Adaptation of video moiré techniques to undersea mapping and surface shape determination

Fixed and variable resolution video moiré techniques have been used to project structured illumination in a model undersea environment and a prototype system has been developed which generates equal depth contours of undersea objects and has applications in sizing, orientation and ranging. An advantage of this system is that the entire field is continously illuminated, and the moiré contours and images are formed at video rates. The spatial frequency of the structured illumination can be continously varied, providing optimal contours for a variety of object sizes. The data can be easily interpreted by eye or processed by computer to obtain surface shape, range and orientation of a known structure.     

透镜焦距及球面曲率的测量

概述目前透镜焦距及球面曲率半径的测量方法，介绍透镜焦距及球面曲率半径的准直光束补偿测量法的原理，指出该方法的关键是对补偿光束准直性的检测，提出一种用准直光束补偿和横向错位进行干涉测量透镜焦距及球面曲率的新方法。     

Binarization of scanning Moiré fringe pattern

This paper describes a digital picture processing method to obtain a binary scanning moiré pattern. In this method, a bias component due to the illumination light distribution is eliminated from the scanning moiré fringe pattern to detect the zero-crossing points of the moiré profile. The binary fringe pattern is then obtained from the zero-crossing points. Experiments indicate that the method is useful for facilitating three-dimensional automatic measurement using moiré topography.     

Applications of Moiré topography measurement methods in industry

Moiré topography is able to determine the shape of an object, by pattern measuring, in a short time. Recently moiré topography has been used in various industrial fields because it has superior features that point measurement doesn't have. As it becomes popular various related techniques that fit specific needs have been developed and there is the prospect that moiré topography will become a more important measurement method and strengthen its position in the near future.

This paper explains the grid irradiation method and the grid projection method as the basis of the moiré topography measurement method and describes the existing status and prospects of utilizing moiré topography measurement in the applied fields of shape measurement, flatness measurement and detection of abnormality.     

An electro-optic holographic technique for the measurement of the components of the strain tensor on three-dimensional object surfaces

An electro-optic holographic technique (also called electronic speckle pattern interferometry or TV holography) for the measurement of displacements and strains on three-dimensional object surfaces is presented. The three components of the displacement vector, in a fixed coordinate system, are separated using four independent illumination beams. The interferometric phase for each illumination beam is extracted using the phase stepping technique. Displacements in the fixed coordinate system are projected on to the object surface using the tensor transformation law. Equations for the computation of surface strains using the projected displacements and the surface geometry are developed. The technique is applied for the measurement of strains on the surface of a cylindrical vessel subjected to internal pressure.     

Two wavelength simultaneous DSPI and DSP for 3D displacement field measurements

A novel optical system is proposed that implements digital speckle pattern interferometry (DSPI) and digital speckle photography (DSP) simultaneously using two wavelength illumination of an object for simultaneous measurement of all three components of the displacement vector field. A collimated red light illuminates both the object and a reference surface in a DSPI configuration which is sensitive to out-of-plane displacement field while a blue light illuminates the object in a DSP configuration which is sensitive to in-plane displacement fields. A color 3-CCD camera records the red and blue lights individually through its red and blue channels, respectively. Two reference images and one image in the deformed state of the object are required for quantitative measurement. Experimental results are presented to validate the system.     

General model to predict and correct errors in phase map interpretation and measurement for out-of-plane ESPI interferometers

Out-of-plane configurations used in electronic speckle pattern interferometry (ESPI), introduce phase error interpretation in extended target objects due to spatial variation of the sensitivity vector. In this paper, a general model to predict and correct the displacement measurement error and error phase map interpretation taking into account the target shape, illumination geometry, and in-plane displacement information, is presented. This model generalizes and extends previous analyses with respect to the sensitivity vector variations. The model is based on a relationship between variable sensitivity vector and the constant one. The importance of illumination geometry and in-plane displacement as factors that introduce the biggest error in the interpretation of the phase map are stressed. An analysis with practical parameters using divergent and collimated beams is presented. In order to predict and correct the error of phase interpretation, the finite element method should be used to obtain in-plane proportional factors to build up the correction function. The analysis shows that the error can be large for common configurations even when using collimated beam. To compute the measurement directly from the phase map, a general sensitivity function taking into account the shape and in-plane displacement information is naturally obtained from the presented analysis.     

On the determination of elastic residual strain fields in rail wheels using moiré interferometry

Moiré interferometry was used to determine the residual strain fields in a rail wheel. Hoop, radial, and shear displacement fields were determined in a whole field manner using a novel four-beam moiré interferometer. Diffraction gratings of 150 lines/mm were employed on the specimen surface. A system of data interpolation utilizing a simple control sample scheme was developed for field environments where sample loading occurs away from the optical device. Agreement was found between the moiré technique and physical measurements of wheel contraction during sectioning. Marginal agreement was found with strain gage measurements.     

电子散斑干涉位移场分离技术及其在三维测量中的应用

提供了一种可将离面位移与面内位移分离的三维位移场测量方法.在双光束电子散斑干涉中增加参考光,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,结合相移技术,可分别得到包含离面和面内位移信息的二幅位相图.只需简单的位相运算就能够将面内位移场与离面位移场分离.在竖直方向上,采用两光束对称照明被测物,直接测量面内位移竖直方向分量.本文将该技术应用到柴油机油泵测量上,得到了油泵的三维位移场.该技术为三维位移场的测量提供了一种新的有效途径,也为柴油机零部件的强度和刚度分析评价提供了新的方法.     

Stereophotogrammetric 3D shape measurement by holographic methods using structured speckle illumination combined with interferometry

We present a unique combination of the numerical three-dimensional (3D) reconstruction of the shape of an object with interferometric deformation measurements. Two cameras record several holograms of an object that is illuminated by structured illumination. This illumination is realized by speckle patterns. To improve the image quality, an inplace speckle reduction technique is combined with the structured illumination to reduce the effect of disturbing subjective speckles which appear in the reconstructed images. Stereophotogrammetric methods are applied to extract the 3D surface information of the object out of the reconstructed images. Since the recording is done by holography and because stereophotogrammetry enables a pointwise correlation between the two views, it is possible to combine other holographic techniques with the reconstructed 3D shape. This is demonstrated by an interferometric deformation measurement of an object cooling down. The resulting interferometric fringes are mapped onto the reconstructed 3D surface. Hence, the proposed method enables automatic and dense matching of interferometric fringe-maps recorded by spatially separated holograms onto the surface of the object, which has not yet been realized by existing techniques.     

Nonparaxial method for computing the gradient field of a wavefront using moiré deflectometry

A nonparaxial method in moiré deflectometry to obtain the gradient field of a wavefront under test is proposed. This method uses only two deflectograms without any information about the phase object. As a result, director cosines of the gradient field are computed from moiré fringe deviations on both deflectograms without ambiguities. The method is proved by using a synthetic wavefront simulation.     

Micromechanical thermal deformation analysis of unidirectional boron/aluminum metal-matrix composite

An experiment was conducted to document thermal deformation of a unidirectionally reinforced boron/aluminum metal-matrix composite. The specimen was subject to an isothermal loading of ΔT = − 100 °C and the deformation on a free surface perpendicular to the fibers was measured by microscopic moiré interferometry. The high sensitivity and spatial resolution made it possible to elucidate the micromechanical behavior of the specimen at the fiber level. Multiplied moiré fringe patterns with a contour interval of 35 nm/fringe contour were obtained for a hexagonal array of fibers and a square array of fibers. The stress-induced strains were determined from the fringe patterns by subtracting the free thermal contraction strains. The results revealed a free surface effect and showed asymmetrical and irregular strain distributions inherent in real (non-idealized) composite materials.     

Optical heterodyne measurement of vibration phase

An optical heterodyne system for measuring a vibration phase, which uses a frequency-shifted and phase-modulated reference beam, is proposed. This system is applicable to a rough surface. By using the light scattered by a standard vibrating point on the object surface as the reference beam (local reference beam technique), effects of the uniform displacement of the object can be eliminated, i.e. only the vibration component is detected with no sensitivity to the uniform displacement. Preliminary experiments demonstrate the validity of the theory.     

Moiré methods in interferometry

The applications of moiré methods to optical interferometry are reviewed. They are used for instrumental error subtraction, reduction of spatial frequency of interferogram fringes, increase of sensitivity and the range of applications of interferometric methods, and various information processing operations. The cases of the reference beam and shear type interferometry are treated separately.     

Three-dimensional profilometry by symmetrical fringes projection technique

A precise interferometric method for three-dimensional surface measurements in a wide dynamic range is presented. The method is based on symmetrical two-spacing projection phase-stepping interferometry. Two approaches are proposed—one with double symmetrical illumination and the other with double symmetrical observation—which reduce influence of shadowing at investigation of complex surfaces and allow absolute coordinates estimation. The theoretical background, experimental check as well as accuracy and sensitivity assessment of the method are discussed. The proposed technique is especially useful for remote, non-destructive in-situ measurements of real objects—for example historical objects and monuments.     

Developments and applications of conjugate-wave holographic interferometry

Conjugate-wave holographic interferometry is an optical technique which was recently proposed by one of the authors for the measurement, in real time, of in-plane deformation.Basically, the technique consists of projecting, from two symmetrical directions, two real images of an object upon the object itself. If the object deforms, interference fringes are observed on its surface. These fringes give a representation of one in-plane component of the point displacement vector and are independent of the out of plane component.On this paper some aspects of the optical principles underlying the technique are analyzed in deeper detail, and more advanced solutions for their practical implementation are proposed. In addition, a few examples of applications on-plane and not-plane specimens are presented.     

Laser speckle decorrelation in NDT

The random noise of the laser speckle field which develops at the focusing plane of an imaging system, is, by now, efficiently used in several interferometric techniques as an information carrier of the macroscopic wavefront distortion induced by the surface displacement field of the object under investigation. The actual noise in this kind of techniques is represented by the speckle decorrelation at the image plane — i.e. the destruction of the carrier — which may be caused by the modification of the texture surface (e.g. by yielding under a severe stress state), but it is inherently produced by the same displacement field under measurement. In the paper the phenomenon of laser speckle decorrelation is numerically simulated and experimentally investigated with the aim of estimating its sensitivity to local deformation and assessing a possible field of application. Satisfactory results in the field of NDT of multilayer fiber-reinforced composites were obtained by reducing the diaphragm of the lens to increase the sensitivity of the imaging system to speckle decorrelation induced by local deformation; unfortunately this simple approach requires a considerable amount of laser power for illuminating the object. Different aperture shapes were therefore numerically simulated which provided improved efficiency and sensitivity and whereby a semi-quantitative analysis of the displacement field could be experimented.     

Position Sensing Grating Interferometer for a Specular Object

A position sensing interferometer is described which is able to sense the position of a specular object from a normal distance. The interferometer consists basically of a diffraction grating, a focusing lens and two plane mirrors. The grating acts as a tool to divide the incident beam and also to combine the reflected beams from an object so that they interfere with each other. An experiment was carried out to verify the principle. Interference fringe patterns caused by variation in the object position were obtained. The sensitivity obtained theoretically was confirmed by measuring the width of the fringes. As a result of theoretical comparison with conventional techniques, it was determined that this interferometric method has the advantage of the high sensitivity and the wide dynamic range being compatible with each other. The setting for sensing is simple and the analysis of the fringe patterns is easy.     

Measurement of nonlinear refraction of dyes doped liquid crystal using moiré deflectometry

Recently the nonlinear effects of the azo-dye doped liquid crystals have attracted much interest. In this paper the nonlinear refractive indices, n₂, of two dyes (Sudan Black B, Sudan III) doped nematic liquid crystal were measured at low laser powers using moiré deflectometry technique. The results show when a nonlinear sample is placed in moiré deflectometry setup, the moiré fringe patterns will rotate around the beam center because of self-focusing effect in the sample. By measuring this rotation, the magnitude and the sign of nonlinear refractive index were calculated. The n₂ values for Sudan black B is larger than Sudan III, because of some differences in molecular structure and molecular polarization.     

具有位移和绝对距离测量能力的回馈干涉系统

提出一种基于半导体激光器泵浦Nd∶YAG微片激光器的回馈干涉系统，适合于非配合目标的位移和绝对距离测量。该测量系统包括移频回馈干涉光学系统，基于泵浦调制的微片激光器调频系统和相敏检波信号处理系统3部分。介绍了移频回馈干涉的原理，给出了位移测量和绝对距离测量应用的原理和测量方法，分析了测量的分辨率。实际测量了发黑工件表面的位移和绝对距离。实验结果表明，该回馈干涉系统灵敏度高，对被测表面反射率的要求很低，兼有对非配合目标的高分辨率位移测量和绝对距离测量的能力。