Application of laser speckle analysis for the assessment of cementitious surfaces subjected to laser cleaning

This paper focuses on the application of laser speckle technique for the assessment of the effectiveness of laser cleaning of cementitious surfaces. Laser speckle-based methods are non-contact, highly resolving techniques for the measurement of displacement, rotation, and strain of an illuminated area on a rough surface. Since the intensity of reflected light depends on the geometrical microstructure and colour of the samples, any alterations of the surface result in different speckle images. Analysis of speckle images presented here is based on the analysis of the distribution of intensity of reflected light obtained in a selected plane, and analysis of statistical parameters describing such distribution (skewness and kurtosis). A wide range of laser-cleaned mortar samples with different geometrical microstructure and moisture content has been subjected to the assessment by He-Ne laser. Laser speckle method has been successfully used to identify the effectiveness of the laser cleaning process with respect to different surface conditions. It appears that the changes in kurtosis and skewness should be mainly associated with the alterations of geometrical microstructure. Whereas, mean light intensity seemed to depend predominantly on the mortar's absorption characteristics (colour).     

Fractal structure of digital speckle patterns produced by rough surfaces

We report on the fractal analysis of digital speckle patterns experimentally generated using an optical setup to record the light scattered from metallic rough surfaces in the normal direction. Using the differential box counting technique, we have calculated the fractal dimension of digital speckle patterns for six samples with different roughness. Our results show a quadratic dependence between the surface roughness and the fractal dimension of the corresponding digital speckle pattern. As an application a method to determine the surface roughness of metallic surfaces is proposed.     

利用散斑图像测量表面粗糙度的计算机模拟及实验

从菲涅耳-基尔霍夫衍射公式出发推导出了粗糙面近场衍射方程。通过对观察面散斑光强统计特性的分析,提出了采用镜射光强分量法测量表面粗糙度的思想。为验证该方法的有效性,首先用计算机模拟产生具有不同统计特性的随机表面,然后对由随机表面产生的散斑场及其光强分布进行计算。计算结果表明,与传统的散斑对比度法相比,散斑镜射光强分量法测量弱粗糙表面粗糙度具有更大的适用范围和更高的测量精度,克服了散斑对比度法易受表面横向相关长度影响的缺点。通过实验对计算机模拟结果进行了验证。     

Neural network painting defect classification using Karhunen–Loeve transformation

This paper deals with the problem of painting defect detection on reflecting surface objects. The problem has been approached with an optical inspecting method. A laser beam hits the object surface. The light scattered from the rough surface generates a digital speckle. The speckle is affected by the painting defect. Using the Karhunen–Loeve transformation, the speckle pattern is transformed into a feature vector. This information is used to train the neural-networks in recovering the defect. The reliability and effectiveness of a prototype is validated by experimental results. At the end, the proposed method is compared with another optical inspection method.     

The autocorrelation of speckles in deep Fresnel diffraction region and characterizations of random self-affine fractal surfaces

This paper studies the correlation properties of the speckles in the deep Fresnel diffraction region produced by the scattering of rough self-affine fractal surfaces. The autocorrelation function of the speckle intensities is formulated by the combination of the light scattering theory of Kirchhoff approximation and the principles of speckle statistics. We propose a method for extracting the three surface parameters, i.e. the roughness w, the lateral correlation length ξ and the roughness exponent α, from the autocorrelation functions of speckles. This method is verified by simulating the speckle intensities and calculating the speckle autocorrelation function. We also find the phenomenon that for rough surfaces with α= 1, the structure of the speckles resembles that of the surface heights, which results from the effect of the peak and the valley parts of the surface, acting as micro-lenses converging and diverging the light waves.     

半导体激光器内自混合散斑干涉

在散斑和半导体激光器的自混合干涉理论的基础上，提出了基于法布里一珀罗腔的自混合散斑干涉模型，对粗糙表面产生的激光器内自混合散斑干涉效应(SMSI)进行了详细的理论研究和分析，得到了粗糙腔条件下的激光动力学的数值解。模拟产生了高斯相关随机表面及其在夫琅禾费面上产生的散斑场，以及该散斑场与相干光场叠加所形成的光场，分析了这些情况中光强的统计特征，与已知实验情况进行了对比，模拟结果与已知统计结果相同。给出了粗糙表面运动时激光器输出增益的变化及其概率密度分布。对由于照明宽度和外腔长度的变化而给粗糙表面运动时激光器输出增益变化带来的影响进行了分析。实验结果和模拟结果相符。     

Numerical computation of in-plane displacements and their detection in the near field by double-exposure objective speckle photography

A model of a rough surface to the scale of the optical wavelength is proposed, with randomly distributed zero-mean Gaussian heights. It is assumed that the aforesaid surface is illuminated by a coherent light beam of homogeneous intensity. An in-plane pure translation of this surface is simulated on a PC by means of the Matlab program. In the near-field optical regions the Fraunhofer approximation and the subsequent FFT are not suitable for application in the generation of a speckle pattern. Hence, with the aim of calculating the translation of the rough surface by means of double-exposure objective speckle photography using the point-wise filtering technique (PWFT), the Fresnel-Kirchhoff integral is first employed to obtain the speckle intensity near the surface on a photographic plate, and then the intensity pattern produced by the diffraction of the plate is determined in the far field (Fraunhofer), which suffices to calculate the modulus of displacement through the Young’s fringes. The results are analyzed according to surface roughness and the sample distance to the recording plane.     

Detection of a small defect on a rough surface

We report experimental results that show that the far-field correlation function is sensitive to a small local change in rough-surface geometry, where the speckle spatial correlation rather than the sample ensemble average is adopted. The angular cross-correlation function of the far-field speckles scattered by one-dimensional random rough surfaces is measured when a polarized beam of light is incident upon the rough surface from vacuum, where one part of the surface used is a thin dielectric film deposited upon a glass substrate and the other part is identical to the first except for a localized defect. We envisage application of this sensitivity property to inspection of a sample with a defect by means of speckle mapping.     

Effects of an external angular dispersion on polychromatic speckle in the far field

The effects of an external dispersion on far field polychromatic speckle produced by a rough surface are investigated theoretically. Under certain assumptions it is found that the speckle pattern will undergo a displacement which does not depend on the RMS roughness of the surface.     

基于多色散斑延长效应的表面粗糙度测量及影响因素分析

粗糙表面在多波长激光束照射下形成的多色散斑场显示出散斑延长效应,利用此效应可以测量表面粗糙度,并且测量结果在一定条件下不受粗糙表面横向特征的影响。通过模拟计算随机粗糙表面的多色散斑场,以空间平均的多色散斑场局部自相关函数研究了平均散斑延长率〈χ〉对表面轮廓均方根偏差σh的依赖关系,分析了测量系统因素,如入射激光波长组合、成像器件光敏单元尺寸和动态范围对测量结果的影响。结果表明,以空间平均的局部自相关函数代替集平均的散斑自相关函数描述多色散斑延长效应是有效的;为达到一定的粗糙度测量精度,应选择合适的入射激光波长组合和合适的成像器件光敏单元尺寸。