Digital Photography in Measuring the Inclination of the Object Surface with Formation of a Speckle Structure

Optical schemes for measuring surface inclination due to shift or deformation are considered. Speckle-interferometry methods are used. The distinguished features of these schemes are connected with using a digital photocamera and a forming speckle structure in nonmonochromatic light. Several technical solutions are proposed making it possibile to measure inclination with an accuracy up to tenths of degree using standard cheap equipment. To form the speckle structure, the pattern of a random binary structure with unit size of several millimeters is used in the base fixed plane. The digital camera was placed in the analyzed plane and turned together with the plane. It registered a reduced image of the pattern. The registration was done for two positions of the surface, the pattern being illuminated by the camera flash lamp. Measuring the width and orientation of the interference fringes formed for the two speckle structures allows one to determine the inclination and orientation of the rotation axis.     

Detection of fatigue damage in steel using laser speckle

We investigated a method to detect fatigue damage of steels without contact using laser speckle. In the earlier stage of fatigue in steels, slipbands appear on the surface and microscopic shear strain is stored in the slipbands. The slipbands appear more densely with progress of fatigue damage. When a laser illuminates the surface of the fatigued steel, light intensity distribution of the laser speckle pattern formed by the reflected light changes with the change of surface properties caused by slipbands. It has been clarified that the width of the speckle pattern broadens corresponding to spatial frequency distribution of the surface profile and thus it is presumed that speckle pattern broadens with increase of slipband density. This shows that we can detect fatigue damage by observing the laser speckle pattern on material surface. The method presented in this paper is based on this phenomenon. We observed change of the speckle pattern during fatigue loading under constant stress amplitude using a steel specimen and the relation between speckle pattern, number of loading cycles and also magnitude of loading was considered. We investigated the possibility of detection of fatigue damage using this method and also proposed a method to estimate fatigue life by observing change tendency of the speckle pattern depending on the number of loading cycles in the earlier stage of fatigue before crack initiation.     

利用电子散斑相移技术测量物体三维面形的方法

为了获得准确的面形测量,提出了一种相移电子散斑干涉技术测量物体面形的测量方法.利用电子散斑干涉产生载波条纹,该载波条纹受到物体表面高度的调制变得弯曲,引起载波条纹相位的变化,可运用相移技术提取物体的相位信息,最后根据高度和相位之间的关系得到物体的面形.介绍了该方法的原理,利用该方法对球冠物体进行了面形测量,证明该方法测量物体面形是可行性的.由于是采用散斑干涉的方法产生干涉条纹,因此该方法测量物体面形具有灵敏度高的优点.     

基于Gabor滤波的散斑条纹图平滑方法

提出了一种基于伽博(Gabor)滤波的散斑条纹图平滑方法.通过加窗傅里叶运算提取散斑条纹图的条纹频率和条纹梯度方向,并利用它们确定具有频率和方向选择性的Gabor滤波器对散斑条纹图进行Gabor滤波.数值模拟和实验结果表明,该方法在滤除散斑噪声的同时能够有效的保留散斑条纹图的条纹结构,为从单幅散斑条纹图中进一步提取条纹相位场奠定了良好的基础.     

Effect of surface roughness on the statistical distribution of image speckle intensity

An experimental investigation is made of the effect of surface roughness on the statistical distribution of image speckle intensity. A speckle pattern is formed by spatially coherent light at the image plane of an object having some surface roughness and its statistical properties are investigated. It is found that the contrast of the speckle is related to the surface roughness. By this relation, a new technique for the measurement of surface roughness is proposed.     

Continual deformation analysis with scanning phase method and time sequence phase method in temporal speckle pattern interferometry

If a laser beam illuminates a continual deformation object surface, it will lead to a temporal speckle pattern on the observation plane. Recording this time-dependent speckle pattern the deformation of the surface of an object can be obtained. Two methods, scanning phase method (SPM) and time sequence phase method (TSPM), have been introduced for measuring the displacement caused by the deformation in temporal speckle pattern interferometry (TSPI). Their principle is that by capturing a series of speckle interference patterns related to the object deformations, the fluctuations in the intensity of the interference patterns can be obtained. Through scanning these fluctuations and estimating both the average intensity and modulation of the temporal speckle interference patterns, the phase maps for whole-field displacements are calculated. In this way one is capable of quantitatively measuring continual displacements simply using a conventional electronic speckle pattern interferometry (ESPI) system without phase shifting or a carrier. The elaboration on the new methods is given in this paper and experiments are performed to demonstrate their performance with a conventional ESPI system.     

Using Speckle Interferometry for Controlling the Units of Multi‐Mirror

Applications of speckle interferometry for measuring displacements of mirrors of multi‐mirror centimeter radiotelescopes are considered and analyzed in modeling experiments. The general principles of speckle photography, speckle interferometry, and the use of two-exposure speckle patterns for the analysis of displacements of rough surfaces (units of the radiotelescope mirrors) are worked out. Experiments are carried out for three possible displacements of the object surface: parallel shifts along the surface itself, rotations, and shifts in the perpendicular direction. The shape and structure of the displaced units do not change. The experiments prove that speckle interferometry is a rather effective and practical tool for measuring shifts and rotations of composite radiotelescope mirrors.     

Using a programmable interferometer for digital phase-shifting speckle interferometry

The use of a programmable phase-shifting interferometer to detect the deformation of scattering surfaces is described. The interferometer is a commercial instrument in use for optical testing applications; the sample surface is imaged as a speckle pattern. The measuring procedure and the program are outlined, and an experimental demonstration on an aluminium plate under load is given.     

Introducing speckle noise maps for Laser Vibrometry

When coherent light scatters from a surface, which is rough on the scale of the wavelength of the light, a speckle pattern is produced. The Laser Vibrometer measures target vibration velocity in the direction of the incident laser beam and typically samples a region of a speckle pattern on its photodetector. Target motions can cause the speckle pattern to change on the photodetector surface, particularly when target motions are non-normal to the direction of the laser beam. This speckle motion modulates the Doppler signal and adds noise to the demodulated output signal. Periodic target motions can cause the speckle noise to become pseudo-random and produce harmonic peaks, with the same fundamental frequency as the genuine target vibrations, which can be indistinguishable from the genuine target vibrations. Typical speckle noise levels are generally considered to be low-level, but they have not so far been adequately quantified. This paper reports preliminary results quantifying speckle noise levels using controlled experimental configurations incorporating periodic in-plane and tilt target motions. Working with commercial Laser Vibrometers, various target surface finishes and treatments are considered and speckle noise maps are produced for each configuration. For a tilting surface, speckle noise has been quantified at approximately 1 μm s^?1/deg s^?1 while, for surfaces with in-plane motion, the sensitivity to speckle noise has been estimated pessimistically at 0.1% of the in-plane velocity. Ultimately, these speckle noise maps will form a valuable practical resource for the Laser Vibrometer user.     

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。     

时间序列散斑干涉技术研究及应用

当用相干激光照射在一个连续位移或变形的漫射物体表面时,在与参考光形成干涉的接收面上即产生一随时间变化的散斑干涉场通过对这一干涉场的时间域进行分析,可实现时变位移场的定量检测本文介绍了基本时间序列散斑干涉场的扫描相位方法和时间序列相位法的相位函数解调原理,探讨了它们的计量特性,并将其用于火箭固体燃料性能的检测.     

Interference fringe-patterns association to defect-types in artwork conservation: an experiment and research validation review

The paper is directed to all classes of optical inspection technologies that are causing a fringe pattern like output, such as interferometry, fringe projection, holography, speckle techniques. The explanations given here are also valid for incoherent fringe projection, moire, and short-coherent techniques, which are producing fringe-like output correlated with the surface structure and the surface change, respectively.     

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.     

地面对激光雷达信号散射的统计研究

从理论角度研究了利用高斯光束照射远场目标时,激光散斑的统计特性.导出了散射光场的自相关函数和光强度的二阶矩的解析表达式,计算了激光散斑的面积.研究表明:在接收面上,光强的分布与目标表面高度的相关长度和均方根高度密切相关,激光散斑面积和散斑光强的相关系数只与激光束腰的尺度有关,与目标的均方根高度和相关长度无关. 关键词： 激光散斑 自相关函数 高斯分布 协方差     

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.     

Sampled-speckle photography for measurement of deformation

Speckles usually are of two kinds: laser speckle and white-light speckle. An additional kind, termed a sampled speckle, is proposed. Whereas laser speckles arise from multiple interference of light scattered from an object illuminated by a coherent laser beam and white-light speckles are a physically generated speckle pattern on the surface of the object, sampled speckles are generated as a result of the sampling of a digital image. The generation of these speckles and their application to displacement measurement are demonstrated.     

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.     

Two-dimensional angular optical scattering patterns as droplets evolve into clusters

Remarkable changes in the two-dimensional angular optical scattering (TAOS) pattern are observed when an acoustically levitated water droplet with inclusions evolves into a dry aggregate. The emergence of polystyrene latex sphere inclusions near the droplet surface has a clear effect on the TAOS. A speckle pattern in the TAOS replaces the continuous contours associated with Mie scattering from a smooth droplet. Once the contours break up, this speckle pattern persists as the levitated droplet evaporates, making the transition from a droplet with inclusions to a dried aggregate. The initial emergence of the speckle pattern is observed for an inclusion volume fraction near 8%.     

Surface roughness measurement using spatial-average analysis of objective speckle pattern in specular direction

The speckle contrast method (SCM) and the light scattering method (LSM) are two of the most promising optical techniques for on-line surface roughness measurement of slightly-rough surface. However, due to the lack of capability in eliminating the influence from the diffuse component of scattered light, SCM and LSM are both sensitive to the variations of surface correlation length. Additionally, for LSM, the presence of speckle noise leads to fluctuations in the measuring results. To solve these problems, an approach based on the spatial-average analysis of the objective speckle pattern in the specular direction, simply called spatial-average method (SAM), is proposed. The SAM establishes the quantitative relationship between a new characteristic parameter extracted from the recorded speckle image and the rms surface roughness, eliminates to a large extent the influence of diffuse light component on the measuring results, and immunizes itself from the speckle noise. The theoretical foundation of SAM is given in details. A computer simulation is then performed to make comparisons among these three methods. Finally an experiment is presented.     

Speckle from a rough surface when the illuminated region contains few correlation areas: The effect of changing the surface height variance

We report speckle contrast measurements taken at the centre of the far-field diffraction pattern of weak, medium and strong diffusers illuminated by a focused laser beam. We study the evolution of curves of speckle contrast as a function of the radius of the laser spot as the variance of surface height is steadily increased for a fixed correlation length. We also present experimental curves of speckle contrast as a function of the distance of the rough surface from the beam focus for three of the diffusers.