Research On Terahertz Radiation Characteristcs And Design Of Contour Extraction Algorithem For Passive Detection Imaging

Electromagnetic waves with frequencies between 0.1 and 10 THz are called terahertz waves.Compared with microwave and millimeter waves,terahertz wave has a higher frequency band and a smaller wavelength,so higher spatial resolution of imaging can be achieved.Therefore,terahertz passive detection and imaging system can be developed.For the far-field application of the system,the loss attenuation of terahertz wave transmission in atmospheric medium is the first thing to be studied for the design of the system.Aiming at the near field application of the system,the radiation characteristics and the radiation bright temperature distribution of the target should be studied.The above research has important guiding significance to the selection of working frequency of the system.Due to the scanning form and channel of passive detection imaging system,the original image is seriously affected by noise,which brings great disadvantage to the subsequent target recognition,positioning and classification.Therefore,the design of a contour extraction algorithm suitable for passive detection image has important practical value for passive detection imaging system..Based on the actual scientific research projects of the research group,the specific work of this thesis is as follows:1.The basic theory and imaging mechanism of terahertz passive detection and imaging technology were studied,and the imaging technology features,system structure and key technologies involved in the terahertz passive detection and imaging system were analyzed.2.Aiming at the far-field application of passive detection and imaging technology,the attenuation of terahertz wave transmission in the atmosphere was studied and analyzed.The relationship between the sky light temperature and frequency was analyzed in combination with the attenuation study,and the conclusion that the sky light temperature exists in the frequency selectivity was drawn.3.Aiming at the near-field application of passive detection and imaging technology,a theoretical analysis model of typical target radiation characteristics was established;The calculation model of radiant light temperature of metal stereoscopic targets in security check scene is proposed and the simulation results are given.The results provide a basis for the selection of system operating frequency of 330 GHz.The penetration rate of common material covering was measured,and the experimental data supported the selection of 330 GHz as the operating frequency of the system.To sum up,the scientific and effective selection of system operating frequency of 330 GHz is demonstrated.4.Aiming at the low SNR of passive detection image and the requirements of the project for contour extraction,the classical algorithm based on anisotropic diffusion filtering was studied,and the y-k-gauss iterative algorithm was proposed to maintain the edge and remove the noise.The experimental results show that the MMW image with serious noise pollution can be processed and the edges can be kept well,which provides a basis for contour extraction.5.Aiming at the limitations of traditional gradient calculation,an improved gradient calculation method with diagonal gradient component is proposed.An improved adaptive double threshold method is designed to select the threshold based on experience.Based on the above improvements,an improved contour extraction algorithm for passive detection images is designed.The millimeter wave image is used to verify the experiment,and the excellent effect is obtained,which can meet the requirements of the project.