Experimental Study On The Optical Coherence Tomography For Detecting The Condition Of Mural Pigment Layer Diseases

The painted mural has a long history in China,and its pigment layer forms vivid patterns,which not only has artistic value,but also has important historical value and research significance because it contains abundant information of ancient society and culture.Unfortunately,by the influence of natural invasion and poor preservation environment,the mural would suffer various diseases such as flaking,hollow,mildew,crack and pigment loss.If these diseases can not be detected and repaired in time,which would lead to mural damage and cause irreversible losses.However,the traditional disease detection methods such as visual inspection,percussion,infrared thermal imaging,spectral imaging and ultrasonic tomography,all have their limitations.They are difficult to satisfy the detection requirements of mural pigment layer and diseases.Optical coherence tomography is a three-dimensional imaging technology with the advantages of non-destructive,high-resolution and non-contact.In general,the axial resolution can reach 3?m-15?m while the imaging depth in air is 2mm.Therefore,it is suitable for mural pigment layer diseases detection whose thickness is tens to hundreds of microns.In this paper,with the background of nondestructive detection of mural pigment diseases,relevant experimental research on OCT mural diseases detection has been carried out.The research work and results are as follows:(1)The paper introduces the principle of spectral domain optical coherence tomography(SD-OCT)and expounds the characteristic of SD-OCT in detecting ancient murals,using the spectral domain optical coherence tomography technology to test the cultural relic murals,by means of analyzing the position and the morphology characteristics of pigment layer and ground layer in the imaging figure,can recognize crack,mildew,flaking,hollow,pigment loss diseases,distinguish the hidden draft line covered by pigment layer and test the effect of repaired area.(2)The Fizeau interference spectral domain optical coherence tomography system has been studied and the transmission characteristics of light wave in graded refractive index multi-mode optical fiber have been theoretically analyzed.Fabricate the imaging probe by use of the graded refractive index multi-mode optical fiber fused with hemispherical no-core fiber.Theoretically and experimentally analyze the optical property of the manufactured probe,and experimentally calibrate the depth in imaging figures.Use the built Fizeau interference spectral domain optical coherence tomography system for imaging the sample of horn craft,pigment layer sample included imitative mural flaking,mural pigment loss,mural bleb.By analyzing the imaging figures,Fizeau interferometer OCT has shown the good performance,which has the advantage of good anti-noise ability and the disadvantage of limited detection depth.(3)Design and manufacture a vibration sensor based on mode conversion of thin-core fiber and fiber Bragg grating(FBG).Analyze and test the mode propagation characteristics.The result has shown that the strain and twist sensor based on the mode conversion has the possibility for the vibration detection of OCT system.