Passive Millimeter Wave Imaging Research Articles

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.

Multi-polarization passive millimeter-wave imager and outdoor scene imaging analysis for remote sensing applications.

Polarimetric passive millimeter-wave (PMMW) measurements can provide additional information about scenes. We report on the development of a 94GHz Multi-Polarization Scanning Imaging Radiometer (MPSIR) for outdoor imaging applications, and the polarization characteristic analysis of experimentally acquired images. The imager angular resolution is measured to be 0.37°, and the thermal sensitivity is about 0.46K. Multi-polarization measurements are realized by rotating the detector around the observation axis direction. Several outdoor complex scenes are selected to passive image, which include the outdoor parking lot and the dormitory buildings in the woods. Various polarization parameter images are generated to analyze the polarization characteristics. Based on the multi-polarization imaging, eight more polarization parameters are created and they are all have the special distribution properties. The experimental results indicate that the multi-polarization imaging and the modified polarization parameters have the great potential for three-dimensional reconstruction, edge detection and image segmentation.

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.

Regularization-Based Semi-Blind Image Deconvolution Using an Improved Function for PMMW Images Application

Image deconvolution is a method for reversing the distortion in an imaging system. It is widely used in removing blur and noise from a degraded image. This is an ill-posed inverse problem, and one should use regularization techniques to solve this problem. Regularization functions play an important role in finding the desired solution. In this paper, a new function in wavelet domain is proposed, which is useful in blind deconvolution. In addition, a simple algorithm is used to obtain the restored image and unknown point spread function. The proposed approach is tested on three standard images and then compared with the previous methods using standard metrics. Real Passive Millimeter Wave (PMMW) images are also used to obtain the sharp deblurred images. Simulation results show that the proposed method can improve the quality of the restored image.

Quadrature Errors and DC Offsets Calibration of Analog Complex Cross-Correlator for Interferometric Passive Millimeter-Wave Imaging Applications.

The design and calibration of the cross-correlator are crucial issues for interferometric imaging systems. In this paper, an analog complex cross-correlator with output DC offsets and amplitudes calibration capability is proposed for interferometric passive millimeter-wave security sensing applications. By employing digital potentiometers in the low frequency amplification circuits of the correlator, the outputs characteristics of the correlator could be digitally controlled. A measurement system and a corresponding calibration scheme were developed in order to eliminate the output DC offsets and the quadrature amplitude error between the in-phase and the quadrature correlating subunits of the complex correlator. By using vector modulators to provide phase controllable correlated noise signals, the measurement system was capable of obtaining the output correlation circle of the correlator. When injected with −18 dBm correlated noise signals, the calibrated quadrature amplitude error was 0.041 dB and the calibrated DC offsets were under 26 mV, which was only 7.1% of the uncalibrated value. Furthermore, we also described a quadrature errors calibration algorithm in order to estimate the quadrature phase error and in order to improve the output phase accuracy of the correlator. After applying this calibration, we were able to reduce the output phase error of the correlator to 0.3°.

Outdoor Passive Millimeter-Wave Imaging: Phenomenology and Scene Simulation

A model for scene simulation is presented, which describes quantitatively the recognized phenomenology of passive millimeter-wave (PMW) imaging in land, sea, and air scenarios. The model describes objects in a scene as facets giving them electromagnetic properties from their electrical permittivities. This enables polarimetric effects in scenes to be reproduced. Illumination from the sky and ground is calculated using radiation transport, as are the emissive and absorbing effects of the atmosphere between the objects and the imager. The model includes the effects of image sampling, diffraction effects of finite imager aperture sizes, and system noise. Phenomena in experimentally acquired images in the 94 GHz atmospheric transmission window are quantified from three scenarios. These are a helicopter landing site as viewed by the pilot, a coastal region showing boats, land and buildings viewed from sea level, and an airfield viewed from directly above. These are compared quantitatively to the simulated scenes from these scenarios, finding agreement between the radiation temperatures to within a few degrees kelvin. These discrepancies are accounted for by the uncertainties in the permittivities of materials in the scene. This paper constitutes the partial validation of a scene simulation capability for PMMW imaging in outdoor scenarios.

Fast Algorithm for Rough-Surface Scene Simulation in Passive Millimeter Wave Imaging

Simulation in the passive millimeter-wave (MMW) imaging of rough surfaces is an indispensable step in the simulation in passive radiation imaging. The multi-layer brightness temperature tracing method (MBTTM) can be used to simulate the image of a rough surface, which has been proven in our previous work. However, it causes a significant decrease in efficiency of the simulation. To solve this problem, a fast MBTTM is proposed in this paper. In the new method, the uniformly emission method and multi-layer densification method are presented. To validate the correctness of the new algorithm, the comparison of the simulation and measurement results are proposed. These results are in good agreement, which indicates that the fast MBTTM can notably shorten the computing time of the simulation of the rough-surface scene.

A Compact Analog Complex Cross-Correlator for Passive Millimeter-Wave Imager

The design, analysis, implementation, and measurement of a compact analog complex cross-correlator for passive millimeter-wave imaging applications are presented in this paper. The correlator uses the “add and square” and “subtract and square” detection scheme by adopting the Schottky diodes and RF distribution network. Meanwhile, in order to realize the correlator with a compact size and low cost, commercially available surface mounting devices and multilayer PCB technique are employed. The relationship between the phase error of the correlator and the phase unbalance of the RF distribution network is also analyzed. Single-frequency test is applied to the correlator, and the results of the measurement reveal that the correlator operates well within the frequency range of 1.5–2.5 GHz. Moreover, a measurement system based on an actual imaging scenario is also developed to characterize the performance of the correlator when injected with correlated broadband noise signals, and the measurement results show that the correlator is well suited for imaging applications. The bandwidth of the current version correlator is around 1 GHz, but it could easily be changed by replacing distributed components.

Using machine learning to detect and localize concealed objects in passive millimeter-wave images

The detection and location of objects concealed under clothing is a very challenging task that has crucial applications in security. In this domain, passive millimeter-wave images (PMMWIs) can be used. However, the quality of the acquired images, and the unknown position, shape, and size of hidden objects render this task difficult. In this paper, we propose a machine learning-based solution to this detection/localization problem. Our method outperforms currently used approaches. The effect of non-stationary noise on different classification algorithms is analyzed and discussed, and a detailed experimental comparative study of classification techniques is presented using a new and comprehensive PMMWI database. The low computational testing cost of this solution allows for its use in real-time applications.

Optimal Target-Oriented Fusion of Passive Millimeter Wave Images with Visible Images Based on Contourlet Transform

Passive millimeter wave (PMMW) imaging system forms images through passive detection of millimeter wave radiations emitted from the objects. This system allows detection of invisible characteristics of a scene. The important characteristic of these waves is their penetration into cloth fibers and bad climatic conditions such as mist, rain and smoke. These waves are able to detect objects (particularly metal objects) hidden in cloths. For this reason, imaging system is applied in security section of the airports. This system has also abundant applications in military, meteorology, navigation industries in low vision and medical industries. In airports and generally the places where there is need for physical inspection of people, millimeter wave scanners are used. These devices are able to detect weapon and other objects hanging on body of the person or hidden in clothes. Due to limitation of dimensions of lens and antenna and limitation of light refraction, images obtained from this system have low resolution. Imaging in this system acts as a low-pass filter. For this reason, high-frequency information of these images is removed; therefore, set of actions is necessary for improving quality of these images. Different methods have been presented for increasing quality of these images which are divided into two classes of retrieval and combination or fusion of image (for example, fusion of PMMW image with visible image). Target of this paper is to present a new method for fusion of millimeter wave images with their visible images with their visible images with contourlet transform and a new method in thresholding; so that the output image includes visual information very close to visible image which contains object hidden from millimeter wave image.

Iteratively reweighted blind deconvolution for passive millimeter-wave images

Passive millimeter-wave (PMMW) imaging frequently suffers from blurring and low resolution due to the diffraction limits and the long wavelengths compared with visual and infrared radiation. Moreover, the observed image is inevitably degraded by the system noise and natural clutter noise. The blur and noise limit the capability of PMMW images in practical applications. In this study, we propose an iteratively reweighted blind deconvolution method for obtaining high quality PMMW images. A weighted least-squares data-fidelity term that is robust to modeling error and a weighted bilateral total variation regularization term are incorporated into the variational blind deconvolution framework. Furthermore, we impose an appropriate smooth constraint for the point spread function of the imaging system. In addition, to improve the practicality of the method, we derive a formula to automatically update the regularization parameter from the data. Comparative experimental results on simulated and real images show that the proposed method is superior to the state-of-the-art methods in terms of both subjective measure and visual quality.

Influence of surface roughness on polarization property in passive millimeter-wave imaging

Influence of surface roughness on polarization property in passive millimeter-wave imaging

Blind image restoration with sparse priori regularization for passive millimeter-wave images

Passive millimeter wave imaging often suffers from issues such as low resolution, noise, and blurring. In this study, a blind image restoration method for the passive millimeter-wave images (PMMW) is proposed. The purpose of the proposed method is to simultaneously solve the point spread function (PSF) and restoration image. In this method, the data fidelity item is constructed based on Gaussian noise assuming, and the regularization item is constructed as the hyper-Laplace function ||x||0.6, which is fitted according to the high-resolution PMMW images. Moreover, a data-selected matrix is proposed to select the regions that are helpful for estimating the accurate PSF. The proposed method has been applied to simulated and real PMMW image experiments. Comparative results demonstrate that the proposed method significantly outperforms the state-of-the-art deblurring methods on both qualitative and quantitative assessments. The proposed method improves the resolution of the PMMW image and makes it more preferable for object recognition.

Polarization-Based Method for Object Surface Orientation Information in Passive Millimeter-Wave Imaging

Surface orientation information is essential to depicting the 3-D structure of an object. This paper analyzes the linear polarization characteristics of several typical objects by simulations and measurements. Then, a polarization-based method is presented to acquire the object surface orientation information from three different linear polarization brightness temperature images by using a 94-GHz imaging radiometer. The experiments are conducted outside, where a wooden plate is placed obliquely on a metal box. Experimental results indicate that our method is capable of achieving the surface orientation information that is beneficial to recognizing the object. This method suggests possible applications for terrain models and object detections.

A Novel Image Fusion Algorithm for Visible and PMMW Images based on Clustering and NSCT

Aiming at the fusion of visible and Passive Millimeter Wave (PMMW) images, a novel algorithm based on clustering and NSCT (Nonsubsampled Contourlet Transform) is proposed. It takes advantages of the particular ability of PMMW image in presenting metal target and uses the clustering algorithm for PMMW image to extract the potential target regions. In the process of fusion, NSCT is applied to both input images, and then the decomposition coefficients on different scale are combined using different rules. At last, the fusion image is obtained by taking the inverse NSCT of the fusion coefficients. Some methodologies are used to evaluate the fusion results. Experiments demonstrate the superiority of the proposed algorithm for metal target detection compared to wavelet transform and Laplace transform.

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.

Multilayer Brightness Temperature Tracing Method for Rough Surface Scene Simulation in Passive Millimeter-Wave Imaging

Simulation in passive millimeter-wave (MMW) imaging of rough surfaces is an indispensable step in the simulation in passive radiation imaging, especially for the rough surfaces of different roughness surfaces. However, little attention has been paid to the simulation of rough surface; based on the existing model of brightness temperature tracing described in previous work, diffused reflection of the rough surface is taken into account in the improved model which is presented in this paper. In the paper, the brightness temperature tracing model of different roughness surfaces has been established. Then, we present a method called multilayer brightness temperature tracing (MBTT) method to obtain the radiation brightness temperature of rough surface. Hence, the discrimination of brightness temperature tracing method is enhanced.

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.

SIMULATION OF MULTI-LAYER ROUGH SURFACES MEDIA IN THE PASSIVE MILLIMETER-WAVE IMAGING

The simulation of multi-layer rough surfaces is an indispensable step in passive radiation imaging, to which little attention has been paid so far. Based on the existing model of brightness temperature tracing described in our recent works, diffused transmission of the bottom layer is taken into account in the improved model which is presented in this paper. Then, a method called multi-layer brightness temperature tracing method (MBTT) is established to obtain the brightness temperature of a rough surface, and the applied range of the simulation in passive millimeter-wave imaging (PMMW) is extended.