Characteristics Of Plywood Detection Based On The THz Time Domain Imaging Technology

The terahertz(THz)wave is an electromagnetic wave(1THz = 1012Hz)in the frequency range of 0.1 to 10 THz.The radiative energy of the terahertz wave is low and does not cause damage to the structure and biological characteristics of the sample and it has the characteristics of penetrating and identifying the fingerprint spectrum.With the rapid development of ultrafast laser technology,a stable,efficient and reliable source of terahertz is generated,which makes terahertz technology widely used in nondestructive detection and military detection and so on.Terahertz domain imaging technology is one of the most important applications of terahertz technology,and it is also an area with huge potential and application prospects.The terahertz time domain system enables fast,efficient and comprehensive detection of samples.Terahertz non-destructive testing has been applied in plywood and wood detection.Most of the research focus on the spectral features of the plywood,the terahertz imaging of the plywood especially the internal structure is limited.So we are unable to fully analyze the surficial and internal information.Also,most of the imaging results of the wood still remain in the gravitational imaging level,the imaging effect is not ideal,and the imaging device is also very complicated.Based on the nondestructive and penetrability of terahertz signals to the biology sample,terahertz imaging was used to detect the three-ply of plates.Our research first used the terahertz time domain imaging to test the plywood.The resolution of terahertz imaging system was determined by studying the basic indexes of terahertz imaging system.Secondly,the terahertz time domain system was used to image the surface of the plywood.we compared with several imaging methods,the best imaging mode was obtained by the maximum.Then the results were analyzed and enhanced by algorithm.The terahertz imaging was then performed on the internal structure of the plywood,and the imaging results were analyzed in conjunction with the internal stratification.Finally,the time domain imaging results was processed by the algorithm to obtain the frequency domain imaging results.The main works include:We determined the basic parameters of terahertz imaging system before the imaging of the plywood.We first identified the instrument's imaging resolution by measuring the minimum distance the system able to distinguish.Finally,the imaging resolution of the sample surface was 0.2 mm was confirmed according to the results of many different sets of samples.The surface of the plywood was imaged by terahertz time domain imaging system.The imaging results could display the alternately dark and bright texture of the plywood surface.Then we used the sharp enhancement algorithm to enhance the edge effect of the terahertz image.The internal structure and thickness of the plywood were analyzed by using the terahertz imaging system to study the inside and the cross section of the plywood combined with the terahertz time domain spectrum.The thickness of first layer is 100mm The error between the result and the true value of the thickness of first layer was below 8%.Due to that the internal could largely absorb terahertz waves,results of the thickness of deep structure were distorted.Our results showed that the terahertz system could only be used to image the surface and shallow structure and the internal deep interface imaging results were distorted.The terahertz time domain imaging was processed by algorithm to obtain frequency information.We also reconstructed image based on different frequency domain information and compared these imaging results.The results suggested that in the low frequency band,the spatial resolution of imaging became high as the frequency domain increases.Due to the SNR became sharpe decline after 1.5 THz,the imaging results varied from the theory after 1.5THz,and the image was distorted in the high frequency band.