The Application Of Wavelet Transform In Terahertz Nondestructive Testing System

Terahertz imaging technology have witnessed great progress in the past decades,and exhibited strong potential for applications in the field of non-destructive detecting and personal screening.Terahertz non-destructive detecting is considered a safe method for the operators and targets,which conforms to the development trend of green nondestructive testing.But spatial resolution and spectral contrast are two major bottlenecks for non-destructed testing of complex samples with current imaging technologies.In this research we use a three dimensional terahertz(THz)imaging system to get the inside structure of the sample,and exploit the wavelet transform algorithm to improve the spatial resolution and the spectral contrast.which provide new insight in terahertz imaging application.This research is concentrate on three dimensional terahertz imaging system,the main research contents are three parts which listed as follows:(1)In order to enhance the terahertz image,a continuous wavelet transform(CWT)algorithm was proposed in this research to process the longitudinal signal.After CWT,an appropriate scale of the wavelet coefficients was chosen,on that scale,the widths of the reflection peaks are narrower and the noise signals are weaken.The “abandon” of other wavelet scales also means “denoise”,all this can make the slice image of the testing sample be “clear” and enhanced.(2)When the thickness is wavelength comparable,the interfaces peaks from upper and lower surface are hard to distinguish because they are very close,which is named “combine peak”.While wavelet transform has strong ability to distinguish that combined peaks.After we reconstruct the three-dimension wavelet coefficients data on the chosen scale,the longitudinal resolution of terahertz detection system can be improved to 1mm.(3)Lipschitz exponent was used as the basis to locate the interfaces of multi-layered sample,which demonstrate to be a reliable tool to describe the feature of signal.This description method is more precise than the description by differentiable.(4)Active terahertz imaging system is difficult to enhance the resolution of the detected image at the time of improving detection speed.To this end,image fusion reconstruction was introduced to process the detected terahertz image.Image fusion reconstruction of multi-layer images with characteristic information enhanced the feature of image effectively.Based on this method,the area of the defect part can be calculated and the edge can be extracted efficiently.This method proves to be a feasible alternative for THz debond detecting of multilayered structures.With this method,the debond defect in the upper and lower interfaces of the 2mm-thick insolation felt in the multi-layered sample could be clearly identified.While the longitudinal resolution was 2mm.For photosensitive resin with simple structure,longitudinal resolution of terahertz nondestructive imaging system reaches 1mm.In order to distinguish small interface and noise peaks,Lipschitz exponent was introduced as a reliable tool.In the last two parts of this paper,image fusion reconstruction of the detection result of multi-layer structure was made,which enhanced lateral resolution of terahertz imaging detection.In Chapter 6,wavelet transform and terahertz nondestructive imaging was combined and applied to terahertz biomedical imaging detection,which could identify the reflector below the chicken skin.The terahertz imaging system also could identify some information from the back of the hand,but this research is only a preliminary study,the application of terahertz imaging in biomedical field is to be developed further.