Electronic speckle pattern interferometry using vortex beams

We show that it is possible to perform electronic speckle pattern interferometry (ESPI) using, for the first time to our knowledge, vortex beams as the reference beam. The technique we propose is easy to implement, and the advantages obtained are, among others, environmental stability, lower processing time, and the possibility to switch between traditional ESPI and spiral ESPI. The experimental results clearly show the advantages of using the proposed technique for deformation studies of complex structures.     

Electronic speckle-pattern interferometer using holographic optical elements for vibration measurements

We report a simple, compact electronic speckle-pattern interferometer (ESPI) incorporating holographic optical elements (HOEs) for the study of out-of-plane vibration. Reflection and transmission HOEs provide reference and object beams in the interferometer. The alignment difficulties with conventional ESPI systems are minimized using HOEs. The time-average ESPI subtraction method is used to generate the fringe pattern and remove background speckle noise by introducing a phase shift between consecutive images. The amplitude and phase maps are obtained using path-difference modulation.     

大剪切电子散斑干涉的载频调制与位移场测量

将电子散斑干涉场的载波调制引入到大剪切电子散斑干涉中,通过对参考物的微小偏转引入载波条纹;利用傅里叶变换法,解调出了变形场的相位,从而实现了物体变形场的精确测量。讨论了大剪切载频的调制机理,理论分析表明,调制条纹的空间频率与参考面偏转的角度成正比;因此,控制参考面的偏转角度可实现不同位移量系统的调制。利用中心加载周边固定圆盘进行了典型实验,实验结果证明在大剪切电子散斑干涉技术中可以通过参考面的旋转高质量地实现电子散斑干涉条纹的调制,求解位移场。该系统具有系统简单,不需要专门引入参考光,条纹质量好等优点。该技术可扩展电子散斑干涉的应用范围,有一定的实际应用价值。     

The design of a non-cube beam-splitter used for electronic speckle pattern interferometry

A non-cube beam-splitter (NCBS) is proposed, by which an incident beam can be separated largely in a direction and then the lights from the test object and the lights from a reference surface placed adjacently to the test object can be combined to construct a simple electronic speckle pattern interferometry (ESPI) system. Two mainly useful quantitative ways, to calculate the phase change of the test object, the phase-shift and the fringe carrier method with Fourier transform, can be achieved in the ESPI system with the NCBS. Experiments with phase-shifting and fringe carrier method are completed. The experimental results show that the monolithic design of the proposed NCBS is effective in ESPI measurement and immunity to vibrations.     

The separation of out-of-plane displacement from in-plane components by fringe carrier method based on large image-shearing ESPI

A fringe carrier method for separating out-of-plane displacement from in-plane components based on large image-shearing electronic speckle pattern interferometry (ESPI) is presented. If the test object is respectively illuminated by two expanded symmetric illuminations in large image-shearing ESPI, two interferometers are formed. Carrier fringe patterns can be introduced by tilting reference surface a small angle. The carrier fringe patterns are demodulated after deformation of the object. Two phase maps, which include out-of-plane and in-plane displacement, can be obtained by using Fourier transform. Then out-of-plane displacement can be easily separated from in-plane displacement by simple operation between two unwrapped phase distributions. The principle of spatial carrier frequency modulation in large image-shearing ESPI is discussed. A typical three-point-bending experiment is completed. Experimental results are offered. The results show that the method offers high visibility of carrier fringes. And the system presented does not need a special beam as a reference light and has simple optical setup.     

The investigation of high frequency vibration modes of PZT-4 transducers using ESPI techniques with reference beam modulation

In this paper the experimental vibration amplitudes together with the vibration modes of the PZT-4 transducer radiating front face are presented for generation in various media. The studies were carried out using an electronic speckle pattern interferometer (ESPI) with reference beam modulation to give resolution down to 20 Å of surface amplitude. Analogies are drawn between these transducers and the resonant vibration modes of thin plates. The relationship between the surface vibration amplitudes and the radiation beam pattern of the transducers is also presented.     

Automatic interferogram analysis techniques applied to quasi-heterodyne holography and ESPI

This paper describes significant developments in methods for the automatic, quantitative analysis of interferograms. All areas of analysis have been considered: fringe field generation, pre-processing, and phase unwrapping.

A new quasi-heterodyne holographic technique is described in which the image is reconstructed using a single beam. The errors in the reconstructed fringe field are mainly linear in form, and an error compensation scheme is proposed. The final error in the phase measurement using automatic analysis is λ/40.

The process of image smoothing by an averaging filter is considered to reduce the effects of random noise. It is shown that by measuring the signal-to-noise ratio of the fringe field an optimum degree of smoothing may be applied. This is demonstrated on holographic and electronic speckle pattern interferometry (ESPI) data.

Two methods for cosinusoidal fringe image combination are compared, using three or four fields. It is shown that an automatic analysis can be achieved using four phase stepped images.

A new algorithm to automatically unwrap the phase of complex fringe patterns is described. The fringe field is segmented into small rectangular areas, called tiles. This allows local data to be obtained on fringe consistency and density. A confidence tree can then be formed to produce an optimal solution for the whole field. Results are presented and discussed for both holographic and ESPI data.     

在电子散斑干涉中利用反相位法进行三维变形测量

提出一种可将离面位移与面内位移分离的三维位移计算方法。在双光束电子散斑干涉系统中增加一路参考光,使这一路参考光为两光束所共用。两束光各自独立地对变形物体进行测量,分别计算相位分布,并对其中之一进行反相位计算。理论分析表明,对二路检测光所得到的相位进行相减运算,就能够较好地减少电子噪声的影响,分离面内位移场与离面位移,实现物体变形的三维测量。介绍该方法的原理,并利用典型实验证实了该方法的可行性。     

基于改进旋滤波的电子散斑干涉图滤波方法

电子散斑干涉术条纹图在成像时不可避免地受散斑噪声调制,去除噪声是散斑干涉条纹处理的一项重要任务。利用散斑条纹图的方向性,提出一种基于模糊方向的旋滤波:在当前点的领域内定义4个模糊方向窗口,将传统旋滤波的一维、精确方向窗口的确定,转变为模糊方向窗口的确定;在确定的窗口内进行低通滤波时,采用自适应加权均值滤波代替传统的中值滤波。利用该方法分别处理模拟散斑条纹图和实验所得的真实条纹图,并与传统旋滤波、双边滤波和小波丢弃子带方法比较。实验结果表明,该改进算法在滤除散斑条纹图噪声的同时,有效保护了条纹的细节信息。     

热传导方程在散斑条纹图相位提取中的应用

去除电子散斑十涉条纹图中的噪声是提取条纹图相位的关键问题.利用热传导方程去除条纹图中的噪声,定性和定量分析了该方法的滤波性能.在此基础上,进一步将MBO算法和热传导方程应用于条纹二值化和相位图平滑过程中,并成功地从单幅模拟条纹图中提取了条纹图的相位.研究结果表明,热传导方程能有效减少散斑条纹图中的噪声,改善二值条纹图的边界.进而获得准确的相位分布.     

Digital phase stepping speckle interferometry

A variation of electronic speckle pattern interferometry (ESPI) is presented whereby phase fringes are produced in contrast to speckle correlation fringes. Digital phase stepping speckle interferometry uses a phase measurement technique developed for holographic interferometry, which involves phase shifting of the reference beam in a conventional speckle interferometer. Saw-tooth phase fringes are produced by subtracting the phases of the speckle fields before and after object deformation in a digital frame store. This enables the sign of the deformation to be distinguished. After application of a special low-pass digital filter, the phase fringes are of adequate quality to be counted automatically.     

Noise reduction in electronic speckle pattern interferometry fringes by merging orthogonally polarised speckle fields

The noise in sawtooth fringes generated by electronic speckle pattern interferometry (ESPI) is investigated. When deformations of depolarising objects are studied, the scattered object light can be decomposed into two orthogonal linearly polarised speckle patterns which are partially decorrelated. Their correlation coefficient decreases with increasing depolarisation coefficient of the object. By suitable merging of the phase distributions of these two speckle fields on the basis of a modulation depth analysis, the rms phase error in the ESPI sawtooth fringes can be reduced significantly.     

Additive-subtractive phase-modulated speckle interferometry: Fringe visibility under partial decorrelation

Additive-subtractive phase modulated speckle interferometry (ASPMSI) is a technique that minimizes the susceptibility of speckle methods to environmental noise while providing fringes of good visibility. The method requires the acquisition of two consecutive video frames of additive-speckle images of the same two deformed states of an object at a rapid enough rate such that ambient noise is not a problem. The additive-speckle images as expected are of very poor visibility due to the presence of the self-interference term. An interframe phase-modulation is introduced and the two additive-speckle images are digitally subtracted to improve the fringe visibility by removing the self-interference term. The ASPM-SI method works with in-plane and out-of-plane deformation sensitive ESPI as well as with displacement-gradient sensitive speckle-shearing interferometry. It is shown that the ASPM-SI scheme has higher visibility than conventional additive-SI and performs consistently better than subtractive-SI schemes in the presence of partial interframe speckle decorrelating optical noise. Furthermore, it is shown that the fringe visibility of the out-of-plane displacement sensitive interferometer which uses a protected reference beam separate from the object beam can be made to be essentially unity even at complete interframe decorrelation.     

一种可实现电子散斑干涉的新型大错位方棱镜

设计了一种可实现电子散斑干涉的大错位方棱镜.将普通方棱镜的一个面磨去一个楔角后变为斜面,该斜面和相邻的一个面镀反射膜,其他二个面镀增透膜.垂直入射的光线经过分光后变成二束光,分别经过平面反射和斜面反射,出射的二束光线就分开了.将该大错位方棱镜置于CCD镜头前,一个物体可以成二个错位的像;相邻的二个物体可叠加成像.错位量由楔角决定,错位量足够大,可实现大错位电子散斑干涉.根据试件大小和成像距离,楔角的大小可选择在1°到10°之间.对中心加载周边固定圆盘进行了电子散斑载频干涉实验,证明了大错位方棱镜能够高质量地实现电子散斑干涉,实现位移场测量.     

基于图切割的相位展开

剪切散斑干涉术中,普遍采取相移法提取相位,只可得到被包裹的相位信息,进一步量化计算之前,必须展开相位,但是大量的散斑噪音使相位展开变得非常困难.本文提出基于图切割理论的相位展开算法,将相位展开等价于整数的最优估计问题,通过最小化能量函数展开相位,无需对散斑包裹相位图进行滤波,就可以从包含大量散斑噪音的包裹相位图中准确地提取出真实相位,极大程度地保留了相位包裹图中的细节信息.对于同一幅模拟的散斑包裹相位图,传统的质量导向枝切法和最小二乘法的均方根误差分别为11.7076和4.9775,新算法的均方根误差则为0.9459,数值模拟与实验结果均验证了新算法优良的抗噪性能.     

A novel interferometric method for contour mapping of optically rough surfaces

A new white light interferometric arrangement for contour mapping of light diffusing but not depolarising surfaces is presented and compared with related electronic speckle and holographic interferometry (ESPI and HI) methods. Using two angles of illumination of the tested surface, and modifying the light beam in the observation pupil it is possible to observe contour fringes with arbitrary sensitivity depending on the geometry of a chosen set-up.     

Speckle interferometry

Illumination of a rough surface by a coherent monochromatic wave creates a grainy structure in space termed a speckle pattern. It was considered a special kind of noise and was the bane of holographers. However, its information-carrying property was soon discovered and the phenomenon was used for metrological applications. The realization that a speckle pattern carried information led to a new measurement technique known as speckle interferometry (SI). Although the speckle phenomenon in itself is a consequence of interference among numerous randomly dephased waves, a reference wave is required in SI. Further, it employs an imaging geometry. Initially SI was performed mostly by using silver emulsions as the recording media. The double-exposure specklegram was filtered to extract the desired information. Since SI can be configured so as to be sensitive to the in-plane displacement component, the out-of-plane displacement component or their derivatives, the interferograms corresponding to these were extracted from the specklegram for further analysis. Since the speckle size can be controlled by the F number of the imaging lens, it was soon realized that SI could be performed with electronic detection, thereby increasing its accuracy and speed of measurement. Furthermore, a phase-shifting technique can also be incorporated. This technique came to be known as electronic speckle pattern interferometry (ESPI). It employed the same experimental configurations as SI. ESPI found many industrial applications as it supplements holographic interferometry. We present three examples covering diverse areas. In one application it has been used to measure residual stress in a blank recordable compact disk. In another application, microscopic ESPI has been used to study the influence of relative humidity on paint-coated figurines and also the effect of a conservation agent applied on top of this. The final application is to find the defects in pipes. These diverse applications demonstrate the power of this technique.     

Evaluation of elastic modulus of cantilever beam by TA-ESPI

The paper proposes an evaluation technique for the elastic modulus of a cantilever beam by vibration analysis based on time average electronic speckle pattern interferometry (TA-ESPI) and Euler-Bernoulli equation. General approaches for the measurement of elastic modulus of a thin film are the Nano indentation test, Buldge test, Micro-tensile test, and so on. They each have strength and weakness in the preparation of the test specimen and the analysis of experimental results. ESPI is a type of laser speckle interferometry technique offering non-contact, high-resolution and whole-field measurement. The technique is a common measurement method for vibration mode visualization and surface displacement. Whole-field vibration mode shape (surface displacement distribution) at resonance frequency can be visualized by ESPI. And the maximum surface displacement distribution from ESPI can be used to find the resonance frequency for each vibration mode shape. And the elastic modules of a test material can be easily estimated from the measured resonance frequency and Euler-Bernoulli equation. The TA-ESPI vibration analysis technique can be used to find the elastic modulus of a material requiring simple preparation process and analysis.     

基于图切割的相位展开

剪切散斑干涉术中,普遍采取相移法提取相位,只可得到被包裹的相位信息,进一步量化计算之前,必须展开相位,但是大量的散斑噪音使相位展开变得非常困难.本文提出基于图切割理论的相位展开算法,将相位展开等价于整数的最优估计问题,通过最小化能量函数展开相位,无需对散斑包裹相位图进行滤波,就可以从包含大量散斑噪音的包裹相位图中准确地提取出真实相位,极大程度地保留了相位包裹图中的细节信息.对于同一幅模拟的散斑包裹相位图,传统的质量导向枝切法和最小二乘法的均方根误差分别为11.707 6和4.977 5,新算法的均方根误差则为0.945 9,数值模拟与实验结果均验证了新算法优良的抗噪性能.     

Measurement of vibrational energy flow in a plate with high energy flow boundary crossing using electronic speckle pattern interferometry

The measurement of vibrational energy flow is an important tool in understanding the vibrational behaviour of structures. In the past, because of transducer constraints, the measurement of vibrational energy flow was mostly restricted to single point measurements. However, recent developments in advanced laser measurement techniques, such as electronic speckle pattern interferometry (ESPI), have gained interest in applying two-dimensional, multi-point measurement techniques to the estimation of vibrational energy flow. This paper addresses the measurement of vibrational energy flow in a plate by using an ESPI based vibrational energy flow measurement technique. A radially symmetric bending wave plate vibration model is introduced and theoretical expressions for energy-based quantities are derived. To assess the accuracy of the measurement method, these theoretical quantities are compared to synthetic results derived from the ESPI energy flow measurement technique. The ESPI measurement technique is also applied to an experimental ‘infinite’ plate. Thus, a specially designed experimental apparatus was constructed so as to minimise undesired wave reflections in the plate and, thus, achieve a high energy flow boundary crossing at the edges of the plate. To reduce the effect of optical noise contamination on the ESPI measured out-of-plane plate displacement data, optimal filters were applied prior to the vibrational energy flow computation. To appraise the accuracy of the experimental method, measured vibrational power on the plate is compared with measured vibrational input power. A difference of less than 1 dB between both quantities indicates that vibrational energy flow within a rectangular plate that contains radially symmetric wave propagation can be measured to a good degree of accuracy if appropriate filtering is applied.