强散射体产生的像面散斑对比度与随机表面及成像系统关系的研究

在对4f光学成像系统中强散射体形成的像面散斑的统计特性的研究中, 首先通过散斑场光波复振幅的一般形式和双重指数函数近似求出散斑光强的系综平均, 然后利用散斑场光波复振幅的实部和虚部的旋转变换法求出散斑光强的方差, 最后得出了散斑对比度与随机表面统计参量和系统参量的直接表达式. 本结果与现有文献中包含随机表面相关面积或散射粒子数目的隐含表达式相比具有明显的改进, 并对标定随机表面的散斑对比度法具有重要意义. 关键词： 随机表面 像面散斑 对比度     

粗糙表面对散斑统计特性的影响

为研究高速摄影中激光相干噪声的影响因素,降低激光照明中散斑噪声对高速摄影成像质量的影响,对激光照射表面的粗糙程度与形成的散斑场统计特性之间的关系进行了研究。基于菲涅尔-基尔霍夫衍射原理,分析了高斯光束入射在随机粗糙表面上经反射形成的散斑图样,通过对大量散斑图样的数据统计,得出了激光散斑的信噪比、自相关函数、概率密度函数与入射表面粗糙度之间的变化关系。计算结果表明:粗糙度的增大使散斑场信噪比降低、相干程度减小、概率密度降低,即表面越粗糙散斑噪声越严重。     

光盘信息符散斑图的统计特性研究

提出了一种用激光散斑测量光盘信息符统计特性的方法.利用散斑统计理论中衍射散斑场的平均对比度与散射屏表面粗糙度近似成线性关系,把光盘等效为弱散射屏,对未刻录、刻录及存在缺陷的CD-R光盘不同区域形成的衍射散斑图进行了对比研究.实验结果表明,该方法具有可量化、全场测量的优点,能够表征光盘信息符的某些统计特性,这为测量光盘刻录质量提供了一种新的方法和途径.     

激光散斑反差与表面粗糙度关系的理论分析

本文从Fresne1-Kirchhoff衍射公式出发,直接由光强分布导出了散斑平均反差与表面粗糙度特征参数的关系公式,采用指数形式和高斯形式的自相关函数,得到极为相符的计算结果。利用激光衍射散斑平均反差测量表面粗糙度,具有精度高和分辨高等优点,可以用于测试精密表面的粗糙度。     

激光光束非相干叠加对散斑噪声抑制情况

为了抑制高速摄影中激光照明产生的散斑噪声对图像信息的干扰,分析了散斑的统计特性,并在此基础上对光束非相干叠加降噪原理进行了分析,讨论了参与叠加的散斑平均光强不等时叠加后散斑对比度的变化情况,对参与叠加的散斑平均光强满足不同分布时叠加后散斑对比度随叠加个数的变化情况进行了模拟。结果表明,参与叠加的散斑平均光强差异越大叠加后的散斑对比度越大,噪声抑制效果越差。并通过实验对理论结果进行了验证。     

高斯相关随机表面及其光散射散斑场的模拟产生和光强概率分析

提出了模拟产生高斯相关随机表面的方法,并模拟产生了这类表面在夫琅禾费面上产生的散斑场.以模拟产生的10000个随机表面作为表面样本系综,对具有不同粗糙度和横向相关长度的表面系综在夫琅禾费面上各点产生的散斑光强系综的概率密度分布进行了分析.发现：(1)散斑场的高斯和非高斯特性在空间上按一定的区域分布,并非整幅散斑图皆为高斯或非高斯散斑;(2)散射表面只包含少数散射颗粒时也可以形成高斯散斑. 关键词：     

空间平均的角度散斑相关粗糙度测量模拟研究

角度散斑相关是一种不受表面粗糙度轮廓的间距特性影响的粗糙度幅度参量测量方法,它的数学模型通常建立在集平均的基础上。通过模拟计算随机粗糙表面的远场散斑场,以散斑图面上的空间平均代替常规的集平均来计算角度散斑相关系数,并应用集平均的数学模型反演粗糙度参量。结果证实了这种空间平均角度散斑相关粗糙度测量方法的有效性,在同一表面只需对少数个区域进行测量并对测得的粗糙度参量取平均,即可获得足够的测量精度。对于Rq大于2.0μm的表面,测量相对误差小于15%。根据最佳测量条件,该方法适用于大粗糙度表面。     

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。     

抛喷丸表面的多波长散斑自相关法粗糙度测量

粗糙表面产生的多波长远场散斑图案的散斑延长现象可以用来进行表面粗糙度测量。以抛喷丸表面为研究对象,具体介绍了多波长散斑自相关表面粗糙度测量方法。根据多波长散斑图案的各向同性径向辐射的特征,提出了适用的数字图像处理方法。探讨了局部自相关函数特征长度的选取、数字图像处理过程中的局部窗口尺寸和散斑图像的饱和曝光比等因素对散斑延长效应的影响。通过拍摄和处理每一样品表面多个位置的多波长远场散斑图像,计算了Ra分别为0.4,0.8,1.6和3.2μm的表面样块的光学粗糙度指标值。结果表明,该光学粗糙度指标能很好反映被测表面的粗糙程度。     

变孔径像面散斑平均光强标定随机表面的理论与实验

采用随机表面的高斯相关模型和统计光学原理,推导出弱散射体在4f系统中的像面上光强的表达式.在此基础上提出了均方偏差粗糙度和横向相关长度的标定方法,该方法用理论结果对实验测量的像面散斑平均光强随滤波孔半径的关系曲线进行拟合,同时测量出被测样品表面的两个统计量.实验上制作了高斯相关随机表面样品,对其表面参数进行了测量,与AFM所测结果符合得较好,这表明该方法具有较高的精度. 关键词： 弱散射体 像面散斑 平均光强     

Relation between statistical properties of surface roughness and the averaged speckle intensity in the diffraction field

A theoretical and experimental investigation is made on the relation between the statistical properties of a rough surface and the averaged speckle intensity in the diffraction field. The theoretical analysis is performed in some detail over a wide range of the random phase variation for various illuminated objects. The result gives an interpretation for the behavior of the averaged speckle intensity obtained experimentally for various states of rough surfaces.     

微粗糙圆柱的各向异性散斑特性研究

分析了高斯光束照射微粗糙圆柱在菲涅尔衍射区形成的散斑图像统计特性,给出了强度起伏自相关函数与表面曲率及粗糙程度等参数之间的关系.根据强度起伏自相关函数的离散化定义,实验并计算了圆柱轴向与径向结构有差异情况下的散斑强度起伏相关函数.结果表明,对于C1和C2圆柱沿垂直于圆柱轴向的散斑尺寸变长,强度起伏自相关函数沿平行和垂直于圆柱轴两个方向的波动相差较大;对于C3和C4圆柱两个方向上的散射特性基本相同;测量C1和C3的结果表明,散斑尺寸和形状依赖于圆柱表面的皱褶和圆柱表面曲率两个因素.研究结果对于如柱型管道、轴承等方面的机械制造的质量控制有重要的应用价值.     

Statistics of speckle focused through a hard-edged pupil

We examine the statistical properties of speckle patterns in and near the far-field, produced by an ordinary optical system having a hard-edged pupil. By applying Fresnel diffraction theory, expressions for the contrast in terms of the statistical parameters of rough surfaces and the geometry of the optical system are derived and discussed. It is shown that the contrast of speckle has maxima and minima in the focal plane and also along the axis of the optical system.     

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.     

Statistical Properties of Laser Speckles Generated from Far Rough Surfaces

The statistical properties of speckle patterns generated from far rough surfaces, under illumination of a Gaussian beam, are investigated. The surface roughness dependence of the first- and second-order moments of intensity is theoretically investigated, and their analytical expressions have been derived and presented. The analysis indicates that the mean intensity distribution on the receiver plane is closely related to the ratio of the lateral correlation length to the surface root mean square (rms) height. On the other hand, the speckle size and the correlation degree of the speckle intensities are found to be independent of the parameter characterizing the roughness of a surface, and are only determined by the laser beam waist.     

Statistical properties of the dynamic dichromatic laser speckle

The statistical properties of dynamic elongated speckle patterns produced in the far-field diffraction region by a moving diffuse object under illumination of the dichromatic laser beam are experimentally investigated by means of a correlation method. The experimental results show that the correlation length of the normalized autocorrelation function of time-varying dichromatic speckle intensity fluctuations depends strongly on the fibrous radial structure of elongated speckle patterns. The qualitative discussions are given for the experimental results.     

Statistical properties of speckle intensity variations in the diffraction field under illumination of coherent light

The statistical properties of speckle intensity variations produced by coherent light in the diffraction field of diffusely transmitting objects are investigated experimentally with intimate relation to their surface roughness. The probability distribution and the average contrast of intensity variations in speckle patterns are studied as a function of the surface roughness of the objects and the receiving plane in the diffraction field.     

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).     

A new method of studying the statistical properties of speckle phase

A new theoretical method with generality is proposed to study the statistical properties of the speckle phase. The general expression of the standard deviation of the speckle phase about the first-order statistics is derived according to the relation between the phase and the complex speckle amplitude. The statistical properties of the speckle phase have been studied in the diffraction fields with this new theoretical method.     

High Accuracy Phase Measurement in Phase-Shifting Speckle Interferometry

A method for accurately measuring information about the deformation of a rough surface object using a phase-shifting speckle interferometer with a television camera and a computer is considered. In this case, the intensity change of the speckle by phase-shifting varies randomly in space because of the statistical property of the speckle. Then, at points with small intensity change, the accuracy of the phase measurement is affected significantly by the quantization error of an analog to digital converter for data recording. To improve the accuracy, the statistical property of the interference speckle must be clarified. This is done theoretically and experimentally, and the experimental results show that higher measurement accuracy can be attained by selecting large amplitude points.Presented at 1996 International Workshop on Interferometry (IWI ‘96), August 27–29, Saitama, Japan.