BACKGROUND OF CREATING INFRARED DETECTORS The technologies for the production of infrared detectors have been developing and improving for over 200 years since the experiment was conducted by English astronomer W.Herschel. During the test, he has found that in the spectrum of the Sun obtained with the help of a prism, the temperature of the reference thermometer rises beyond the boundary of red light, and its values differ from those of other thermometers. The first operating electro-optical converter was developed by Holst of "Philips" (Holland) in 1934. Since the mid‑1930s, publications on IR technology have completely disappeared from the open press. The first competition of the great powers – England, the USSR, Germany and the USA – has begun in the field of night vision. "Holst Glass" was finalized to serial production by EMI (England), and from 1942 to 1945 they were produced in the amount of several thousand pieces for the needs of the British army. At the end of the war in the American zone of the German occupation, a production line for the production of Kiel-IV homing heads with photodetectors from lead sulphide was discovered at the Karl Zeiss plant. Large studies on photodetectors have been started in the United States in 1940 after the organi3Ation of the National Council for Defense Research under the US President, who supervised the issues of optics and infrared technology. The night sights for small arms are known, successfully applied when the Americans landed on the island of Okinawa. In 1944 the use by the German army of the direction-finding equipment for detecting offshore targets from the shore (heated pipes of English ships from distances of up to 10 km) became known. Later it was reported that the German coastal defense forces were using infrared thermal direction-finders at a wavelength of 10 µm using the bolometric effect. In these devices, radiation from targets (ships, boats, etc.) was collected by a parabolic mirror on a blackened plate, heating it, which led to an increase in resistance. The moment of change of resistance testified the finding of the ship in the field of view of the direction finder. About a hundred of such instruments were manufactured at the Karl Zeiss plant. In the Soviet Union, P.V.Timofeev and V.I.Arkhangelsky, as well as academicians S.I.Vavilov and A.A.Lebedev from the Leningrad State Optical Institute were engaged in the development of night vision devices. By the beginning of the war, the Black Sea Fleet had 15 sets of ship borne night vision systems. The equipment developed in 1943–1944 was intended mainly for engineering troops. By decree of the State Defense Committee, a special purpose motorized engineering regiment was formed, which was armed with night sight devices "Alpha", "Gamma", "Comet" and infrared spotlights "OSA‑1" and "OSA‑2". The device "Alpha" with searchlight station "OSA‑2" allowed detecting the target in the dark at a distance of up to 250 meters, and the device "Gamma" – up to 150 meters. The device "Comet" was intended for the search of passes in minefields in the night conditions. For the purposes of long-range observation, a prototype of the night vision device "Elephant" was developed, which, under IR illumination, allowed seeing a human figure at a distance of up to 450 meters. For the armament of the engineering regiment, 7 searchlight stations, 100 "Alfa" devices, 78 "Gamma" devices and 363 "Comet" devices were manufactured. In 1943–1945, the regiment conducted more than 300 thematic drills that showed the suitability and effectiveness of the developed devices for night observation of the enemy’s forward edge of defense, for forcing water obstacles, for indicating passageways in minefields, etc. By 1944, domestic sight for night shooting "Iskra" has been developed. However, in combat conditions, these devices never got a chance to demonstrate their performance .
The intensity of the processes of research, development and exploring of the thermal imaging area has significantly increased in the last 50 years. Today, considering the physical principles of detecting thermal radiation and existing technologies, it is possible to classify IR detectors into groups (Fig. 1). The operational principle of all thermal detectors is based on a change in the electrical characteristics of the receiver material due to the energy of absorbed thermal radiation. In microbolometers, an increase in the temperature of the receiver changes its electrical conductivity, thermal electromotive force appears for the thermopile, the value of the surface charge changes for the pyroelectric receiver, and the thermionic thermal diodes have the value of the internal thermionic emission current. The technology of manufacturing thermal detectors has reached a certain degree of perfection and predetermined a number of advantages due to which sensors of this type occupy a dominant position in the market in quantitative terms. Their advantages are the simplicity of the design and the lack of cooler. There is practically no need for servicing. Microbolometers do not require cooling; they achieve Noise Equivalent Temperature Difference (NETD) of 40–50 mK for an aperture value of one . When passing through the Earth’s atmosphere, the thermal radiation is weakened due to the absorption of gases by molecules, as well as the scattering of molecules and particles (aerosols) by rain, snow, fog, smoke, smog, dust. The "spectral windows" of the Earth’s atmosphere are known: • 0.3–1.3 µm (visible range) – "large window" • 1.5–1.8 µm (IR range) – "first window" • 2.0–2.6 µm (IR range) – "second window" • 7.0–15.0 µm – "thermal IR range" or "third window" Molecular absorption is the main reason for the attenuation of radiation (in the absence of strong smoke and other causes), and the most intense radiation is absorbed by water vapor, carbon dioxide and ozone (Fig. 2). Between the spectral bands, there are bands of complete absorption of IR radiation by the atmosphere, mainly by carbon dioxide CO2 (2.6–2.9, 4.2–4.4 µm) and water vapor H20 (5–8 µm) . ASTRON‑3A MODULE SPECIFICATIONS Multispectral video-thermal imaging modules ASTRON‑3A (Fig. 3) are the basic elements of building a distributed railway track and railway facilities protection system against possible threats. The modules are designed taking into account the considerable experience of operating thermal imaging systems of ASTRON (Tabl.1) type for the protection of extended objects of Gazprom and RZD. Application of the well-proven thermal imaging systems ASTRON‑320A100, ASTRON‑320A75 in the module, which are serially produced during the last years, allows to guarantee the stable operation of the module within three years (or more than 30,000 operating hours).
Multispectral module ASTRON‑3A works in two ranges of radiation: visible and far infrared (7–14 µm). In the module, in addition to thermal imaging, a video channel with a range of vision and recognition of up to 1000 meters is provided. Viewing angles of video and thermal imaging channel are identical. The video channel is used mainly in the daytime. However, the use of low-level sensors in the visible range cameras allows using the optical channel at low illumination (up to 0.001 lx), which is especially effective during the morning and evening temperature equali3Ation. It is possible to switch cameras to fog monitoring mode. Due to the presence of two channels, the detectability of the module is significantly increased at any time of the day and under any weather conditions. DESIGN FEATURES OF ASTRON‑3A MODULE The rigidity and reliability of fastening the module to the support of the contact network (when used in the systems of Russian railways) ensures that the module does not vibrate or jitter during the passage of high-speed trains. Adjustment to the field of view is performed without damaging the module. The construction of the housing contains germanium windows with clearing and diamond-like coating to avoid spectral loss in the IR range when they are contaminated. A special design is provided for protecting germanium windows from contamination during the passage of trains. Direct heating of the germanium window is provided with simultaneous thermal calibration of the matrix for snow thawing.
Anti-vandal feature. Thanks to the mirror in the thermal imaging compartment, the optical axis of the thermal imaging module has been changed. When the external protective window is destroyed as a result of mechanical external objects (stone, bullet, etc.), the removable module is quickly replaced with the window. The cost of a removable module is tens of times less than the cost of a germanium lens and a thermal imager, which remain intact at the same time. Increased reliability. Important features are as follows: no need for field work inside the module, high tightness between the compartments and the outside of the housing, tightness of power connectors and fiber optics, built-in protection against overvoltage in the power circuit, dielectric strength of the fiberglass housing, electrical insulation with high-voltage insulators with the overlap voltage greater than the discharge voltage of the arrester. All this ensures the reliability of the module in extreme conditions of the railway. Microbolometric matrices that have passed military acceptance, with performance of up to –40 °C, exclude their degradation with possible power outages in winter. Fog low-level cameras in the visible range are equipped with ultrafast optics to recognize threat objects at distances up to 1000 meters ASTRON‑3A MODULE HARDWARE The module contains, in addition to the units necessary for thermal imaging and visible surveillance, equipment to protect the module itself from electromagnetic interference, electrical and magnetic interference, surges and ripples of voltage, maintaining various units in the required temperature range, window blowing and direct heating of the window and some other elements (Table 3).
The design of the multispectral module ASTRON (Fig. 4) includes the main assemblies: • the upper compartment (1) contains a low-level fog camera in the visible range, the lens and the window with a brush; • the lower compartment (2) contains the prism for turning the optical axis, the alignment system, the germanium protective window. Thermal radiation from the object through the germanium window falls on the mirror prism and is directed upwards on the lens of the thermal imaging unit. The location in the lower compartment eliminates dust and condensation on the germanium lens. Adjustment to the object is carried out by changing the position of the mirror prism. The prism is made in the form of a cassette and can be easily replaced with a germanium window. • the sealed compartment (3) contains a high-power germanium lens, bolometric matrix, capture board, video analytics unit, surge protection devices, etc. • the module housing is mounted on high-voltage support insulators (4) withstanding voltage exceeding the operating voltage of the arrester on the lightning protection line (ground) of the railway contact system. The housing itself is made of fiberglass and does not require grounding. For the convenience of installation, the housing of ASTRON‑3A module has seats for fixing insulators on three sides, which are mounted on the support plate of the contact network. This allows the module to be fixed depending on necessity on the left, right side of the support or front. Seats on three sides of the module housing under the insulators are hermetically sealed, which allows the module to be installed on either side of the support of the contact network and in any viewing direction from the railway track. At the bottom of the housing, the module contains two sealed IP67 connectors for connecting fiber and power cable. The supply kit includes a return connector for the connector with the connected fiber and a power cable of the required length, coordinated with the installation company. The delivery is possible with the clutch connected to the fiber and with a piercing clamp for connecting the power wire to the line of the self-supporting insulated wire. The optical cable clutch has a hermetic design, which ensures the operation of the module in the open air and does not require additional protection. HIGH-APERTURE GERMANIUM LENSES OF ASTRON‑3A MODULES Germanium passes the spectrum of radiation in the range of 2–16 µm and has a high refractive index, which makes it possible to obtain a high optical power of devices in the range 8–12 µm. In thermal imaging ASTRON‑3A modules, specially designed lenses are used to detect people in the conditions of poor visibility over long distances. The experience of the creation by Russian enterprises of infrared telescopes for obtaining and identifying IR radiation from low-luminosity stars allowed us to create ultra-light lenses for thermal imagers. It seems to us that the importance of such a parameter as the diameter of the lens is underestimated by CCTV specialists and the specialists using security thermal imagers. With small diameters, the lens is not able to collect the required amount of thermal IR radiation from the far object for the bolometric pixel response. The size of the lens becomes particularly important in the thermal imager, since the bolometer is, in other words, a thermistor, heated by infrared rays. The sensitivity of the entire thermal imager depends on how many rays will get from the object to the bolometer.
It is generally accepted that it is problematic to apply microbolometers to obtain images at long distances, since the larger optics at a higher price are required. Our experience shows that the current level of development of the technology of growing large-diameter (with a diameter of more than 100 mm) single crystals of germanium by Chochralski method with high structural perfection makes it possible to obtain the necessary lenses at a quite acceptable cost price on an industrial scale . Germanium windows that come into contact with aggressive external environment have a special "diamond-like" coating. It prevents damage to the germanium window when exposed to dust and abrasive particles. NOMENCLATURE OF MODULES AND DISTANCE OF HUMAN DETECTION ON RAILWAY TRACKS The nomenclature of modules and the distance of human detection on railway tracks are given in Table 3. The distance of human detection is determined by the intelligent analytics built into the module. The parameters indicated in the table are presented based on operational experience on the main course of the railway. The data may differ from those calculated by the Johnson method, since the actual operation takes into account the specific features of the railway infrastructure
Intelligent video and thermal analysis ASTRON‑3A module uses built-in intelligent video and thermal analytics. Analytics setup has a user-friendly interface, understandable to any computer user. All settings are made from the Management Center. All standard threat models, developed by experiment, are configured. The setting is intuitively clear and consists in the calibration of the horizon and perspective, taking into account the actual dimensions of the human dummies and the allocation of control zones with the definition of functions
Features of ASTRON‑3A analytics Features of analytics consist in terms of operation and specificity of the thermal imaging: detection of objects up to 2–3 pixels at a resolution of 324 Ч 256 thermal imaging sensors, a significantly wider range of analyzed temperatures than 256 gray standard television images, features of locomotive and trains recognition, with dust, snow storms, accompanying the passage of high-speed trains, jitter of the analyzed image.
Recognition of human and objects Intelligent analytics of video and thermal imaging is able to recognize not only the type of object (human, dog, object, train, etc.) but also determines its dimensions and speed of movement (Fig. 6). Specifically, the recognition data by the built-in analytics (objective factor) are indicated in the table of the model range of the modules.
Features of simultaneous registration in the visible and thermal spectrum Due to the presence of two channels, the detectability of the module increased significantly during the day, morning and evening. A separate analysis of the video image duplicates the thermal imaging channel, complementing it. The high detectability of the module, which manifests itself at any time of the day and in any weather, provides the possibility of detecting objects that fell from the overpasses to railway tracks (Figs. 6 and 7). Standard solutions for two-track and four-way lines have been developed.
Connecting fiber optic links and setting up network equipment A distribution coupling is used to connect the fiber optic cable, which allows to conduct transit fiber-optic lines and to establish the necessary optical lines for the module operation. The adjustment of all components of the network equipment does not require the work inside the module and is performed via Ethernet TCP/IP network from any access point.
Servicing of the thermal imaging module The thermal imaging module does not require any special maintenance for the entire duration of the work. If you need special flashing of the matrix or change the output parameters, the algorithms for processing the temperature fields of work are carried out by connecting through a sealed plug of the power cable containing service buses for adjusting the matrix. The analytical unit is serviced via Ethernet network from any access point .
CONCLUSION Multispectral module ASTRON‑3A is a unit of full factory readiness, installed on the supports of the contact network and connected to the system via fiber optic network (Fig. 8). ASTRON‑3A module is specially designed for the needs of the railway tracks based on serially produced thermal imaging and video blocks. The module is fully adapted to work in the conditions of the railway. Technical solutions found during the development can be applied both to the improvement of existing thermal imagers and to the development of new generation thermal imagers.