrared region of the spectrum are used as a sensitive element.
First, let us characterize these sensitive elements in general terms.
A comparison of the sensitivity of thermal photodetectors and photodetectors based on semiconductor photosensitive matrices based on the internal photoelectric effect is shown in Fig. 2 (1 — thermal detectors, 2 — photon detectors). |
Due to objective reasons related to the peculiarities of the physical processes of operation of microbolometric elements (absorption of broadband radiation and high noise level), the detectability of these devices is orders of magnitude less than the detectability of semiconductor photosensitive matrices operating on the basis of the internal photoelectric effect, sensitive to radiation with a certain wavelength.
To assess the differences between thermal imagers made on these and other sensitive elements, we will evaluate their detection ability.
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The detectability of semiconductor photosensitive elements fluctuates within 109–1015 cm•Hz1/2W-1. The detectability of bolometers fluctuates within 107–108 cm•Hz1/2W-1. In addition to the fact that this is a very significant difference in itself, a comparison of these values can be used to determine which thermal imager with which sensitive element should be used when recording an object located at different distances. Bolometers are used in IR systems for observation and recognition of objects at short distances (10–1500 m). Initially, thermal imagers based on bolometers were developed for IR sights, binoculars, IR surveillance systems for vehicles, and unmanned small-sized IR surveillance systems.
Thermal imagers based on semiconductor sensitive elements with high detection capability provide “visibility” for tens of kilometers and are used in serious complexes.
Thermal imagers based on thermal photodetectors for image visualization (thermal imagers based on bolometers)
The operating principle of thermal photodetectors is based on recording changes in the properties of a material when its temperature changes due to the absorption of optical radiation. There are different types of thermal photodetectors based on different effects. Among them, the most common are:
a) bolometers, using changes in the resistance of a thin metal, semiconductor or superconducting film;
b) thermoelectric detectors such as thermocouples or thermopiles, using the effect of the occurrence of thermo-EMF at the contacts of two metals;
c) pyroelectric receivers based on the pyroelectric effect in pyroelectric crystals, including ferroelectric crystals, near the Curie temperature;
d) optical-acoustic receivers (OAR), sometimes called pneumatic IR detectors or Golay elements, using periodic expansion and compression of gas when it is heated by amplitude-modulated optical radiation absorbed by a thin membrane. |
A widely used method is to measure the total radiation energy by using the phenomenon of a change in the electrical resistance of a temperature-sensitive element when it is heated due to the absorption of the measured radiation flux. This principle underlies the operation of bolometers. In order to determine the spectral composition of radiation using a bolometer, it is used together with a spectrometer. For spectral measurements, the sensitive element of the bolometer is made in the form of two identical strips. Radiation is directed to one element, and the other is used to compensate for changes in the ambient temperature and interference. The temperature-sensitive element is usually a thin (0.1–1 μm) layer of metal (nickel, gold, bismuth, etc.), the surface of which is covered with a layer of black having a high absorption coefficient in a wide range of wavelengths, or a semiconductor with a high temperature coefficient of resistance (0.04–0.06 °C or more), or a dielectric. Historically, thermal photodetectors appeared before semiconductor detectors of optical radiation based on the generation of electron-hole pairs. Bolometers, like all thermal receivers, are broadband receivers of constant intensity radiation. Since bolometers use a heat-sensitive element, they are used specifically as receivers of infrared (thermal) radiation. The standard sizes of the sensitive matrix of bolometers are 320 x 240 or 160 x 120, while the resolution is the same, but, as in the case of video cameras, a larger matrix allows you to capture a larger field of view with the least distortion. Larger matrices are expensive. Now there is a tendency to switch to matrices of 640 x 480.
The technology for creating bolometers (VOx microbolometers) was developed abroad by Honeywell in the mid-80s of the last century under a contract with the US Department of Defense. Since the 1990s, this technology has been used by Raytheon, Flir, and some others — BAE Systems, L-3 Communications, DRS Technologies, InfraredVision Technologies Corp., NEC, Institut National d’Optique (INO), ULIS.
At present, these technologies are constantly evolving, in scientific and technical publications you can find many publications devoted to the problems of improving and choosing the optimal technology.
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The essence of the question is as follows. As already mentioned, each element of the sensitive matrix of the microbolometer consists of a thin layer sensitive to temperature changes and deposited on a substrate for thermal insulation. The temperature-sensitive element, for example, based on modifications of vanadium oxide VOx and two electrodes connect the temperature-sensitive material and the reading circuit on the substrate. The emitted IR energy received by each detector of the microbolometer increases the temperature of the detector. The change in temperature induces a change in the resistance of each detector, which is recorded by a multiplexing integrated circuit located on the same semiconductor substrate. An important factor in achieving high technical characteristics of microbolometer matrices is the choice of a temperature-sensitive layer with a high temperature coefficient of resistance (TCR) and a low level of excess noise, as well as ensuring good absorption of radiation in the working spectral region. Sensitivity is mainly limited by the thermal conductivity of each pixel. The operating speed is determined by the ratio of thermal capacitance and thermal resistance. Reducing thermal capacity increases not only the operating speed, but also thermal fluctuations — noise. Increasing thermal conductivity increases the operating speed, but reduces sensitivity (and in order for it to increase, it is necessary to increase the TCR and base resistance).
Semiconductor films have a high TCR. An approach to solving some problems of VOx microbolometers is based on the use of amorphous (non-crystalline) silicon as a temperature-sensitive material. Amorphous silicon is characterized by a higher TCR value, which provides a higher level of sensitivity. However, amorphous silicon has a higher base resistance, so there is a problem of matching the high output impedance with the input impedance of the reading chips. Also, semiconductors are characterized by excess current noise. In this regard, the choice of material for the sensitive element is a multifaceted task.
Multilayer, so-called sandwich structures can significantly increase the absorption coefficient of infrared radiation. Sandwiches built in the form of optical resonators absorb 80% of radiation at a wavelength of 8 μm. Absorption of planar structures of 50–80% in the band of 8.5–10 μm can be achieved.
The image visualized by thermal imagers based on bolometers has the same appearance as the image obtained by thermal imagers based on semiconductor matrices.
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Thermal imagers based on the internal photo effect for image visualization
When electromagnetic radiation (including optical radiation) passes through semiconductors, free electrons are generated in them. In the case of the internal photoelectric effect, for intrinsic absorption, the photon energy must be no less than the width of the so-called band gap of the semiconductor (Eg), i.e. for intrinsic absorption of photons with the formation of electron-hole pairs, the following condition must be met: h Eg, where h is the photon energy
- radiation frequency (λ = c/ν)
h is the Planck constant.
The long-wavelength limit of photoconductivity is determined by the ratio:
λ = hc/Eg=1.24/Eg(eV)
This is the maximum wavelength of radiation that will be absorbed by a semiconductor with a given band gap with the formation of electron-hole pairs.
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Different semiconductor materials have different band gaps: GaAs – 1.4 eV, CdSe – 1.8 eV, CdS – 2.5 eV, ZnS – 3.7 eV, Ge – 0.7 eV, Si– 1.1 eV, GaP – 2.3 eV, SiC – 2.4–3.1 eV, etc. Consequently, different materials convert radiation with different wavelengths into electrical signals. Fig. 3 shows the spectral characteristics, and Fig. 4 – the absorption coefficients of photodetectors made of different semiconductor materials. We see that they are all sensitive to radiation with different wavelengths. The cheapest and most widespread material on earth, silicon, is used to record the visible spectral range. Photosensitive matrices based on it are used in video surveillance cameras. The maximum spectral characteristic of silicon devices is located at a wavelength of λ = 0.85 μm – in the near IR region.
That is, thermal imagers register distant IR radiation invisible to the eye with a wavelength of 1.0–14 μm, using semiconductor materials with a band gap corresponding to this radiation spectrum, mainly indium antimonide and arsenide, PbSe, as well as mercury and cadmium tellurite. As already mentioned, any object whose temperature differs from absolute zero has thermal radiation. Moreover, it is different for different objects: forests, water, roads, walls, houses, people, human clothing, different parts of a person's face, etc. emit different radiation. There are even thermal maps of the area (Fig. 5). This is the revolutionary breakthrough made with the development of thermal imagers.
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A thermal imager does not transmit the visible contours of a person's face, but shows the temperature distribution on the face. If, for example, you put your hand to your chest and then quickly remove it, the thermal image will record your image with your hand on your chest. The human eye or a regular CCTV camera will not reflect anything like this. That is, a thermal image often reflects a picture that is not very familiar to us. That is why designs containing a regular video camera and a thermal imager in one housing and transmitting two images are widespread.
The more sensitive the thermal imager, the more accurately it records the temperature distribution. One of the main parameters characterizing the quality of thermal imagers is detectability.
Detectability of thermal imagers
For infrared detectors, the most frequently used quality criterion is the specific detectability (D*).
D*= A1/2 B 1/2/NEP [cm Hz1/2/W]
— NEP – the root-mean-square power of incident radiation required to obtain a signal-to-noise ratio equal to 1 in a frequency band of 1 Hz;
— A – the area of the photodetector, B – the bandwidth of 1 Hz
Fig. 6 shows typical values of detection ability for photoresistors and photodiodes. Photodiodes are designated PD. At wavelengths near 0.5 μm, the highest efficiency is demonstrated by a CdS photoresistor, while at λ = 10 μm, HgCdTe photoresistors are preferable. These are very expensive materials, and their compounds are obtained as a result of complex technological processes. Therefore, the cost of photosensitive elements based on them is several orders of magnitude higher than that of silicon, which is primarily reflected in the final cost of thermal imagers.
The detection ability of infrared photodetectors depends on the type of semiconductor material. For various types of silicon photosensitive matrices used to visualize visible radiation in video cameras, such a difference is, of course, not observed.
It should also be noted that to detect radiation in the mid, far and ultra-far IR ranges, photoresistors are cooled to low temperatures (77 K and 4.2 K). At such temperatures, thermal effects that cause thermal noise are reduced, and the gain and detection efficiency increase. Most often, electric microcryogenic units operating on the Split-Stirling cycle are used for these purposes. This also makes a very significant contribution to the cost of thermal imagers.
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Thermal imager resolution
Just like the photosensitive matrices of CCTV cameras, the photosensitive matrices of thermal imagers have a different number of sensitive elements (pixels) and are characterized by different resolution. The higher the resolution, the more expensive the thermal imager. Modern technologies make it possible to manufacture semiconductor thermal imaging matrices with megapixel resolution. However, their use in the civilian video surveillance market is limited by both price and political factors.
Thermal imagers in the security systems market
On the one hand, the actively developing Russian market is of interest to many foreign manufacturers. On the other hand, thermal imaging devices are primarily a product of the defense industry, and the supply of high-quality products to Russia is limited.
Very often, due to the specificity of thermal imaging, which complicates the identification of the terrain for visual perception, for security surveillance, for combat operations and special operations, a color video camera is built into the body of the thermal imager. An example of such a design is shown in Fig. 9. The figure shows a thermal imager with a video camera of the Oculus RC-5126 brand, broadcasting 9 fps and 25 fps as an option, manufactured by Infinity and distributed in Russia by STA+. The price of such a thermal imager with a short-focus lens (5.8 mm) is about 35,000 euros, and with a long-focus lens (25 mm) — more than 44,000 euros.
The SVS company produces the C-allview device. This is a high-quality video surveillance camera with IR illumination and a thermal imager mounted in one case. The thermal imager in this case costs more than 000. Fig. 8 shows images obtained with this device.
Quite high-quality Israeli thermal imagers are supplied to Russia by LB Sky Global, a division of Group LB. This company supplies thermal imagers with a photosensitive element based on both cooled semiconductor matrices and bolometers. The EYE SES thermal imager supplied by this company has a photosensitive element based on bolometers operating in the wavelength range from 7 μm to 14.0 μm, with a range of up to 1 km. The EYE SEC thermal imager can be mounted on a controlled rotating platform to provide surveillance within a radius of 360 degrees. To detect targets at a distance of at least 1 km and their subsequent recognition at a distance of about 500 m, the EYE SEC thermal imager is equipped with a 75 mm motorized lens, providing a horizontal field of view of 6.1 degrees and a vertical field of view of 4.6 degrees. The cost of this thermal imager is about 000. |
The same company supplies the POP (Plug-in Optronic Payload) thermal imaging module, which is a cooled 3rd generation thermal imager and a high-resolution color video camera installed in one housing on a gyrostabilized rotating platform. The sensitive cooled matrix based on indium antimonide InSb records radiation with a wavelength of 3-5 microns. The range of such a thermal imager is up to 6 km, the cost of this module is about 0,000.
The American company Flir Systems, which produces thermal imagers, is conducting a very active advertising campaign on the Russian security systems market. These thermal imagers have special limiters built in. The price level for thermal imagers from this company fluctuates in a very wide range. Thus, the Patriot thermal imager with a range of 100 m and a resolution of 160 x 120 pixels costs about 00, the SR-19 and SR-100 thermal imagers have a sensitive element based on vanadium oxide, provide visibility at short distances and cost about 000. The Poseidon thermal imager, designed for use in marine environments, is housed in a sealed case, the external elements of which are made of stainless materials with increased corrosion resistance. Its cost is about 000. Thermal imaging system ThermoVision 2000/3000MS is a multi-channel video surveillance system on a high-speed and precise rotary device with a thermal imaging channel, a video channel and an optional laser rangefinder, has a long-wave narrow-band QWIP detector with a 320 x 240 pixel FPA matrix. According to distributors, this system provides visibility up to 40 km and costs 250,000 euros.
Sources:
S. Zi. Physics of semiconductor devices. Moscow, Mir, 1984. |
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