Abstract:This paper briefly discusses the classification of eavesdropping technology and the principle of infrared thermal imaging, analyzes the feasibility of applying infrared thermal imaging technology in the field of anti-eavesdropping, and emphatically introduces a design scheme of the anti-eavesdropping equipment based on cooled infrared focal plane detector. The scheme has certain technical advantages and good detection effect for some eavesdropping devices. The design of this scheme is based on users and usage scenarios, and focuses on man-machine adaptability, so it has a certain market prospect.

With the development of eavesdropping technology, all kinds of eavesdropping devices are emerging. News reports often say that eavesdropping devices were found in hotel sockets, chandeliers and switches, causing privacy leaks. There is even a risk of eavesdropping in some important places, such as troops, corporate executives and even the head of state conference rooms. Since the establishment of diplomatic relations between the United States and the Soviet Union in 1933, the United States Embassy in the Soviet Union has been under the focus of Soviet intelligence. In 1945, the United States, Britain and the Soviet Union held a summit in Yalta to establish a new post-war order. Due to the importance of the matter, before and after the meeting, the United States carried out a large-scale thorough inventory at the Embassy in the Soviet Union, and found that more than 100 disguised microphones were hidden. This shows that even heavily guarded embassies may be eavesdropped, let alone ordinary meeting places. Therefore, how to search hidden eavesdropping devices quickly and effectively is a big problem at present.

The most widely used eavesdropping mode ——— FM transmitter eavesdropping [1]: FM transmitter is used as the collecting end of sound information, FM radio is used as the relay end of sound, and then the signal is transmitted directly, recorded or encoded by short wave. This method can make the volume of the transmitter very small, hidden in the room (such as the interior of the conference table, decoration or the back of the display board, etc.) has a strong concealment. In addition, even if the eavesdropping activity was discovered, there was no clue to the eavesdropper. According to records, a company executive's office found six eavesdroppers, daily work stolen countless trade secrets, shocking.

Aiming at the most widely used FM transmitter eavesdropping device, this paper designs an anti-theft device based on infrared thermal imaging. It can quickly and efficiently search for hidden eavesdroppers.

Search principles of infrared thermal imaging for eavesdropping devices

Beyond the visible light, there is an invisible ray ——— infrared. Any object in nature radiates infrared rays in real time. Different objects, even different positions of the same object, will have different infrared intensity, so the surface of the object will produce temperature difference [2-3]. As an instrument that can detect infrared intensity in real time, infrared detector converts infrared signal into processing electric signal. Then through a series of image processing, such as non-uniformity correction, image enhancement, filtering and so on, different gray values are given to different temperatures, and finally a black and white temperature distribution map is displayed on the screen (see figure 1 for imaging effect).

FM transmitter eavesdropping device needs power supply when working. In order to improve concealment, most eavesdropping devices use batteries to power. The discharge current of the battery is only milliampere, which causes the temperature of the battery to rise. With the increase of temperature, the eavesdropping device hidden behind the wall, furniture and decorative panels will transfer heat to the shelter, resulting in the local temperature of the latter being obviously higher than the surrounding surface temperature. At this time, if the temperature difference is higher than the minimum discernible temperature difference (Minimum Resolvable Temperature Difference,MRTD) of the infrared thermal imager, the abnormal temperature point can be quickly observed by the thermal imager, thus guiding the user to make further investigation.

Simulation of Search Effect of Infrared Thermal Imaging

Based on the eavesdropping buried in the superficial position of furniture, the simulation model of heat conduction between the eavesdropping device and the plank was established by using the Flotherm V9.3 heat analysis software (see figure 2).

Simulate the following scenarios in the software and input the relevant parameters into the software:

(1) The size of the eavesdropping device is set to 50 mm×30 mm;; and

(2) The power consumption of the eavesdropping device is 7.5 mW; when it is working properly

(3) The shell close to the side of the battery is plastic with a wall thickness of 1 mm and its thermal conductivity is 0.15 W/mK;; and

(4) The thickness of the wiretapping device close to the board is 10 mm, and the thermal conductivity is 0.125 W/mK;; and

The ambient temperature (the initial temperature of the plank) is 25℃ at room temperature and 10 min. for heat conduction

Fig .3 is the obtained heat conduction simulation diagram. it can be seen that within 10 min, the heat of the eavesdropping device as a heat source quickly penetrates the 10 mm thick planks and spreads around. A maximum temperature of 25.3℃ at the center point and a temperature difference with the surrounding area

0.3 ℃。

Design of infrared thermal imager

In this paper, a fast and efficient hand-held anti-theft infrared thermal imager is designed by using high sensitivity refrigeration infrared focal plane detector as the core device and referring to the shape of commercial digital camera.

The working principle of the thermal imager is as follows: the infrared radiation from the eavesdropping device enters the human optical lens and focuses on the sensitive element of the infrared detector through the optical system with selected focal length. Under the continuous operation of Stirling refrigerator, the infrared detector is cooled

By about 77 K, the sensitive element converts the infrared signal into a tractable electrical signal. Output after bias and drive circuit

The analog signal is pretreated with DC reduction and amplification. and then it is converted into high-precision digital signal by analog-to-digital sampling, and a series of digital image processing are carried out, including non-uniformity correction, blind element filling, histogram processing, pseudo-color processing, image enhancement, contrast adjustment and brightness adjustment [4]. The final output infrared video can completely reflect the thermal characteristics of the target gray or pseudo-color image, thus assisting the observer to quickly identify the suspicious position and further search the eavesdropping device. Figure 4 is a schematic diagram of signal processing.

The main technical parameters of the thermal imager are as follows:

(1) Detector type :320×256 m2 HgCd focal plane detectors.

Working bands :3~5 m.

Quality :≤2 kg.

(4) MRTD (0.83cyc/mrad):≤0.1 K.

View angle :≥11"×8.8".

Refrigeration start-up time :≤8 min.

Taking into account the use of scene, functional requirements and user operating habits, the infrared signal is collected by combining the manual focusing optical lens with the infrared detector; the dual-core architecture of FPGA( field programmable gate array +DSP( digital signal processing) is used to realize many functions such as image processing, photo taking and video recording; the display effect is further improved due to the use of a high-definition 7 in liquid crystal display screen; and the thermal imager is light and compact after by a lightweight PVC( polyvinyl chloride) shell.

Moreover, the product's built-in 32 G memory card can support continuous recording of more than 9 h of video or shooting more than 30,000 pictures. By the USB( universal serial bus) interface of the product, the stored data can be exported to the computer for viewing. The product composition is shown in figure 5.

At present, anti-theft infrared thermal imager has been developed successfully. and it has strong detection ability for the eavesdroppers buried behind the common media in the office and conference room such as display board, metal filing cabinet, foam plastic, A4 paper, plank, cement board, etc., so it is more practical. The actual imaging effect is shown in figure 6. The experimental results show that the hand-held anti-theft infrared thermal imager can help the operator to quickly and effectively identify the eavesdropping device hidden on the back of the typical indoor medium (display board, metal cabinet, documents, thin wall, etc.) to avoid the leakage of important information.

Conclusion

Based on the introduction of eavesdropping devices and the analysis of anti-theft demand, a hand-held anti-theft infrared thermal imager based on infrared thermal imaging technology is proposed. The product has the characteristics of portability and real-time, and is suitable for fast, efficient and wide range search for eavesdropping devices hidden in the room. It has good detection effect on the most widely used FM transmitter eavesdropping device.

After successful development, the product has entered the batch trial production stage and is expected to be delivered to users by the end of 2020. With the upgrading of non-cooled focal plane detector, its detection sensitivity will be improved accordingly, and it is expected to replace the refrigeration focal plane detector as the core device of the product in the future. In this way, the volume, weight and cost of the product will be further reduced. Product delivery will effectively eliminate privacy leakage, so it has a very broad market prospects.

ZIP Technology has been at the forefront of visual information and targeting technology for years. A leader in the design, development and production of innovative thermal imaging technologies and advanced electro-optical solutions, its products are used across the military, security, law enforcement, search & rescue, and commercial aviation markets.

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