数字散斑亚像素小角位移测量的曲面拟合法

与传统测量方法相比,数字散斑相关法由于其目标特征单元网格划分的灵活性,能够更好地满足不同场合小角位移的测量需求。针对该方法亚像素小角位移测量的曲面拟合参数选择问题,研究了亚像素测量图像小角旋转前后的九点二次曲面拟合法,并根据计算机生成模拟散斑进行模拟实验分析,得到最佳误差效率优化条件下的曲面拟合法求解亚像素小角位移的最佳散斑尺寸3.5 pixel、计算窗口尺寸41×41 pixel和拟合窗口尺寸3×3 pixel。实验验证了上述测量参数的有效性,为进一步的曲面拟合法数字散斑成像角位移测量提供参考。     

Adaptive optics interferometer using superspeckles for high resolution deformation measurement

Digital speckle pattern interferometry (DSPI) is a well-known technique for measuring deformations, but it is limited in range by speckle decorrelation. We developed an adaptive interferometer with increased measurement range by combining the DSPI set-up with a pupil filter that generates optical superresolution. As a result, a decrease in the transverse speckle size is observed accompanied with a substantial increase in the longitudinal speckle size. The increase in the speckle correlation length along the optical axis allows for obtaining an out-of-plane deformation measurement over a significantly extended depth. An experimental verification of this concept of superspeckles is presented with an intensity-only LCD spatial light modulator (SLM) acting as a pupil filter. Comparison of speckle correlation fringes with and without the superresolution filter confirms the proposed concept. The present work demonstrates a doubling of the measurement range when using the superresolution interferometer. A standard set-up in which the speckle length is comparably elongated by stopping down the lens has a similar loss of intensity, but its lateral resolution is only half as good.     

Assessment of speckle-pattern quality in digital image correlation based on gray intensity and speckle morphology

In digital image correlation (DIC), speckle patterns are generated on the surface of a specimen to resolve uniqueness issues. Thus, speckle patterns significantly affect the accuracy of image correlation. To assess the quality of speckle patterns, the standard deviation of gray intensities within each speckle (SDGIS) is introduced as a new metric. On the basis of the cumulative distribution of SDGIS, speckle-pattern quality measurement (ρ) is proposed, which integrates the features of gray intensity and speckle morphology. Twelve speckle patterns are generated by changing the spraying time and nozzle sizes of an airbrush because these are associated with the speckle volume fraction and speckle size, respectively. In addition, three displacement fields are used to investigate the effects of speckle patterns on the accuracy of the DIC results. For the 12 speckle images associated with the three displacement fields, the correlation results demonstrate that the proposed speckle-pattern quality measurement is inversely proportional to the averaged error of the subset method. This is statistically confirmed by evaluating the correlation coefficient and p-value. Furthermore, the error of the subset method is more affected by speckle patterns than the subset size when the subset size is sufficiently large.     

Speckle intensity mean value dependence on the number of scatterers

The number of scatterers contributing to the speckle amplitude can be determined by the spot size of the incident light or by the amplitude point spread function of the optical system. The speckle intensity mean value measurement results with two methods of choosing the number of scatterers applied are presented. The values of the rms phase deviation and correlation length obtained from two methods agree well. The properties of both methods are discussed. The possibilities for the roughness measurement are suggested.     

Digital ultrasonic speckle phase-shifting method

The digital ultrasonic speckle phase-shifting method (USPM), which is introduced in this paper, can be applied to the measurement of small displacement that is smaller than speckle size at the test point compared to traditional ultrasonic speckle correlation method (USCM). Using USPM, a digital ultrasonic reference signal is introduced to interfere with the ultrasonic speckle signal, which is picked up at the test point on an object surface and is referred to as the object signal. As the phase of the reference signal is shifted several times using the software and then they superimpose with the object signal respectively, the phase of the object signal can be calculated according to the intensities of the superimposed signals. If the object surface moves a small distance, the phase variation of the object speckle can be detected by the same process. As a result, the displacement of the object surface can be measured. Based on the feature of ultrasonic speckles, inner surface displacement of an object can be measured using this proposed method. In this case, the effect of outer surface roughness to the measurement accuracy of USPM is examined experimentally. The experimental results show that the measurement is successful when the displacement is smaller than half of the speckle size at the test point and the roughness parameter R_a of the outer surface of the specimen is less than about 5.47 μm.     

Gaussian pre-filtering for uncertainty minimization in digital image correlation using numerically-designed speckle patterns

This paper discusses the effect of pre-processing image blurring on the uncertainty of two-dimensional digital image correlation (DIC) measurements for the specific case of numerically-designed speckle patterns having particles with well-defined and consistent shape, size and spacing. Such patterns are more suitable for large measurement surfaces on large-scale specimens than traditional spray-painted random patterns without well-defined particles. The methodology consists of numerical simulations where Gaussian digital filters with varying standard deviation are applied to a reference speckle pattern. To simplify the pattern application process for large areas and increase contrast to reduce measurement uncertainty, the speckle shape, mean size and on-center spacing were selected to be representative of numerically-designed patterns that can be applied on large surfaces through different techniques (e.g., spray-painting through stencils). Such “designer patterns” are characterized by well-defined regions of non-zero frequency content and non-zero peaks, and are fundamentally different from typical spray-painted patterns whose frequency content exhibits near-zero peaks. The effect of blurring filters is examined for constant, linear, quadratic and cubic displacement fields. Maximum strains between ±250 and ±20,000 µε are simulated, thus covering a relevant range for structural materials subjected to service and ultimate stresses. The robustness of the simulation procedure is verified experimentally using a physical speckle pattern subjected to constant displacements. The stability of the relation between standard deviation of the Gaussian filter and measurement uncertainty is assessed for linear displacement fields at varying image noise levels, subset size, and frequency content of the speckle pattern. It is shown that bias error as well as measurement uncertainty are minimized through Gaussian pre-filtering. This finding does not apply to typical spray-painted patterns without well-defined particles, for which image blurring is only beneficial in reducing bias errors.     

散斑自相关分辨率测量和修正

散斑相关是许多基于散斑的光学测量和成像技术的基础,决定了光学系统的分辨率。当前散斑尺寸（颗粒度或分辨率）的理论描述不够精确,也缺乏实验验证。探究了散斑图样自相关尺寸的影响因素,与相同数值孔径物镜聚焦进行比对,揭示薄散射介质的“散射透镜”性质。通过散斑自相关和透镜聚焦尺寸的多组测量,结果表明,截趾函数会影响其分辨率,需要根据具体光路对阿贝判据做修正。对基于散斑的测量和成像技术具有一定的参考价值。     

Particle Imaging Using a Transmission Wide‐Field Phase Confocal Microscope

A phase‐sensitive wide field transmission microscope, combining the advantages of both interferometric and confocal techniques, has been developed and applied to analysis of particulates, both in dry powder form and in suspensions. The microscope has also been used in detecting defects in crystals. Confocal operation is achieved by superimposing speckle illumination of a reference beam in a Mach‐Zehnder interferometer with a matched speckle pattern of the object beam. It is shown that the phase measurement enables particle size to be determined even when the particle is smaller than the focal spot size. The data acquisition time is below 1ms, making the system suitable for dynamic process measurement. The experimental results are in good agreement with modelled results giving rise to the possibility of simultaneous determination of both the size and refractive index of small particles.     

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。     

Digital image correlation with gray gradient constraints: Application to spatially variant speckle images

As a carrier of local deformation information, speckle pattern inside a subset is usually crucial for surface displacement acquisition based upon a digital image correlation (DIC) method, since both accuracy and precision of DIC method are closely related to the amount of speckle information in a subset. Although some comprehensive theoretical frameworks have been developed to estimate the quality of local speckle patterns, it is still a great challenge how to effectively integrate the subset speckle information into the well-developed correlation criteria used for DIC. By means of a well-designed square window function, we here propose the concept of continuous subset in order to modulate subset size in a continuously derivable manner. Afterwards, we further develop a new constrained zero-normalized sum-of-squared differences (CZNSSD) criterion and construct the corresponding iterative algorithm, based on which the subset size involved can be automatically determined according to the necessary amount of speckle information. Numerical results of synthetic speckle images indicate that the set of algorithm can enhance the accuracy and precision of displacement measurement, especially for spatially variant speckle images.     

Deformation sensing with a multimode POF using speckle correlation processing method

A deformation sensing technique with a multimode plastic optical fibre based on intensity speckle patterns’ correlation coefficient measurement has been presented. Influence of the average speckle size on results of deformation measuring has been studied and discussed. The presented sensing technique provides a good linear response to the applied deformation in a relatively wide operation region. It is shown that the proposed technique is highly sensitive, low-cost and simple to implement in practice.     

Measurement of the local displacement field generated by a microindentation using digital speckle pattern interferometry and its application to investigate coating adhesion

This paper presents a technique to investigate the adhesion of thin coatings which combines digital speckle pattern interferometry and an indentation test. The proposed approach is based on the measurement of the local displacement field produced by a microindentation introduced on the coated surface of a specimen. It is experimentally demonstrated that the buckling of the coating generated by the microindentation depends on its adhesion to the substrate. Experiments carried out in specimens with different conditions in the coating–substrate interface show that digital speckle pattern interferometry can be used to determine the size of the buckled region and to give a measurement of the coating adhesion strength.     

基于自相关函数的激光散斑统计半径的测量

为了测量激光散斑的统计半径,运用统计光学理论推导了二维近场散斑光强的归一化自相关函数,并提出了通过求解散斑图像自相关拟合函数的系数来计算激光散斑统计半径的一种测量方法,利用该测量方法编制的测量软件可以方便地计算出激光散斑的统计半径.通过对尺寸已知的模拟散斑的测量,测量结果与实际尺寸相吻合,证实了该测量方法的可行性.     

Fringe analysis by automatic image fitting and its application to speckle pattern photography

A new image processing technique is described that has been found well suited for the fully automatic evaluation of Young's fringes in speckle pattern photography. The basic idea is fringe pattern analysis by creating synthetic images and fitting them to the image picked up by a TV camera. High speed fringe generation and comparison is achieved using a videoprocessor. A theoretical discussion and experimental results show the efficiency of this method for subfringe evaluation, i.e. the measurement of displacements well below the mean speckle size.     

Effect of speckle pattern on laser Doppler velocimeters

The effect of speckle pattern on laser Doppler velocimeters is studied theoretically and experimentally. We have found that dynamic speckle patterns can cause error in velocity measurement. Increasing sampling time and using proper signal processing circuits can eliminate this bad effect caused by speckle pattern.The accuracy of velocity measurement is better than 2.1％ in a wide velocity range(4.66-468.86 mm/s)when speckle effects are taken into account.     

高分辨激光显微测量系统的应用

高分辨显微测量系统在观察样品表面细微结构和普通光学难以分辨的极限尺寸样品的测量中实现了高分辨、高精度和自动化的综合显微分析。文中采用激光光源提高显微测量精度,并消除了激光散斑的影响,采用高分辨CCD工业相机对光盘的道间距及表面参数进行了实时显微测量分析。     

散斑面内形变场检测系统设计

数字散斑测量是一种非接触、全场测量的高精度光测方法。该系统由散斑生成装置、数据采集装置和数据处理装置三部分组成。散斑生成装置将扩散的激光束投射到待测物体上,用接收屏接收物体散射生成的散斑图像。数据采集装置主要由CCD和数字采集卡组成,CCD拍摄散斑图像,并将其传输给数字采集卡,经采集卡转换成数字图像,以矩阵形式输入到数据处理装置。数据处理装置根据光学相关识别技术设计的检测程序对采集到的数字图像进行相关运算,通过对相关峰的准确定位来获取物体的形变信息。     

基于Contourlet变换的自适应SAR图像相干斑噪声抑制算法

为了保持高岛分辨率合成孔径雷达(SAR)图像中的纹理结构,提出了一种基于BivaShrink模型的Contourlet 域SAR图像相干斑噪声抑制算法.联合当前层和父层的Contourlet系数,通过计算局部方差一致性范数和区域能量比,自适应地确定方差估计区域的形状和大小.从而对原始图像方差进行最优估计.实验结果表明,算法在噪声的去除和结构信息等细节的保持上均不同程度的优于小波BivaShrink去噪算法和Contourlet 阈值去噪算法,主观效果和数值指标都有较好改进.     

数字散斑技术在混凝土单轴压缩试验中的应用

数字散斑技术作为一种新兴的测量方法,它具有非接触、全场量测、精度高、操作简单等特点,随着计算机技术的迅猛发展,数字散斑技术得到了广泛的应用与研究。针对混凝土在外荷载作用下具有明显的非均匀变化的特征,借助传统的测量方法很难得到混凝土表面的全场变形结果的问题,结合数字散斑相关方法测量技术,开展了混凝土单轴压缩试验,拍摄了混凝土表面在单轴压缩过程中的位移与应变矢量场,为分析混凝土试件的损伤区域奠定了基础;与电测法测量结果进行了对比,其测量结果表明,这两种测量方法误差相对较小,且数字散斑技术能更早的发现混凝土表面的破坏,验证了数字散斑相关方法的准确性和可靠性。借助数字散斑技术可以更加方便准确的得到材料表面的变形场,这也将在一定程度上促进土木工程领域的发展,具有广阔的应用前景。     

The variation of speckle size with film density: An example of Babinet's principle

The variation of the size of photographically recorded laser speckle with film density is compared with the predicted speckle size. The ten percent increase in the recorded speckle size and high average film densities is due to the non-linear film response and is an example of Babinet's Principle.