Active Millimeter-wave Imaging Research Articles

Enhancing concealed object detection in Active Millimeter Wave Images using wavelet transform

In contemporary security detection systems, the utilization of millimeter-wave radar has assumed a central role owing to its non-contact and innocuous nature. This study addresses the challenging issues of detecting low-resolution and small targets in active millimeter wave (AMMW) images concealed detection. We identify a prevalent drawback in existing detectors, specifically the adoption of strided convolution or pooling layers, leading to a loss of crucial details that adversely impacts the detection rate. To overcome this limitation, we introduce a novel convolutional structure, termed Wavelet-Conv, which maintains information integrity while segregating features from high and low frequency bands, effectively replacing the unfavorable design. Furthermore, we harness the wavelet transform to enhance channel and spatial attention mechanisms, enabling more effective utilization of frequency band features and ensuring interpretability in the computational process. In this pursuit, we integrate the proposed Wavelet-Conv and Wavelet-Attention modules into the YOLOv8 framework, culminating in a unified model, termed Wavelet-YOLO. Through rigorous experimental validation on two AMMW datasets, our approach exhibits superior performance by significantly enhancing the recall and mean average precision (mAP) of small targets in AMMW images, while maintaining competitive inference speed. Extensive experiments demonstrate the outperformance of our proposed method over existing state-of-the-art approaches.

Design and 3D printing of the Powell lens for sub-terahertz imaging.

The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

Artifact suppression using cross-circular polarization for millimeter-wave imaging

Millimeter-wave imaging is widely applied to radar detection, personnel security inspection, environmental perception, and so on. Multipath artifact is a common phenomenon in active millimeter-wave (AMMW) imaging and is difficult to be removed. The suppression methods in image reconstruction and post-processing stage are usually complex or time-consuming. This report proposes a cross-polarization method to suppress multipath artifacts between equivalent two-cylinder structures before image reconstruction. The physical mechanisms of co-polarization and cross-polarization imaging are revealed. The suppression effectiveness is demonstrated by conducting W-band AMMW imaging simulation and measurement experiments of a typical multipath scenario at the central frequency of 100 GHz (97 ∼103 GHz). The proposed method can directly reconstruct the image with artifact suppression and reduce the pressure of post-processing.

Active Millimeter Wave Imaging Using Leaky-Wave Focusing Antenna

In this article, an active millimeter-wave imaging system using two rectangular waveguide leaky-wave focusing antennas deployed in a quasi-monostatic transmitter and receiver pair is proposed. The broadband design method of the focusing antenna is presented, where the phase constant is derived from the radiation direction of a rectangular waveguide leaky-wave antenna. To eliminate the effect of mutual coupling between the transmitting and receiving antennas and scattering from the surrounding environment, the scattered field from the object is obtained by subtracting the incident field from the total field. The 1-D detection of conducting sphere, conducting and dielectric cylinders, and a conducting plate is performed by experiment to validate the imaging capability of the proposed system. The 2-D imaging of a conducting cylinder in free space and positioned in front of a human body phantom representing the human body is finally performed to simulate the practical application of the system.

A Two-Sided, Reflection-Based K-Band 3-D Millimeter-Wave Imaging System With Image Beat Pattern Mitigation

Active millimeter-wave (mmWave) imaging illuminates a scene with a source of mmWave energy and forms images based on the reflected energy. When imaging objects (such as packaged goods in commerce) that have a mixture of opaque and translucent parts, two-sided reflection imaging can be used to overcome the occlusion by the opaque objects that would plague a transmission imaging approach. However, the two-sided reflection imaging approach suffers from wavelength-dependent beat patterns when coherently combining the images from each side. We present and experimentally verify an approach to successfully mitigate this image beat pattern. The presented two-sided <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> -band 3-D mmWave imaging system achieves system resolutions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\delta }\text {x} = 17.07$ </tex-math></inline-formula> mm, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\delta }\text {y} = 4.62$ </tex-math></inline-formula> mm, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\delta }\text {z} = 7.26$ </tex-math></inline-formula> mm, with an accuracy of 0.8 mm in the y- and z-directions. It is shown that the system is not only capable of producing beat-pattern-free 3-D images of the products inside an optically opaque corrugated cardboard shipping box but also the two-sided approach with image beat pattern correction showing an increased image volume even if portions of the scene are occluded.

Precise Localization of Concealed Objects in Millimeter-Wave Images via Semantic Segmentation

Existing concealed objects detection methods in active millimeter wave (AMMW) images are mainly based on bounding boxes. In this paper, we consider the problem of precise localization of concealed objects in AMMW images with the use of semantic segmentation networks. To improve the performance of the detection and localization of concealed objects, we propose a method with two steps. In the first step, we build a two-class semantic segmentation network to segment concealed objects in pixels from the images with the complex human body background, while in the second step, we use connected components extraction to detect and localize concealed objects in the segmented image. To improve the performance of the detection and localization of small objects, the network we built is composed of stacked dilated convolution blocks to enlarge the receptive field while keeping the resolution of associated feature maps unchanged. In addition, we give a rule for design of the associated dilation rates and the expand-contract dilation (ECD) assignment strategy for the pattern of the dilation rates. In the numerical experiments, we use the universal evaluation metrics, such as the AP (average precision) @ IoU (Intersection over Union)=0.5 and mIoU (mean value of IoU) to evaluate the performance of precise localization. The experiment results show that our method outperforms the existing ones for precise object localization in AMMW images, where the improvement of the AP@0.5 is about 38% and that of the mIoU is about 27%.

Millimeter-Wave Coherent Imaging of Moving Targets by Using Complex-Valued Self-Organizing Map and Auto-Encoder

Active millimeter-wave imaging systems using complex-valued self-organizing map (CSOM) have potentially wide applications in moving-target imaging by acquiring and classifying complex textures. However, interference in the coherent observation causes harmful effects such as amplitude fluctuation and phase distortion, resulting in the deterioration of the visualization quality achieved by the CSOM. In this article, we propose the introduction of feature refinementZ using a complex-valued auto-encoder (AE) into the feature extraction process of our imaging system to suppress these effects. We show that the complex-valued AE is extremely useful for extracting features appropriately even under such adverse influences, and improves the clustering performance. We also experimentally investigate the influence of the number of hidden-layer neurons on the AE performance to discuss the robustness of our system.

Surface-roughness measurement based on scalar field correlation with active millimeter-wave imaging system.

Active millimeter-wave (MMW) imaging is of interest because it has played an important role in the area of personnel surveillance over the past decades. Through reconstructing reflectivity, potential threats can be recognized based on shape. Besides reflectivity, diverse physical characteristics should be explored so that supplementary information can be used to assist recognition. This paper presents a surface-roughness measurement method using holographic data that has already been acquired during security checks. Based on scalar diffraction theory and speckle metrology, a simple mathematical relation between the correlation of holographic fields and surface roughness has been derived. Consequently, another kind of information, that is, the surface-roughness estimate, can be used to help differentiate similarly shaped articles. Results of simulations and laboratory experiments have shown its validation and potential in application to MMW imaging systems.

A low-complexity method for concealed object detection in active millimeter-wave images

A low-complexity method for concealed object detection in active millimeter-wave images

Improved-resolution millimeter-wave imaging through structured illumination.

A resolution-improved active millimeter-wave (MMW) imaging structure, based on the theory of structured illumination, is proposed in this paper. The structured illumination is a well-defined concept for surpassing the diffraction limit in optical microscopy, where imposing grating patterns on the targeted object could help in moving the object frequency spectrum along the desired direction in the spatial frequency domain. This frequency shift helps in passing different parts of the object's frequency spectrum through the diffraction filter. The combination of resultant images provides a framework to pass a wider frequency band of the object, thereby achieving super-resolution. This idea has not yet been employed for MMW image resolution improvement due to practical limitations in producing the desired grating patterns. Therefore, a desired fringe pattern is produced here and tailored for a MMW imaging system through antenna array synthesis. In the proposed scheme, the structured illumination has been implemented for improving the MMW image resolution. Furthermore, an adaptive approach has been proposed in order to generalize the structure for resolution improvement in all required directions in a very fast manner. Electromagnetic simulation results show at most twofold improvement in the image resolution through the proposed MMW imaging structure.

Research on an artificial dielectric material for millimeter-wave imaging application.

Material made of artificial molecules fabricated from cage-shaped granules of conductor (CGC) is introduced and its electrical and magnetic characteristics are presented. Its refractive index, calculated using complex relative permittivity and complex relative permeability, is 1.504 at 35 GHz. A two-element lens, consisting of a pair of spherical plano-convex lenses, was designed and fabricated by embedding CGC in poly(methyl methacrylate). The active millimeter-wave imaging system was constructed with the two-element lens by having the curved surfaces face each other. Millimeter-wave (MMW) images of a right trapezoid and twin bars were obtained. The image quality was acceptable, proving that the CGC material has the ability to refract MMWs in MMW imaging and that the ability contrasts with that of some traditional high polymer material.

Performance Dependence on System Parameters in Millimeter-Wave Active Imaging Based on Complex-Valued Neural Networks to Classify Complex Texture

Millimeter wave exhibits relatively straight propagation as well as high penetration into dielectric materials, such as plastics, cloth, and paper. In security imaging, we use these features to discover weapons concealed under clothes. Self-organizing map (SOM), a type of neural networks, can map high-dimensional data on any dimension with unsupervised learning. It is utilized for clustering and visualization of high-dimensional data. Previously, we proposed a millimeter-wave imaging system for moving targets consisting of a 1-D array antenna, a parallel front end, and a complex-valued SOM to deal with complex texture. Experiments demonstrated its high performance in the visualization. In this paper, we investigate the dependence of the visualization performance on its configuration parameters as well as processing parameters. We reveal the effect of the modulation-frequency number and the window size. We also discuss the effective depth range for visualization and a tradeoff relationship between the measurement time and the visualization quality.

Optimal Non-Invasive Fault Classification Model for Packaged Ceramic Tile Quality Monitoring Using MMW Imaging

Millimeter wave (MMW) frequency has emerged as an efficient tool for different stand-off imaging applications. In this paper, we have dealt with a novel MMW imaging application, i.e., non-invasive packaged goods quality estimation for industrial quality monitoring applications. An active MMW imaging radar operating at 60 GHz has been ingeniously designed for concealed fault estimation. Ceramic tiles covered with commonly used packaging cardboard were used as concealed targets for undercover fault classification. A comparison of computer vision-based state-of-the-art feature extraction techniques, viz, discrete Fourier transform (DFT), wavelet transform (WT), principal component analysis (PCA), gray level co-occurrence texture (GLCM), and histogram of oriented gradient (HOG) has been done with respect to their efficient and differentiable feature vector generation capability for undercover target fault classification. An extensive number of experiments were performed with different ceramic tile fault configurations, viz., vertical crack, horizontal crack, random crack, diagonal crack along with the non-faulty tiles. Further, an independent algorithm validation was done demonstrating classification accuracy: 80, 86.67, 73.33, and 93.33 % for DFT, WT, PCA, GLCM, and HOG feature-based artificial neural network (ANN) classifier models, respectively. Classification results show good capability for HOG feature extraction technique towards non-destructive quality inspection with appreciably low false alarm as compared to other techniques. Thereby, a robust and optimal image feature-based neural network classification model has been proposed for non-invasive, automatic fault monitoring for a financially and commercially competent industrial growth.

Improved Two-Dimensional Millimeter-Wave Imaging for Concealed Weapon Detection Through Partial Fourier Sampling

Active millimeter-wave imaging based on synthetic aperture focusing offers certain unique and practical advantages in nondestructive testing applications. Traditionally, the imaging for this purpose is performed through a long procedure of raster scanning with a single antenna across a two-dimensional grid, leading to a slow, bulky, and expensive scanning platform. In this paper, an improved bistatic structure based on radial compressive sensing is proposed, where one fixed transmitter antenna and a linear array of receiving antennas are used. The main contributions of this paper are (a) reducing the scanning time, (b) improving the output quality, and (c) designing an inexpensive setup. These improvements are the result of the underlying proposed simpler scanning structure and faster reconstruction process.

Range resolved mode mixing in a large volume for the mitigation of speckle and strategic target orientation requirements in active millimeter-wave imaging.

In spite of many reports of active millimeter-wave imaging in the literature, speckle and requirements for cooperative target orientation significantly reduce its practical usefulness. Here we report a new technique, range resolved mode mixing (RRMM), which significantly mitigates both of these issues. It also provides a three-dimensional (3D) image. RRMM accomplishes this by combining multimode illumination (which eliminates the requirement for cooperative target orientation) with range resolution (which provides statistical independence of speckle patterns for averaging and the 3D image). The use of a 5W extended interaction klystron amplifier results in large signal margins in the 50 m scale atrium of the Physics Department at Ohio State University. It appears that there are a number of scenarios out to a range of 1 km for which this approach is useful to provide 3D images, with minimal speckle, and no requirement for cooperative target orientation.

A Broadband T/R Front-End of Millimeter Wave Holographic Imaging

A broadband T/R frond-end of active holographic imaging system is presented. Compact autodyne mode circuit structure front-end is adopted to achieve higher signal to noise ratio and higher reliability, which is beneficial to the after-end imagining. The factors that influence the dynamic range and the transverse resolution ratio of holographic imaging system have been analyzed. Wide-band oscillator, wide-band low noise amplifier and the tapered slot antennas are implemented to meet the requirements of the holographic imagining system. According to the measured results, the output power is uniform in the broadband working frequency. The sub-harmonic suppression is better than 25 dBc from the frequency of 28 GHz to 33 GHz. The isolation between antennas channel is greater than 20 dB. The experimental result shows that the performance of the front-end is good enough to meet the needs of active millimeter-wave holographic imaging system.

Elimination of speckle and target orientation requirements in millimeter-wave active imaging by modulated multimode mixing illumination

Active imaging can provide significantly larger signal margins in the millimeter-wave spectral region than passive imaging, especially indoors-an important application for which there is no cold sky illumination. However, coherent effects, such as speckle, negate much of this advantage by destroying image clarity and target recognition. Moreover, active imaging demonstrations often use strategically chosen target orientations to optimally reflect power from the active illuminator back to the imaging receiver. In this paper we will discuss and show experimental results for a new active imaging approach that largely eliminates coherent effects and the need for optimized target orientation. The work described uses a synthesized harmonic multiplier chain to drive a 5 W extended interaction klystron at 218.4 GHz, a mechanical mode mixer to illuminate and modulate many modes, and a heterodyne receiver coupled into a 60 cm scanning mirror. Large signal margins were obtained in this ~50 m range work, showing paths to imaging at ~1 km, imaging with considerably less powerful illuminators, and the use of focal plane arrays.

A Novel Fully Electronic Active Real-Time Imager Based on a Planar Multistatic Sparse Array

The interest in active millimeter-wave and microwave imaging systems is increasing due to their utilization in security, medical, and industrial applications. Imaging systems using monostatic arrays suffer from the high number of antenna positions, making a fully electronic solution very costly and impractical. Multistatic arrays, however, offer the chance for high reduction factors in the number of antennas, and then allow for fully electronic solutions. Here, a new array architecture is presented, which is suitable for real-time operation and is capable of delivering high image quality in means of resolution and dynamic range. Various technological solutions were developed for the imaging purpose, which are introduced in detail. An example array for operation from 72 to 80 GHz is demonstrated. The imaging system hardware is introduced, along with several imaging results of humans with concealed objects.

A 160-GHz low-noise downconversion receiver front-end in a SiGe HBT technology

A 160-GHz SiGe-HBT (Heterojunction Bipolar Transistor) down-conversion receiver front-end for use in active millimeter-wave imaging arrays and D-band communication applications is presented. The monolithic front-end consists of a three-stage low-noise amplifier providing 24 dB of gain and a Gilbert-cell mixer capable of operating from a −8-dBm LO signal. A fully differential architecture compatible with balanced on or off-chip antennas is used to avoid the need for on-chip baluns in antenna-integrated applications. The implemented downconversion front-end consumes 50 mA from a 3.3 V supply and requires a 0.1 mm2 die area (excl. pads) per channel. With a 160-GHz input signal and an Intermediate Frequency (IF) of 1 GHz, the implemented front-end yields a 25-dB conversion gain, a −30-dBm input compression point, and a 9-dB/7-dB (with/without auxiliary on-chip input balun) system noise figure.

A 0.25-.MU.m Si-Ge Fully Integrated Pulse Transmitter with On-Chip Loop Antenna Array towards Beam-Formability for Millimeter-Wave Active Imaging

This paper presents a 100-120-GHz pulse transmitter chip with a 54×24 on-chip loop antenna array for the purpose of beam-formability in portable millimeter-wave (mm-wave) active imaging applications. We present a new idea for silicon-based mm-wave pulse beam-forming by using voltage-varied CMOS inverter chain. This 4-mm×4-mm transmitter chip is designed and fabricated in a 2.5-V 0.25-µm 4-metal-layer Si-Ge Bi-CMOS process. The 30-µm × 30-µm loop antenna located on the top-metal layer operates as an coil in an integrated mm-wave pulse generator. Each of on-chip pulse generators employing under-damped/over-damped conditions to produce mm-wave pulses includes an R-L-C circuit, a bipolar junction transistor (BJT) operated as a switch and a CMOS inverter chain circuit for shaping the rising edge of the input clock. Simulation results by ADS 2009 and HSPICE show that loop antenna' inductance and resistance at 80-120-GHz are 51pH and 3Ω, respectively. A simulation performance of an integrated 13×6 loop antenna array illustrates the variation of maximum radiation angles depending on different phase values between array's elements. By using an mm-wave power meter, a 90-140-GHz standard horn antenna and a Schottky diode detector, several measured radiation patterns of this loop antenna array chip are achieved. From the measurement result, we demonstrate the possibility of an integrated mm-wave pulse generator for the purpose of beam-forming by changing power supplies of inverter chains.