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.     

Automatic Analysis of the Phase Object Using Projection Moire Deflectometry

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Digital speckle pattern interferometry (DSPI) with increased sensitivity: Use of spatial phase shifting

This paper presents a real-time digital speckle pattern interferometry system with twofold increase in sensitivity for the measurement of in-plane displacement and first order derivative of out-of-plane displacement (slope). Spatial phase shifting technique has been used for quantitative fringe analysis. The system employs a double aperture arrangement in front of the imaging system that introduces spatial carrier fringes within the speckle for spatial phase shifting. For in-plane displacement measurement, the scattered fields from the object are collected independently along the direction of illumination beams, and combined at the image plane. For slope measurement, a shear is introduced between the two scattered fields. Experimental results on an edge clamped circular plate subjected to in-plane rotation for in-plane displacement measurement and central loading for slope measurement are presented.     

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.     

云纹干涉载频调制技术及分析

在物体变形场的测量中,云纹干涉的载频调制技术有重要作用.在云纹干涉中,对称光照明产生的衍射光沿试件栅表面的法线传播并产生干涉.通过改变照明光的入射角度,可实现变形条纹场的空间调制.通过光程差的分析说明了载波条纹产生的机理,得出了载波频率与照明光入射角度变化之间的关系式.利用三点弯曲加载结合二维载频调制实验,给出了实验结果.证明了在初始条纹较少的情况下,载频调制技术能有效地测量物体的位移场.     

A fringe carrier technique for unambiguous determination of fringe orders in shearography

Shearography is an interferometric method which measures displacement derivatives. This paper presents a technique which eliminates the ambiguity in shearographic fringe interpretation. The technique is based on generating a carrier fringe pattern. Introduction of the carrier fringes results in the formation of a fringe pattern which has monotonically increasing fringe orders, thus allowing fringe orders to be determined in a straightforward manner and without ambiguity. The phase change due to deformation alone is then obtained by simply subtracting the known undeformed carrier fringe orders from the deformed carrier fringe orders. Elimination of fringe ambiguity has paved a way for automatic data acquisition in shearography.     

基于优化电子散斑光谱的三维物体测量研究

用激光光束直接照射到测试表面,再用CCD采其变形前后表面散斑颗粒干涉形成的条纹,条纹图解析为测量点的位移量和变形量,进而得到其离面位移,在优化算法的时候采用π位相技术获取另一个π相移的变形条纹图像,将面内位移与离面位移分离,为了消除零级分量,让投影光栅移动1/2个周期.通过Matlab和四步位相算法给出了三维空间模型,得出变形后物体的离面位移数据.实验仿真数据表明其能够稳定地测量物体变形场三维分量,误差较低.     

Increasing the measurement sensitivity in comparing the microrelief of diffusely scattering surfaces

A method is developed for optical processing of photographs of projected fringes, which significantly increases the measurement sensitivity in comparing the macrorelief of diffusely scattering surfaces. The method is tested experimentally in studying the displacements of a thin aluminum plate due to plastic deformation of the surface. A pattern of lines of equal displacement is obtained for a diffusely scattering surface of the plate with an eightfold enhancement of the sensitivity, with the distance between neighboring contour lines reduced to 15 μm. Zh. Tekh. Fiz. 67, 67–72 (March 1997)     

Microscopic surface contouring by fringe projection method

This paper describes the use of optical fringe projection method for 3D surface profile and deformation measurement of micro-components. In this method, sinusoidal linear fringes are projected on a micro-component surface by a grating phase shifting projector and a long working distance microscope (LWDM). The image of the fringe pattern is captured by a high-resolution CCD camera and another LWDM and processed by phase-shifting technique. A simple procedure is described which enables calibration of the optical set-up for subsequent quantitative measurement of micro-components of unknown shapes. This method is relatively simple and accurate, and is capable of conducting fully automated measurements. In this paper, two micro-components, a micro-mirror (0.1 mm×0.1 mm) and a micro-electrode pad are used to demonstrate deformation measurement and microscopic surface contouring.     

Phase shifting nano-moiré method with scanning tunneling microscope

In this paper, a new nano-moiré method using scanning tunneling microscope (STM) is proposed. This method is capable of measuring nanoscopic deformation of matter. The formation mechanism of the STM moiré fringe and the phase shifting technique used in STM moiré fringes are explained in details. Typical experiments are conducted with the crystal lattices of freshly cleaved highly oriented pyrolytic graphite, are used as specimen grating, to generate STM moiré fringe patterns. Phase shifting is realized in four steps from 0 to 2π by controlling the PZT in the STM system to shift the specimen in the vertical direction. This method provides a new way for disposal of moiré fringes pattern in the nano-moiré measurement.     

载频调制大剪切电子散斑干涉系统

大剪切电子散斑干涉技术不需要引入参考光，具有条纹质量好等特点。提出将干涉场的载频调制技术引入到大剪切电子散斑干涉中，可形成具有载频调制功能的新的电子散斑干涉系统。该系统具有对测量环境的隔振振动要求低，能方便定量求解物体的变形场等优点。首先讨论大剪切载频的调制机理，然后利用中心加载、周边固定的圆盘进行典型实验，设计了可用计算机控制且可对参考物进行精确偏转的步进电机系统，进而实现了对电子散斑干涉场的自动控制调制。最后，利用傅里叶变换法对调制条纹进行解调，解调出变形场的相位，并通过相位与位移的转换计算，得到精确的物体变形场。实验结果证明，该系统能够调制电子散斑干涉场，求解物体的位移场。     

电子散斑干涉载频调制形貌测量方法的分析

根据电子散斑干涉载频调制测量物体形貌的基本原理,物体表面的微小偏转可引入包含物体高度信息的载波干涉条纹。将物体的偏转视为变形,对物体变形与载波干涉条纹之间的关系进行了讨论,得出了离面位移引入载波和面内位移引入含有物体高度信息相位的结论。发现利用典型电子散斑干涉系统测量物体形貌效果最好,并通过实验得到了验证。     

Digital speckle interferometry for assessment of surface roughness

In this work, the principle of interferometry is used to assess the surface roughness of the machined surfaces. Interferometry produces an interference fringe pattern when two or more light waves interact with each other. It is one of the important tool for precision optical metrology and testing. Well-known advantages of the phase shifting interferometry include high measurement accuracy, rapid measurement, good result even with low contrast fringes and that the polarity of the wave front can be determined. In fringe projection techniques, a known optical fringe pattern is projected onto the surface of interest. The fringe pattern on the surface is perturbed in accordance with the profile of the test surface, thereby enabling direct derivation of surface profile.In this work, an attempt has been made to assess the surface roughness using a speckle fringe analysis method of five frame phase shift algorithm for machined surface (ground surface). As these fringes are too noisy, advanced filtering technique has been used so as to reduce noise and to get improved wrapped phase map from the phase shifted fringes. A phase unwrapping software has been developed using discrete cosine transform (DCT) to generate the three-dimensional (3-D) profiles. Finally, it is compared with R_a values measured using a mechanical stylus instrument, showing good agreement.     

Young's fringes analysis with a computer image-processing system

A two-dimensional Fourier transform analysis is presented for numerically processing the Young's fringes diffraction pattern from a double- exposure speckle photograph. The fringe spacing and orientation are determined using only one Young's fringes pattern without any other diffraction halo patterns. This algorithm is based on the 2-D FFT of the fringe pattern with 256 × 256 pixels. Therefore, it enables automatic analysis to be performed on specklegram with noisy and poor fringe patterns. The measurement of deformation by the laser speckle method can be carried out with high accuracy, reliability and speed.     

Measurement of complex surface deformation by high-speed dynamic phase-stepped digital speckle pattern interferometry

We describe a high-speed digital speckle pattern interferometer incorporating a line-scan camera and a waveguide phase modulator for the measurement of complex deformation (vibration phase and amplitude) at audio acoustic frequencies. Experimental data show continuous phase-stepped recovery of out-of-plane surface deformation in one dimension, obtained at 100 kHz with 2pi/20-rad (0.02-mum) displacement resolution, for surface velocities of 3.2 mm s>(-1) .     

Nanoscale deformation measurement by using the hybrid method of gray-level and holographic interferometry

This study extends the use of holographic interferometry to measure the nanoscale out-of-plane displacement with high surface resolution. It is noted that if the deformation is less than half of the optical wavelength, it is hard to find an obvious fringe pattern. Under such a situation, in general, the phase shift method is used. However, it needs to take more than 3 images for phase shifting and phase reconstruction In this paper, a more simple hybrid method of gray-level and holographic interferometry is used to extract fringe skeletons, in which it just needs to take one or two images for the normal deformation measurement directly, even if there exists no obvious fringe pattern. The displacement field with high surface resolution can also be obtained. The proposed method yielded a theoretical precision of 0.15 nm for out-of-plane displacement with a monochromatic CCD camera of 10-bit gray scale (1024 gray scales) sensitivity and microscale surface resolution for millimeter scale object with 640×480 pixels image resolution by an He–Ne LASER (632.8 nm wavelength) light source. The gray-level method is proposed to calculate the non-obvious interferometry fringe by traditional holographic interferometry hologram, and the result showed that this method works for this purpose.     

单幅对称变形相位图分离二维变形分量的相移电子散斑干涉技术

提出了利用相移电子散斑干涉测量物体二维变形分量的方法.单光束照明的传统电子散斑干涉技术,测量得到的是一幅物体变形的混合相位场.当物体具有对称变形时,可由这一幅相位图求得二维变形分量.方法是将该相位图镜像翻转得到第二幅相位图,通过二幅相位图的叠加、复位和分离运算,获得物体的二维变形场的分量值.利用三点加载的简支梁进行了实验,给出了实验结果,并与对称光照明实验结果进行了对比,验证了该方法的正确性.     

Mode power distribution effect in white-light multimode fiber extrinsic Fabry-Perot interferometric sensor systems

Theoretical and experimental results have shown that mode power distribution (MPD) variations could significantly vary the phase of spectral fringes from multimode fiber extrinsic Fabry-Perot interferometric (MMF-EFPI) sensor systems, owing to the fact that different modes introduce different extra phase shifts resulting from the coupling of modes reflected at the second surface to the lead-in fiber end. This dependence of fringe pattern on MPD could cause measurement errors in signal demodulation methods of white-light MMF-EFPI sensors that implement the phase information of the fringes.     

散斑剪切干涉条纹特性的实验研究

本文通过位移位相存在的真实性实验、小孔滤波的必要性实验、位移梯度的解相关性实验,证实了散斑剪切于涉术的条纹是由物体变形所引起的“位移位相”和“位移微分位相”以及由位移和位移微分共同决定的“附加位相”之和的余弦条纹。实验证实这种余弦条纹的分布,随着滤波孔的位置而变化,它的衬比随着位移和位移梯度的增大而下降。特别是,当在象面上一点相遇的两个物点的象面散斑位移之差的模值大于散斑的直径时,两者相遇的象点上的条纹消失。     

Measurement of curvature distribution using digital speckle three-shearing aperture interferometry

Digital speckle three-shearing-aperture interferometry used for the measurement of curvature distribution fields of a deformation object is proposed for the first time. In this method, pure curvature distribution fringes without containing slope distribution fringes can be obtained by using digital speckle three-shearing-aperture interferometry. Two specklegrams, one before and the other after object deformation, are recorded by a CCD camera and stored in a computer. A 2-D fast Fourier transform (FFT) and a 2-D inverse FFT (IFFT) are performed on the two specklegrams. The subtraction of the transformed specklegrams will produce curvature distribution fringes, and the speckle noises in curvature distribution fringes can be removed after these fringes pass through low-pass filtering. Results from theory and experiments are in good agreement.