Simulation Imaging And Image Processing Of Pulsed Terahertz Diffraction Based On FDTD

In general,when pulsed terahertz nondestructive testing is used to detect materials with internal defects,diffraction at the boundary of the material defect often causes boundary effects,which leads to the blur of terahertz imaging of defect boundary and the difficulty of accurate identification of defect features.At present,there are few specific analyses on pulsed terahertz diffraction theory and the boundary effect of pulsed terahertz imaging in terahertz imaging technology.Therefore,the boundary effect of pulsed terahertz nondestructive testing is studied by employing the finite-difference time-domain method and the quality of terahertz imaging is improved in this paper.The main tasks are as follows:(1)The study on the diffraction effect of pulse terahertz is carried out theoretically.A circular aperture diffraction model is established based on the finite-difference time-domain method,and the terahertz band diffraction phenomenon is described from the perspective of near-field imaging and far-field imaging at different distances.Pulsed terahertz circular aperture diffraction experiment is designed and carried out.The experiment results show that the position of the receiving screen is fixed,as the diameter of the circular hole increases,the higher the energy of the light wave passing through the circular hole,and the energy is distributed in a Gaussian distribution;when the diameter of the circular hole is constant,the frequency-domain waveform will redshift with the increase of the distance from the same receiving screen of the beam axis;at a certain frequency,as the diameter of the circular hole increases,the denser the alternation of light and dark rings.The experimental and simulation results are mutually verified.(2)From the perspective of practical engineering,research on boundary effects in pulse terahertz non-destructive testing is carried out.Based on the finite-difference time-domain method,the imaging blur caused by the bubble boundary effect in the adhesive layer is explored from the time-domain waveform,B-scan imaging and multispectral imaging technology.Terahertz nondestructive testing experiment is performed on the adhesive layer.As is shown in the experiment: when the wavelength is equal to the bubble size,there are light and dark stripes at the edge of the bubble,and the existence of boundary effect makes the real boundary of the bubble difficult to distinguish;the bubble edge stripes do not form a ring,which is due to the irregular shape of the bubble,which proves that the essence of boundary effect is diffraction phenomenon.(3)In view of the blurring problem of pulse terahertz imaging caused by the boundary effect,research is carried out from the perspective of image processing.HDPE samples with different diameters circular holes are designed,and the time-domain feature defect recognition imaging technology is proposed.The kurtosis and skewness algorithms,boundary detection operator and hole filling algorithm are combined used in processing the image boundary,to achieve the detection and recognition of circular holes with minimum diameter of 3mm.The frequency-domain feature high-resolution imaging and defect recognition technology are proposed by utilizing the standard resolution plate,and the highresolution multi-resolution imaging algorithm is adopted.A spectral algorithm combined with image sharpening technology is used to process the edge of the resolution plate,which reaches the resolution limit of 0.5mm in width.This paper aims to provide theoretical support and processing means for the boundary effect of terahertz waves in the dielectric defect.It explains why the defect boundary cannot be determined due to the blur of terahertz imaging.The image edge detection algorithm is used to improve terahertz imaging quality and defect recognition ability of the image.