Research On Active Millimeter-Wave Fast Imaging Algorithm For Security Screening

Personnel security check in public places is necessary to prevent terrorist attack and protect public safety.Due to the capacity of penetrating common clothes and nonionizing characteristic,the active millimeter-wave imaging techniques have gained lots of attention among various checking methods,and it has huge development potential and broad market prospect.In this dissertation,the millimeter-wave near-field imaging algorithms and relevant techniques for personnel security check are developed,including single-tone holographic auto-focus imaging algorithm,wideband holographic imaging algorithm,wideband diffraction tomography and fast imaging algorithms for SIMO and MIMO system.Besides,a W-band holographic imaging system is presented to verify the effectiveness of the proposed algorithms.Firstly,the wavenumber domain algorithm for single-tone holographic imaging is introduced.In order to overcome the shortcoming that the single-tone holographic imaging system cannot guarantee the target is focused because of the short depth of focus,we propose an auto-focus method based on the amplitude integral value of the imaging results,which can adjust the focal plane to the place where the strong scattering center locates.Moreover,in order to overcome the shortcoming that the single-tone holographic imaging system can only focus on focal plane,we propose a multi-section auto-focus method,which divides the imaging area into several sections and auto-focus respectively,then combines the imaging results from each section to form a complete image.The proposed method enables the single-tone holographic imaging system to focus on the targets at different places simultaneously.Secondly,the wavenumber domain algorithm based on Stolt interpolation for wideband holographic imaging is introduced.A W-band holographic imaging system based on a planar mechanical scanner is established.Experiments have been done to verify the effectiveness of the techniques.Thirdly,the algorithm for wideband diffraction tomography is introduced.The algorithm is faster and accurate than Stolt interpolation based algorithm,because it completely avoids interpolation by replacing the 3-dimensional Stolt interpolation loop with loop in range dimension.Thereafter,the tomography can be combined with auto-focus to narrow the imaging area to a limited but sufficient depth of focus.Depending on the simulation and experiment,the processing time is quarter of that with linear interpolation.Finally,the fast imaging algorithms for SIMO and MIMO system are introduced.The algorithm processes the transmitting delay in space domain,and processes the receiving delay in wavenumber domain,which can be regarded as the combination of back-propagation algorithm and wavenumber domain algorithm.The algorithm is suitable for near-field operated SIMO and MIMO system to reconstruct the image with less processing time.