Passive Millimeter Wave Imaging System Research Articles

Passive millimeter-wave imaging system with variable focal length optical configuration

Passive millimeter-wave imaging system with variable focal length optical configuration

Ka Band Low Channel Mutual Coupling Integrated Packaged Phased Array Receiver Front-End for Passive Millimeter-Wave Imaging

This paper presents a Ka band eight-channel integrated packaged phased array receiver front-end for a passive millimeter-wave imaging system. Since multiple receiving channels are integrated in a given package, the mutual coupling issue affecting the channel will deteriorate imaging quality. Therefore, in this study, the influence of channel mutual coupling on the system array pattern and amplitude phase error is analyzed, and the design requirements are proposed according to the results. During the design implementation, the coupling paths are discussed, and passive circuits in the path are modeled and designed to reduce the level of channel mutual coupling and spatial radiation. Finally, an accurate coupling measurement method for a multi-channel integrated phased array receiver is proposed. The receiver front-end achieves a 28~31 dB single channel gain, a 3.6 dB noise figure, less than −47 dB of channel mutual coupling. Furthermore, the array pattern of the two-dimensional 1024 channel system composed of the front end of the receiver is consistent with the simulation, and the receiver’s performance is verified by a human-body-imaging experiment. The proposed coupling analysis, design, and measurement methods are also applicable to other multi-channel integrated packaged devices.

High-resolution 96-GHz (3 mm) passive millimeter-wave imaging system and the effect of scanning resolution on images

A high-resolution 96-GHz (3 mm) passive millimeter-wave imaging system is constructed. The system has a 96-GHz radiometric receiver, which is placed on the focal point of a 50-cm diameter parabolic reflector designed and created. A specially written program for the system controls the antenna via control hardware in desired steps by a 2-axis positioner motor. A balance weight has been calculated and mounted to the rear of this parabolic reflector antenna to prevent possible damage to the rotor of the positioner motor due to the antenna’s weight. The maximum field of view of the system is 120 deg in azimuth and 90 deg in elevation. The angular scan step can be varied from 0.1 deg to 1 deg. Although the system can shoot 1190 × 890 samples (pixels) images, which is close to the HD quality (1280 × 720 pixels) in the full field of view, the actual resolution of the image is limited by the antenna’s beamwidth (BW). Some measurements were performed to investigate the effect of scanning steps on PMMW image quality for different angular scan steps. It is observed that the PMMW image scan with the smallest step gives a detailed image that is the closest to the scene’s optical image as expected. Even though the angular scan step angle is less than half of the antenna’s BW at Nyquist sampling, it is observed that the image quality still improves due to averaging effect of the received signals giving a better estimated S/N. The resolution and scan step values of our system are compared with that of some systems found in the literature and it is shown that ours has the highest value among them. This system can be used to obtain high-resolution images in remote sensing and other applications.

Coupling Effects Analysis and Suppression in a Highly Integrated Ka-Band Receiver Front-End MMIC for a Passive Millimeter-Wave Imager System

This paper presents the coupling effects analysis and suppression of a highly integrated receiver front-end MMIC for a passive millimeter-wave imager system. The receiver MMIC consists of a low-noise amplifier, double-balanced image-reject mixer, frequency quadrupler, and analog phase shifter. In order to integrate these devices into a compact single chip without affecting the core performance, coupling problems need to be solved. We analyze the influence of coupling effects on the image rejection ratio, and propose corresponding solutions for three different coupling paths. (1) The coupling in the LO-RF path of the mixer is solved by designing a double-balanced mixer with high isolation characteristics. (2) The coupling between the LO chain and the LNA from space and dielectric is suppressed by optimizing the two main transmission lines spacing and adding isolation vias. (3) The coupling caused by the line crossing is restrained by designing a differential line crossover structure. The design and implementation of the MMIC are based on 0.15 µm GaAs pHEMT process. The receiver chip has 6.1~8.7 dB conversion gain in 32~36 GHz, less than 3.5 dB of noise figure, and more than 35 dB of image rejection ratio. The measurement results show that the receiver MMIC is especially suitable for high-sensitivity passive millimeter-wave imaging systems.

A Compact Broadband Analog Complex Correlator with High Correlation Efficiency for Passive Millimeter-Wave Imaging System

In this paper, the design, fabrication, and measurement of a compact broadband (4–8 GHz) analog complex correlator for a passive millimeter-wave imaging system are presented. To achieve high sensitivity and high integration of the imaging system, the wideband and miniaturization of the correlator are required. The correlator achieves wide bandwidth by using the add-and-square method, which is composed of a six-port circuit and a detection circuit. In order to realize the miniaturization of the correlator, the six-port circuit is realized on the chip base on the 0.15-μm gallium arsenide (GaAs) process. The influence of mismatch of the detection circuit that employs zero-bias Schottky diodes on the correlator is also analyzed to guide the design of the correlator. The measurement results of the designed chips and detector are consistent with the simulation result. Finally, a Sweep-frequency test is applied to the designed correlator, and the measurement results show that, within the frequency range of 4–8 GHz, the correlation amplitude fluctuation is less than 1.9 dB and the correlation efficiency is larger than 99%, which reveal that the correlator is suited for interferometric passive millimeter-wave imaging applications.

Passive Millimeter-Wave Imaging for Burns Diagnostics under Dressing Materials.

This paper presents a feasibility study of using a passive millimeter-wave imaging (PMMWI) system to assess burn wounds and the potential for monitoring the healing process under dressing materials, without their painful removal. Experimental images obtained from ex vivo porcine skin samples indicate that a ThruVision passive imager operating over the band 232–268 GHz can be used for diagnosing burns and for potentially monitoring the healing under dressing materials. Experimental images show that single and multiple burns are observed throughout dressing materials. As the interaction of millimeter-wave (MMW) radiation with the human body is almost exclusively with the skin, the major outcomes of the research are that PMMWI is capable of discriminating burn-damaged skin from unburned skin, and these measurements can be made through bandages without the imager making any physical contact with the skin or the bandage. This highlights the opportunity that the healing of burn wounds can be assessed and monitored without the removal of dressing materials. The key innovation in this work is in detecting single and multiple burns under dressing materials in noncontact with the skin and without exposing the skin to any type of manmade radiation (i.e., passive sensing technology). These images represent the first demonstration of burns wound under dressing materials using a passive sensing imager.

Design of high-resolution near-field focusing metasurface lens

A high-resolution near-field focusing metasurface lens (MSL) was proposed to meet the high-resolution and miniaturization requirements of focusing devices in the Ka-band passive millimeter wave (PMMW) imaging system. Based on coaxial ring structure with polarization insensitivity, the design of a single refraction surface phase gradient MSL capable of focusing on plane waves is completed. Furthermore, the near-field focusing is realized by the method of phase superposition. The overall size of the lens s is 80 × 80 mm2, composed of 25 × 25 unit with the thickness of only 0.24λ. The feed source used a tapered slot antenna based on substrate integrated waveguide. The simulation results indicate a good spatial resolution of the presented lens, which can focus the electromagnetic wave radiated from the feed antenna at 70 mm with the focal spot size of 9 mm. The lens can effectively reduce the system volume and achieve high resolution imaging when utilized in PMMW imaging system.

Experimental Demonstration and Calibration of a 16-Element Active Incoherent Millimeter-Wave Imaging Array

In this article, an active incoherent millimeter-wave imaging array is presented, along with its calibration procedure. Active incoherent millimeter-wave imaging uses the transmission of incoherent signals from multiple transmitters to mimic the properties of thermal radiation, enabling interferometric image reconstruction that can be realized in a snap-shot mode, without beamsteering. Due to the use of transmitters, the sensitivity requirement on the receivers is significantly relaxed compared with passive millimeter-wave imaging systems that detect low-power thermal radiation, making it possible to use standard commercial hardware, therefore decreasing the cost considerably. No exact knowledge of the transmit illumination is needed; thus, the coordination of the transmitters is minimal, further simplifying the system implementation. In this work, a 16-element Ka-band millimeter-wave imager is built and presented using commercial components and in-house fabricated antennas, along with a calibration procedure to account for amplitude and phase variations in the hardware. Experimental 2-D snapshot image reconstructions are presented.

An efficiently-designed wideband single-metalens with high-efficiency and wide-angle focusing for passive millimeter-wave focal plane array imaging.

Wide-angle, high-efficiency, wide-band, and ultra-compact focusing blocks are crucial for implementation and future evolution of passive millimeter-wave focal plane array imaging systems. The spherical or doublet metalens can attain high-efficiency, wide-angle field-of-view (FOV) but suffer from fabrication difficulties, complex assembly, and low compactness. Here we present an efficient single-metalens design capable of performing high-efficiency diffraction-limited wideband focusing over a wide-angle FOV. This single-metalens design can greatly mitigate the Seidel aberrations by a rational allocation of amplitude-phase of the electromagnetic waves. A proof-of-concept metalens at millimeter-wave band (33 GHz-37 GHz) confirms the validity of our design.

A Multi-Thresholding Method Based on Otsu’s Algorithm for the Detection of Concealed Threats in Passive Millimeter-Wave Images

Abstract In this study, an algorithm to the detection and imaging of hidden arms for passive millimeter-wave (PMMW) imaging systems is proposed. This technique is; in fact, an improved version of our previously developed auto-classification algorithm by extending it by exploiting the Otsu’s multi-level thresholding method. The detailed derivation and the brief steps of the proposed algorithm are given. The proposed algorithm is tested and validated by real PMMW images obtained by a real radiometric imaging system. Resultant measured images are obtained with the employment of signal and image processing procedures of the suggested technique. It is demonstrated by the constructed PMMW images that proposed technique successfully detects a concealed metal threat and also predicts its size by drawing the shape outline based on Otsu’s multi-level thresholding routine that was specially tailored to our auto-classification technique.

Antipodal Vivaldi Antenna for Passive Imaging Technology in Ka Frequency Band

Passive Millimeterwave Imaging (PMI) Technology is used to view scene in foggy air conditions or to see concealed weapon under clothes that cannot be seen other technology because of the materials of weapon. There are some frequency windows to work with millimeterwave technology as 35, 96 and 220 GHz and it is near 35 GHz frequency for this work. Antenna is one of the most important part in PMI Technology, consequently; major importance has been given to this segment. Antipodal Vivaldi Antenna has 50.07 ohm input impedance for matching to the forward circuit, and -39.23 dB input reflection coefficient, S11, and 6.82 dB gain, is designed for 35.18 GHz in Computer Simulation Technology Studio Suite Program (CST). After the antenna is connected to the other circuits such as amplifier or detector, the created system will be integrated into a scanner which is capable of scanning azimuth and elevation axes. As a result, PMI System will be formed and ready to view the scene or the hidden things in haze air conditions.

WAVELET FUSION FOR CONCEALED OBJECT DETECTION USING PASSIVE MILLIMETER WAVE SEQUENCE IMAGES

Abstract. PMMW imaging system can create interpretable imagery on the objects concealed under clothing, which gives the great advantage to the security check system. Paper addresses wavelet fusion to detect concealed objects using passive millimeter wave (PMMW) sequence images. According to PMMW real-time imager acquired image characteristics and storage methods，firstly, using the sum of squared difference (SSD) as the image-related parameters to screen the sequence images. Secondly, the selected images are optimized using wavelet fusion algorithm. Finally, the concealed objects are detected by mean filter, threshold segmentation and edge detection. The experimental results show that this method improves the detection effect of concealed objects by selecting the most relevant images from PMMW sequence images and using wavelet fusion to enhance the information of the concealed objects. The method can be effectively applied to human body concealed object detection in millimeter wave video.

A thin wideband high-spatial-resolution focusing metasurface for near-field passive millimeter-wave imaging

In this paper, we present a flat transmission-type focusing metasurface for the near-field passive millimeter-wave (PMMW) imaging systems. Considering the non-uniform wavefront of the actual feeding horn, the metasurface is configured by unit cells consisting of coaxial annular apertures and is optimized to achieve broadband, high spatial resolution, and polarization insensitive properties important for PMMW imaging applications in the frequency range from 33 GHz to 37 GHz, with the focal spot as small as 0.43λ0 (@35 GHz). A prototype of the proposed metasurface is fabricated, and the measurement results fairly agree with the simulation ones. Furthermore, an experimental single-sensor PMMW imaging system is constructed based on the metasurface and a Ka-band direct detection radiometer. The experimental results show that the azimuth resolution of the system can reach approximately 4 mm (≈0.47λ0). It is shown that the proposed metasurface can potentially replace the bulky dielectric-lens or reflector antenna to achieve possibly more compact PMMW imaging systems with high spatial resolution approaching the diffraction-limit.

A Novel Otsu Method Based on Prior Area Information for Concealed Target Detection in PMMW Images

A novel double-threshold Otsu algorithm based on prior area is proposed to improve the detection accuracy of small target in this paper. The proposed algorithm makes good use of the area information of each region in images to improve the accuracy of target detection, and can handle scenes containing small target which has low computational complexity, and achieves robust detection for concealed target in Passive Millimeter Wave (PMMW) images. Experiments are conducted on PMMW images collected by PMMW imaging system. The experimental results show that the proposed algorithm can effectively detect the concealed targets, and has better performance in target detection than other traditional threshold segmentation algorithm based on Otsu.

RF Performance of Layer-Structured Broadband Passive Millimeter-Wave Imaging System

Low profile and simple configuration are advantageous for RF module in passive millimeter-wave imaging system. High sensitivity over broad operation bandwidth is also necessary to detect right information from weak signal. We propose a broadband layer-structured module with low profile, simple structure, and ease of manufacture. This module is composed of a lens antenna and a detector module that consists of a detector circuit and a broadband microstrip-to-waveguide transition. The module forms a layer structure as a printed substrate with detector circuit is fixed between two metal plates with horn antennas and back-short waveguides. We developed a broadband passive millimeter-wave imaging module composed of a lens antenna and a detector module in this work. The gain and the antenna efficiency were measured, and the broadband operation was observed for the lens antenna. For the detector module, peak sensitivity was 8100 V/W. Furthermore, the detector module recognized a difference in the absorber’s temperature. The designs of the lens antenna and the detector module are presented and the RF performances of these components are reported. Finally, passive millimeter-wave imaging of a car, a human, and a metal plate in clothes is demonstrated in this paper.

Antenna array layout optimization of collaborative passive millimeter-wave imaging system

Based on the advantages of phased array and synthetic aperture imaging method in passive millimeter-wave imaging,a collaborative passive millimeter-wave imaging system was developed for real-time and high resolution imaging in human security application. In the horizontal orientation,by applying annealing algorithm,different aperture horn antennas were determined for the improvement of the phased array performance,which was generally compared with the maximum main beam effective and most smoothable sidelobe level. In vertical orientation,the synthetic aperture,namely,the sparse antenna array was designed by the criterion of the most uniform redundant baseline,while baseline absence should be avoided. Finally,simulation results reveal that with the optimized antenna array the requirement of increasing singal-to-noise ratio( SNR)in power measurement in the collaborative passive imaging system can be fulfilled.

근거리 수동 밀리미터파 이미징 시스템을 이용한 영상 측정과 영상처리

본 논문에서는 근접한 거리의 수동 밀리미터파 이미징 시스템을 설계하고 테스트하였다. 제안한 수동 밀리미터파 이미징 시스템은 안테나와 저잡음 증폭기, 다이오드 검출기로 구성된 94GHz 밀리미터파 이미징 센서와 2축 직교 로봇, 데이터 수집장치, 이미징 프로그램으로 구성된 제어 장치로 되어있다. 금속 공구나 IC 카드, 플라스틱 등의 물체에 대해서 영상을 측정하고, 2축 직교 로봇은 래스터 스캔을 통해서 2차원 영상을 얻을 수 있다. 60초 이내로 20×20 픽셀의 영상을 얻어서 출력하고, 후처리를 위해서 영상을 저장한다. 측정 영상의 해상도 개선을 위해서 여러 가지 보간법을 적용하여 성능을 확인하였다.

Segmentation of Concealed Objects in Passive Millimeter-Wave Images Based on the Gaussian Mixture Model

Passive millimeter wave (PMMW) imaging has become one of the most effective means to detect the objects concealed under clothing. Due to the limitations of the available hardware and the inherent physical properties of PMMW imaging systems, images often exhibit poor contrast and low signal-to-noise ratios. Thus, it is difficult to achieve ideal results by using a general segmentation algorithm. In this paper, an advanced Gaussian Mixture Model (GMM) algorithm for the segmentation of concealed objects in PMMW images is presented. Our work is concerned with the fact that the GMM is a parametric statistical model, which is often used to characterize the statistical behavior of images. Our approach is three-fold: First, we remove the noise from the image using both a notch reject filter and a total variation filter. Next, we use an adaptive parameter initialization GMM algorithm (APIGMM) for simulating the histogram of images. The APIGMM provides an initial number of Gaussian components and start with more appropriate parameter. Bayesian decision is employed to separate the pixels of concealed objects from other areas. At last, the confidence interval (CI) method, alongside local gradient information, is used to extract the concealed objects. The proposed hybrid segmentation approach detects the concealed objects more accurately, even compared to two other state-of-the-art segmentation methods.

SHAPE FEATURE ANALYSIS OF CONCEALED OBJECTS WITH PASSIVE MILLIMETER WAVE IMAGING

Millimeter wave (MMW) imaging has found rapid adoption in security applications such as concealed object detection under clothing. However, the imaging quality is often degraded due to resolution limit and low signal level. This study addresses shape feature analysis following concealed object detection. The object region is extracted by multi-level segmentation. Shape features are composed of several descriptors which are object area, perimeter, major and minor axes of the basic rectangle, rectangularity, compactness, and eccentricity. In the experiments, three objects (gun, hand ax, and plastic bottle containing liquid skin aid) concealed under clothing are captured by the passive MMW imaging system. The extracted shape features are compared with the true features from the object model showing good accuracy.

LINEAR POLARIZATION SUM IMAGING IN PASSIVE MILLIMETER-WAVE IMAGING SYSTEM FOR TARGET RECOGNITION

In passive millimeter-wave imaging systems used indoors, the radiometric temperature contrast is barely enough for coarse object detection, being usually insu-cient for recognition due to the absence of cold sky. The image contrast results from a combination of emissivity and re∞ectivity which are dependent on the dielectric constant of objects, the angle of incidence, and the polarization direction. To improve the capability of target recognition, we proposed the linear polarization sum imaging method which is based on the combination of the difierent polarization images for increasing the intensity contrast between the target area and the background area. In order to capture the linear polarization sum images of a metal sphere, a metal and a ceramic cup, we designed W-band quasi-optical imaging system which can generate the polarization dependent images by manually changing the linear polarization direction of its radiometer receiver from 0 to …=2 by the step size of …=8. The theoretical and experimental results of the linear polarization sum imaging show that it is capable for achieving good image quality enough to recognize the target.