The millimeter wave refers to an electromagnetic wave having a frequency between 30 and 300 GHz, and the wavelength is on the order of millimeters, and its position in the electromagnetic spectrum is as shown in FIG. It can be seen from Fig. 1 that the frequency of the millimeter wave is one order of magnitude higher than that of the microwave, and at the same time has a certain overlap with the terahertz band (generally, the range of the microwave band is 3 to 30 GHz, and the range of the terahertz band It is 0.1 to 10 THz). Because millimeter wave can penetrate common clothing, paper and plastic and other shielding materials, and does not have ionizing radiation, it is safe for the public, so it is especially suitable for human safety inspection.

Figure 1 The position of the millimeter wave in the electromagnetic spectrum
The realization of the millimeter wave holographic imaging technique relies on the heterodyne mixing technique, which measures no longer the millimeter wave intensity but a complex signal containing amplitude and phase information. In the actual imaging system, the millimeter wave antenna emits a millimeter wave signal to the imaging target, and the millimeter wave signal reflected by the target is received by the receiving antenna, and then the amplitude and phase of the reflected signal are obtained by the heterodyne mixing technique, and combined by line scan. The method of mechanical movement can obtain the complex reflection signal on the two-dimensional aperture to form a hologram, and then invert the image by the image reconstruction algorithm to obtain the three-dimensional complex reflectance image of the target. Common aperture scanning methods are flat scanning and cylindrical scanning (as shown in Figure 2).

Figure 2 Plane scan (a) and cylindrical scan (b)
Millimeter wave technology has gained more and more attention in the past 30 years, and the research team at home and abroad has grown steadily, and a large number of scientific research achievements have emerged. However, the progress of this technology in commercial applications has been very slow. At present, one of the most successful commercial application directions of millimeter wave imaging technology is the security door for human security inspection applications. The reasons for this are mainly the following two advantages: First, the wavelength of such technology can just penetrate clothing. Obtaining body surface information, and the current detector chip process level is also sufficient to support the analysis and image reconstruction of the reflected signals in the band; second, such techniques do not cause electromagnetic radiation similar to X-ray penetration technology to the human body, although the technology The image analysis effect is not as clear as the X-ray imaging technology, and it is not widely used in the inspection of the article package. However, when the human body is safely tested, its ion-free radiation characteristics become the biggest technical selection advantage.
The millimeter-wave holographic imaging system applied to human body security can be traced back to the 1995 patented Real-time Holographic Surveillance System, which proposes a single-frequency array type millimeter wave holographic imaging device. The device uses the electronic switch of the antenna array to complete the one-dimensional scanning, and controls the mechanical movement of the antenna array to complete the scanning in the other direction, thereby realizing the 35 GHz single-frequency millimeter wave holographic imaging based on the plane scanning. In 1996, Sheen of the Pacific Northwest National Laboratory (PNNL) improved the system (Real-time Wideband Holographic Surveillance System), and extended the single frequency to broadband to make the imaging system have distance resolution. In turn, the speckle effect in the single-frequency hologram image is weakened, and the image quality is greatly improved. In 2001, PNNL Lab published its further research in the field of millimeter wave holographic imaging, including its 27-33 GHz plane-scanning wide-band holographic imaging system and the proposed millimeter-wave holographic reconstruction algorithm based on spherical wave expansion. This paper is a pioneering article in the field of millimeter-wave human security screening. Subsequently, PNNL proposed a cylindrical scanning holographic imaging system for human security imaging. The system uses a line array of about 2 m high to scan the human body for cylindrical scanning. After 2008, PNNL Lab transferred the patent for the millimeter-wave cylindrical scanning imaging system to L3 for commercialization, so there was L3's ProVision series. In China, the application of millimeter wave holography technology to the human security inspection industry is still in its infancy. In 2013, under the auspices of the National Science and Equipment Development Project of the Ministry of Science and Technology, Tsinghua University and Tongfang Weishi Technology Co., Ltd. jointly developed the first generation of independent intellectual property rights of millimeter wave holographic imaging human body scanners, compared with similar foreign countries. The product, which is at the same or slightly superior level in terms of technical indicators, cleverly circumvents the patent barriers of competitors and highlights the technological strength of China's advanced manufacturing enterprises.
From the market feedback, the US L3 company's Provision series active millimeter-wave human body security instrument has been mass-replaced metal security doors and X-ray (backscatter) security doors at the US airport since 2011. So far, it has been sold worldwide. Thousands of units have achieved sales of hundreds of millions of dollars. German R&S companies and some domestic millimeter wave human security products have also matured and formed sales. (Author: Jin Yingkang Pu)

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