Possibilities of the thermal imaging method of non-destructive testing in solving anti-terrorist problems
KOVALEV Aleksey Alekseevich
KOVALEV Aleksey Vasilyevich, professor, doctor of technical sciences
Source: Magazine «Special Equipment» No. 6 2007
Everything related to terrorism and security is currently discussed with great enthusiasm, at different levels, with different final results. This is undoubtedly necessary and relevant, but it is necessary to understand that security issues require careful research, study and analysis with mandatory prediction of the final result. Such a process is usually called diagnostics.
Diagnostics in its broad sense is the basis of security. The entire life of a person, from beginning to end, for many centuries has been accompanied by diagnostics — from slapping a newborn baby to make him scream, to putting a mirror to the mouth to make sure that the person is still breathing. Both medicine and security must be accompanied by diagnostics using known methods and means of research. The overwhelming majority of all emergency events and terrorist attacks are due to the lack of effective diagnostics. Diagnostics is the technical and methodological support of security, its main intelligence. If we talk about the anti-terrorist aspect of security, then in this case anti-terrorist and forensic diagnostics (ATC) can be defined as a set of principles, methods and means of preventing and warning terrorist acts and other criminal manifestations, or, in other words, as an active component of the technical support of the security of the country, society and the prevention of terrorism.
The purpose of this work is to further examine the capabilities of non-destructive testing (NDT) methods to prevent terrorist attacks and improve public safety, more specifically, to examine thermal imaging testing methods, their features, stages of development, and modern hardware arsenal. It is well known that the high information content and broadest potential capabilities of ND methods are due to the use of virtually the entire frequency range of the electromagnetic spectrum, which allows the creation of technical means capable of seeing in optically opaque media. The process of seeing is carried out by visualizing, using optical-electronic systems, images invisible to the human eye, created in the X-ray, ultraviolet, infrared (IR) and other ranges of the electromagnetic spectrum.
Optical images and patterns are the highest form of receiving, storing and transmitting information, as well as its most convenient, optimal form for human perception. If you look at the graphic image of the electromagnetic radiation spectrum, it is easy to see that the visible range, that is, the range in which a person sees without the use of technical means, occupies only a small part of it. Naturally, in order to obtain more information about the world around us or about individual objects, it is necessary to carry out vision in other ranges.
The IR range of the spectrum is quite interesting and informative, which is due to the fact that it is here that the main share of the electromagnetic radiation of most objects of natural and artificial origin around us is concentrated. The IR range covers wavelengths from 0.76 to 1000 μm (which corresponds to frequencies from 300 to 0.3 THz). This fairly wide region of the spectrum is conventionally divided into five intermediate ranges: near (0.76 — 1.1 μm), short-wave (1.1 — 2.5 μm), medium-wave (3.0 -5.5 μm), long-wave (8 — 14 μm) and far (15 -1000 μm). Sometimes the first two ranges are combined into one for convenience (0.76 — 2.5 μm).
IR ranges of 3 — 5.5 and 7 — 14 µm are the working zones of the thermal imaging method of non-destructive testing. It should be noted that the more informative range of 8-14 µm is of particular interest, completely coinciding with the widest window of atmospheric transparency and corresponding to the maximum emissivity of the observed objects in the temperature range from -50 to +500° C.
The thermal imaging method of control is based on the fact that any processes occurring in nature and human activity are accompanied by the absorption and release of heat, changing the internal energy of the body, which in a state of thermodynamic equilibrium is proportional to the temperature of the substance. As a result, the surfaces of physical bodies acquire a specific temperature distribution. The main way to implement the thermal imaging method is to create hardware that ensures the conversion of temperature distribution or IR radiation into a visible image. The implementation of the capabilities of the thermal imaging method, which ensures both the detection of internal defects in various objects and an effective solution to the problem of night vision, detection of hidden (camouflaged) objects or the implementation of search activities in difficult weather conditions, led to the creation of a wide range of thermal imaging equipment — portable, mobile, stationary. The operating principle of thermal imaging devices is based on the two-dimensional conversion of their own thermal radiation from objects and terrain (or background) into a visible image. Thermal imaging technology has a number of advantages and capabilities that are unique to it: detection of remote heat-emitting objects (targets) regardless of the level of natural illumination, and to a certain extent — from thermal or other interference (smoke, rain, fog, snow, dust, etc.) The development of thermal imaging technology began in the early 60s of the 20th century with research and development of devices in two main areas:
- using discrete radiation receivers together with image scanning (scanning) systems;
- using equipment without mechanical scanning based on two-dimensional IR receivers.
Today, we can conditionally distinguish four generations of development of such technology.
Zero generationbased on the use of single cooled receivers and two-dimensional (line and frame) scanning using a scanning optical-mechanical system;
First generation — on the use of line receivers and simplified frame scanning;
Second generation— on the use of grouped several lines (with time delay and accumulation) and a low-speed scanning system. The second generation includes vacuum devices with electronic scanning of the receiving target — pyrocones.
The fundamentally new third generation is based on the use of «simultaneously viewing» — focal-plane (FPA — Focal Plate Area) and two-dimensional solid-state multi-element (matrix) radiation receivers (MDR), that is, without the use of optical-mechanical scanning systems.
In recent years, the development of thermal imaging technology has been mainly based on the use of uncooled multi-element MPI, the physical characteristics of which are very high and are practically not inferior to cooling systems. Modern thermal imaging systems (TIS) have small weight and size characteristics and energy consumption, provide silent operation and high quality of thermal imaging, a wide dynamic range when operating in the broadcast television standard mode, digital processing in real time, communication with a computer, etc. and are divided into two main classes:
- observational (showing);
- measuring, or radiometric (thermographs).
Observational TPS are designed to detect, recognize and visualize remote heat-emitting objects (or targets) against the background of thermal interference. Such systems can be supplemented with autonomous channels, which usually contain a scaled television channel and a channel for remote temperature measurement with laser target designation, as well as laser rangefinders.
Fig. 1. Results of detection of various objects by a thermal imaging system
Fig. 2. Spectral transmittance of the atmosphere
Such an addition to observation TPS allows them to partially perform measuring functions. Measuring (radiometric) TPS are used mainly for qualified thermal diagnostics of various industrial facilities, equipment, buildings, structures, mechanisms, etc. Each of these classes of TPS has its own specific practical application (market niche) and its operational capabilities. Below we will consider observation TPS, which occupy a special place in solving search and inspection tasks. TPS that perform such functions along with observation are called search and inspection, or simply search. Search TPS provide the ability to see at significant distances regardless of the level of natural illumination, the intensity of light interference, the degree of transparency of the atmosphere. These devices are capable of registering thermal radiation from objects through media that are not transparent to visible and near IR radiation, but transparent to thermal radiation: foliage, camouflage nets, a small layer of earth, a pile of objects, etc., which makes it possible to detect disguised or hidden objects. Thermal imaging search systems can be used for round-the-clock all-weather surveillance, reconnaissance, aiming, target tracking, facility security, customs control, for solving forensic problems, driving vehicles, searching for the wounded and injured as a result of military actions or natural disasters, for detecting mines, etc.
The capabilities of search TPS for detection and recognition of equipment and people at significant distances are demonstrated by Fig. 1, which shows the results of detection of various objects by a thermal imaging system based on an uncooled bolometric thermal imaging matrix of 320×240 elements (pixel size 50 μm) with a lens with a focal length of 100 mm. The maximum achievable results of detection and recognition are presented here. In practice, the same results will look somewhat more modest, which is explained by non-optimal control conditions, reduced transparency of the atmosphere and a number of other factors that reduce the characteristics of the equipment. These factors will be discussed in more detail below.
The influence of the atmosphere on the process of propagation of IR radiation when observing remote objects is expressed as a weakening of the object's own radiation and is caused by two main factors:
- absorption of radiation, as a result of which its energy is converted into other types;
- weakening or scattering of radiation.
The effect of these factors is a weakening of the signal energy from the control object, a decrease in the image contrast, a distortion of its spatial structure, which ultimately leads to a deterioration in image quality and a decrease in the range of vision.
Indeed, according to spatial-frequency representations based on the mathematical apparatus of the Fourier transform, the spatial distribution of the brightness of any object can always be represented as a certain set (spectrum) of spatial frequencies, each component of which has a certain amplitude and phase. The layer of the scattering medium (atmosphere) transmits each of these components with a certain transmission coefficient depending on the spatial frequency. As a result, the original spatial-frequency spectrum is distorted.
A quantitative description of all the factors affecting the quality of an IR image and the viewing range is beyond the scope of this paper, as it is a rather labor-intensive and voluminous process. We will only give an example demonstrating the classic picture of atmospheric transmission for a 1 km long ground path (Fig. 2). The appearance of the presented picture generally depends on the concentration of absorbing substances in the atmosphere, which changes with the height of the path (often with its length), as well as a number of other factors.
Conventionally, depending on the range, TPS are divided into three groups:
- Short-range TPS: up to 0.7 — 1 km for a full-length human figure and up to 1.5 — 2 km for a car;
- Medium-range TPS: 1.2 -1.5 and 2 -, respectively. 4 km, and also up to 8 km by plane;
- TPS with increased range, exceeding the values corresponding to the average range.
The first group of search TPS includes handheld portable thermal imagers weighing up to 2 kg, small-sized sights for small arms, helmet-mounted and head-mounted observation devices. The second group of search TPS includes portable or temporarily tripod-mounted observation devices. The third group of search TPS are stationary devices equipped with long-focus optics, as well as mobile or watercraft-mounted observation systems.
Modern search TPS are based on uncooled IR radiation converters, which are focal-plane two-dimensional multi-element matrices capable of perceiving temperature contrasts of up to 50 — 80 mK.
In the spectral range of 8-14 µm, uncooled TPSs use large-format microbolometric (MB) focal matrices and multi-element receivers based on pyroelectrics or ferroelectrics as converters.
The main advantage of MB systems is the absence of cooling, which makes them economical in terms of power consumption, lighter and cheaper than cooled TPS. MB systems are capable of reaching operating mode in a few seconds. The second important advantage of MB systems compared to other uncooled systems, such as pyroelectric radiation receivers (PDR), is the ability to operate without mechanical modulators. MB systems are noiseless, which is important for work in covert surveillance conditions. The third important advantage is the sensitivity of MB receivers in a wide spectral range. Currently, the most common range of 8-14 μm is used in practice, but MBs are potentially suitable for creating promising multispectral systems.
The format of the MB-matrices of the overwhelming majority of models, especially at the initial period of their serial production, was of two types: 320×240 and 160×120 elements, and the temperature sensitivity, equal to the minimum equivalent noise temperature difference NETD (Noise Equivalent Temperature Difference), was 100×150 mK. In this case, the size of the matrix element was 50×50 µm. Somewhat later, matrices with the format of 640×480 elements appeared, the pixel size decreased to 28×28 µm, and NETD reached 50 mK. The Defense Advanced Research Projects Agency (DARPA) Advanced Uncooled Thermal Imaging Program (AUTI) plans to create a 1280×960 microbolometer with a 15 µm pixel size and a temperature resolution of 10 mK. Currently, 640×480 µm matrices with a 17 µm pixel size are already being produced (L-3 Communication, USA), and by 2009 this company is expected to begin serial production of 1280×1024 matrices with a 17 µm pixel size and a sensitivity of less than 20 mK. The use of such matrices will significantly improve the spatial and temperature resolution of the TURS.
As for another type of uncooled matrix IR receivers — PPI, then in sensitivity they are somewhat inferior to MB matrices. NETD PPI does not exceed 80 mK, the typical value is within 100 — 150 mK. The most commonly used format is 320-240 elements. Recently, matrices of 640-512 and 512-256 pixel formats have appeared. There is an area of application of pyroelectric matrices, where they have a clear advantage over bolometric ones — these are round-the-clock surveillance systems, during the operation of which there is a possibility of direct solar exposure to the sensitive area of the matrix. The probability of failure of the pyroelectric matrix in this case is significantly lower than that of the bolometric matrix.
Table 1. Johnson criteria
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