06/25 2024 453
Exploring the Role and Significance of Terahertz Imaging in Security Inspection
In daily life, we often see criminals carrying prohibited items and committing crimes in reality or online. Especially in public places with a large crowd and high mobility, there is often a greater risk of casualties and difficulties for the police to capture criminals. Therefore, security inspection devices have emerged. We often see the presence of security inspection devices in public places such as subways, libraries, shopping malls, stations, airports, and so on.
Security inspection refers to a personal and luggage inspection item that people must undergo before entering a public place. Its purpose is to ensure people's safety and avoid potential security threats. Security inspection is usually carried out through metal detectors, X-ray machines, or other detection tools. Generally speaking, security inspection is implemented and supervised by security personnel, police, or other law enforcement agencies. Its purpose is to ensure public safety and minimize factors that may threaten public safety.
Nowadays, the commonly used "metal door" type of security inspection in the market has many shortcomings. If people even have metal buttons on their clothes, the "metal door" will still sound an alarm. Therefore, it must be combined with "body search", but this also brings problems such as inefficiency and privacy infringement. X-ray security inspection cannot directly scan the human body due to the radiation hazard of X-rays to the human body. Therefore, finding a safe and efficient security inspection method is an urgent public issue.
With the development of technology, people have invented a method of using terahertz imaging technology for security inspection. Terahertz (THz) waves refer to electromagnetic waves with frequencies ranging from 0.1-10 THz (1 THz=1012 Hz) and wavelengths ranging from 3 mm-30 μm, as shown in Figure 1. It has many advantages: low photon energy, which does not cause ionization damage to biological tissues; many biological macromolecules exhibit strong absorption and resonance in this frequency band, thus displaying the "fingerprint spectrum" characteristics of substances; able to penetrate non-metals and non-polar materials; and sensitive absorption characteristics to water. Therefore, terahertz technology has broad application prospects in the fields of life science, communication technology, and physical science.
Figure 1: Terahertz Wave Band
In recent years, with the continuous progress of terahertz technology, the performance of key devices such as terahertz sources and detectors has gradually met the needs of near-field imaging in security inspection and other fields. Depending on the imaging mode, terahertz security inspection imaging can be divided into passive and active types. Passive imaging systems use the differences in terahertz waves generated and reflected by objects and human bodies to perform comparative imaging, without emitting any radiation or energy themselves. Active imaging systems emit terahertz wave signals and detect the echo signals scattered by the target, thereby obtaining the target image. Passive and active imaging have their respective advantages and disadvantages. Passive imaging has a simple structure and no radiation safety issues, but its imaging quality is greatly affected by the environment. Active imaging is not affected by temperature and surrounding radiation, and its environmental adaptability is much stronger than passive imaging. Active imaging has high resolution and clearer images, but the system structure is complex and costly.
In 2003, Kawase et al. successfully detected and identified illegal drugs (methamphetamine, MDMA, aspirin, etc.) hidden in envelopes using terahertz imaging technology. In 2005, Karpowicz et al. from Rensselaer Polytechnic Institute in the United States built a miniaturized continuous-wave terahertz imaging system that can be applied to security inspection. This system achieved imaging detection of knives hidden in briefcases.
In 2014, during the 9th China International Defense Electronics Exhibition, the 38th Research Institute of China Electronics Technology Group Corporation launched the first terahertz human security inspection device in China based on passive imaging principle - Bowei [5]. It can achieve rapid security inspection within 1-2 seconds. The system has a field of view range of 1 m x 2 m, an imaging speed of 3-5 frames/s, and can complete security inspections for more than 500 people per hour. This product is currently being used by the Shanghai National Convention and Exhibition Center.
In 2018, the Technical Laboratory of the French Atomic Energy Commission developed a passive terahertz imaging system with a working frequency of 2.5 THz [6]. The system has a field of view range of 30 cm x 20 cm at a distance of 4 meters, and the imaging speed can reach 1 frame/s.
In 2019, ThruVision, a British company, launched the LPC8 terahertz passive human imaging system [7], which can detect objects with a minimum size of 3 cm x 3 cm. The operating frequency of the system is 0.25 THz, the optimal imaging distance is 2.5 m-4 m, and the field of view range at 3.5 m is 2.1 m x 0.9 m.
British Digital Barriers has developed a new THz security inspection system ThruVision series product [8], which can image people within a safe distance of 3-15 m, and quickly detect most of the suspicious materials hidden on people's bodies, such as metal, ceramic, plastic, currency, liquid, gel and powder. The product operates in the 0.25 THz band, and the system imaging at a speed of 6 frames per second.
In 2020, Zhongdian Bo Micro Terahertz Information Technology launched the TeraSnap series of products [9], which are used for passive human safety checks. The imaging speed can reach 10 frames/s and the detection distance can reach up to 8 meters. This product has been widely used for safety inspections in multiple occasions such as the World Military Games and the Boao Forum for Asia. Southeast Asia, Hong Kong, and Russia have also made strong purchases of this product, and its product image is shown in Figure 6. At present, there are many research institutions and units engaged in the research and development of terahertz security equipment both domestically and internationally, actively carrying out the research and development of terahertz security equipment. The imaging speed and distance have been greatly improved, and it is gradually entering practical applications.
With the continuous in-depth research on terahertz technology, the application of terahertz technology in the field of human imaging security inspection is becoming increasingly mature. After being applied to security checks, this technology provides more practical protection for public safety and greatly reduces the loss of human and material resources, which has a positive significance for social security.