Phase-object analysis with a speckle interferometer

We describe the characteristics of a double-exposure specklegram obtained through a double-aperture system, by introduction of a wedge in front of one aperture in one exposure. It is assumed that a uniform displacement of the diffuser is produced between exposures. The average intensity distribution and visibility of the interferometric fringes in the Fourier plane are analyzed. An alternative interferometric technique for phase-object detection is proposed.     

旋转孔径散斑照相逐点分析法

本文提出了旋转孔径散斑照相的逐点分析法,其杨氏花纹一般来说不是等距离的直条纹,而是与位移的动态过程相联系的曲线条纹.本文解释了这种条纹的意义,给出了理论分析和实验证明.     

数值孔径与数字剪切散斑干涉条纹亮度的关系

分析了数字剪切散斑干涉条纹图的形成理论，并获得了条纹亮度与摄象机数值孔径等参数的精确关系式，从理论分析和实验验证均得出在采用小的摄象机数值孔径和３ｍＷ Ｈｅ－Ｎｅ激光器的情况下，仍可获得比较满意的数字剪切散斑干涉条纹图，理论工作及其结论对于进行数字剪切散斑干涉实验的最佳参数选择具有指导作用。     

Dynamic behaviour of speckle cluster formation

In this work, we analyse the speckle cluster structure generated when a coherently illuminated diffuser is imaged by introducing a multiple aperture pupil mask in front of the lens plane. We demonstrate that the speckle cluster originates from the complex speckle modulation generated by multiple interferences among the wavefront passing through each aperture. The auto-correlation function of the intensity distribution when using a multiple aperture pupil arrangement is calculated. Besides, we demonstrate that the autocorrelation function and the intensity corresponding to a single scattering element of the input are coincident. This result allows interpretation of the dynamics behaviour of the speckle cluster formation by considering the result obtained by a single scattering element. Then, we determine the pupil mask geometrical parameters that control the cluster behaviour and therefore the condition for obtaining a highly repetitive cluster structure that we define as a 'regular cluster'. The theoretical simulations based on the random walk model are in agreement with the experimental results supporting the validity of our approach.     

Measurements of axial displacements undergone by a diffusing object in speckle photography

The speckles of the image plane of an object are both radially shifted and decorrelated when the object is axially translated through an amount ?. We demonstrate that this radial shift, which is related to the position of the pupil of the optical system, disappears when the pupil lies in the back focal plane of the imaging lens. However, if the irradiance of the image plane is twice recorded on a photographic plate which is laterally shifted through ζ_H between the exposures, the minimum value of the contrast of the Fourier fringes exhibited by the plate after processing, gives the value of ? if ? is less than a particular value, ?_M which will be defined. Also we propose a new speckle measurement method in which the fringes are automatically removed when ? is greater than ?_M. We record the image of the object illuminated in convergent light through an amplitude diffuser placed in the Fourier plane of the object. The mean speckle speckle size of the diffuser is equal to the mean size of the speckles generated by the object in its Fourier plane.     

Longitudinal Correlation Properties of Speckles Produced by Ring-Slit Illumination

Spatial correlation properties are studied for speckle patterns produced in the diffraction region behind a lens which is located at a focal distance from a diffuser with an aperture under illumination of the coherent plane-wave light. It is shown theoretically that, if a ring-slit aperture of a negligible slit width is used, the speckle field produced has a perfect correlation in the longitudinal direction. An experiment was performed using a circular aperture and ring-slit apertures having different ratios of the inner to the outer radius and the speckle pattern produced using the ring-slit aperture with the smallest slit width had the longest longitudinal correlation in all cases examined. The statistical stationarity and symmetry of the speckle field, as well as variation in the speckle size, are also discussed.     

五孔剪切相机同时测量二维曲率,扭率和斜率

本文首次提出一种散斑剪切相机。该相机能把曲率、扭率和斜率的二维信息同时记录在一张双曝光散斑图上。对散斑图进行滤波分析时在频谱面上出现十三块互不重叠的衍射晕。在适当的衍射晕上滤波即可获得二维曲率、扭率和斜率的等值全场条纹。     

Cluster speckle structures through multiple apertures forming a closed curve

In this work, cluster-like speckle patterns are analyzed. These patterns are generated when a diffuser illuminated by coherent light is imaged by a lens having a pupil mask with multiple apertures forming a closed curve. We show that the cluster structure results from the complex modulation produced inside each speckle which is generated by multiple interferences of light through the apertures. In particular, when the apertures are uniformly distributed along a closed curve, the resulting image speckle cluster replicates the pupil aperture distribution. Experimental results and theoretical simulations show that cluster features depend on the apertures distribution and the size of the closed curves.     

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.     

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.     

频闪散斑照相法

本文讨论了频闪散斑照相法的基本原理,给出了简谐振动和非简谐周期振动散斑图的全场分析的平均光强分布和条纹可见度的解析式.理论和实验表明频闪散斑法较之时间平均散斑法有着明显的优点.     

研测动态问题的旋转孔径散斑照相法Ⅰ:双孔、四扇孔

本文提出一种研测动态问题的实验新方法——旋转孔径散斑照相法。该法设备简单,利用一个旋转孔径装置,能在一张散斑图上记录下物体动态变形的全过程,然后在全场波滤分析时把各瞬时的信息分离出来,可得任意时刻的瞬态位移场。转孔法能应用于振动和非周期性动态问题,并以三个实验证实其可行性。实验结果表明,散斑条纹清晰可靠,定量分析与其它方法所得结果一致。     

Measurement of dynamic crack tip displacement field by speckle photography and interferometry

Speckle interferometry has been used to measure the transient displacement field around a crack due to stress wave loading. Double exposure interferograms are recorded with a pulsed ruby laser, and correlation fringes formed by spatial filtering of the developed film. A new technique is presented for reducing the noise level in spatially filtered speckle interferograms. Independent fringe patterns, obtained from one interferogram but with the filtering aperture sampling different regions of the Fourier transform plane, are digitised by a CCD camera and combined numerically. It is demonstrated that accurate analysis of fringe patterns by the Fourier transform method can be carried out, even if no carrier fringes are present. The measurements are compared with those obtained by speckle photography. Least-squares fitting of the theoretical displacement field is used to calculate the stress intensity factor at the crack tip as a function of time.     

Code retrieval via undercover multiplexing

The purpose of this research is to develop an undercover multiplexing technique to give additional protection for optical information encryption. We employ the double random phase mask as our basic optical encryption system. The holographic storage medium of choice is a photorefractive crystal. To achieve the multiplexing we use the aperture size of the pupil in the optical system, as it governs the speckle size. We introduce such variation in order to produce a decorrelation between two consecutively stored speckle patterns. Each stored speckle pattern is associated to an input encrypted image, thus producing a multiplexing of the encrypted information. We implement this operation without altering the setup architecture and the random phase masks. This multiplexing is our undercover operation to encipher a true code behind a fake code. Under this approach, the user can only recover the bulk information stored in the volume hologram. However, he cannot recover the true code without the additional information on the pupil size key, even if accessed in position of the original decoding mask.     

A semi-automatic method for measuring interferometry fringes: prototype design and first results

A device for automatically obtaining the information contained in holographic interferometry fringes is proposed. The method may be applied to any kind of interferometric fringes. Accuracy has been demonstrated by contrasting the results obtained with our prototype system and those with the single-beam speckle interferometry technique for the measurement of displacements in a plane. The results show good correlation between the two series of measurements.     

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.     

Development of Matlab filtering techniques in digital speckle pattern interferometry

In interferometric fringe pattern analysis, specular and speckle fringe patterns are the two main divisions. While specular fringes are characterized by quality fringes, speckle (that obtains due to the diffuse scattering of the coherent radiation from an optically rough surface) fringe patterns are characterized by noisy fringes. This paper concentrates on this aspect and the Matlab based filtering methods to improve the quality of speckle fringe patterns by developing the appropriate software. Further, the newly developed software “Macurv” will be presented which can give the second order derivative (curvature) fringe information. A software with several functions is written using Matlab. The objective of the software is to provide a more effective way for the post-processing of speckle interferometric fringes. The algorithm and functions of the developed software “Macurv” will be explained.     

Measurement of tilt of a diffuse object by double-exposure speckle photography using speckle fanning in a photorefractive BaTiO3 crystal

A technique has been proposed for the measurement of tilt of a diffuse object using speckle fanning in a photorefractive BaTiO₃ crystal. Free space geometry has been used for recording in which the axis of rotation of the object lies away in three-dimensional space. The method is based on recording of two exposures, one before- and another after the tilt, thus producing speckle correlation fringes due to overlapping of two speckle fans. We consider a generalized case of tilt, generating curved fringes with center shifted in the transverse direction. The axis of rotation lies perpendicular to the optical axis in three-dimensional space having the transverse and axial components of the shift in the center of rotation with respect to the diffuser plane. Fourier transform of the curved fringes produces correlation output. Measure of separation between the correlation peaks in transverse and axial directions provides information about the tilt angle, the direction of tilt, and the distance of rotation axis from the optical axis. Experimental results have been presented to validate the theoretical predictions.     

带旋转毛玻璃干涉成像系统的统计分析

 从统计光学出发,建立了带旋转毛玻璃干涉成像系统的信噪比理论模型,分析旋转毛玻璃参数、光场相干性与系统信噪比之间的关系。模拟计算和实验结果表明：旋转毛玻璃降低了干涉成像系统中光场的相干性,增大了曝光时间内采集的不相关散斑场个数,从而抑制了相干噪声;旋转毛玻璃表面粗糙度的增大,相关长度的缩短,转速的提高都会减小光场的相干时间。为了得到较理想的系统信噪比,应控制旋转毛玻璃参数使旋转周期约等于CCD曝光时间,并使光场相干长度为测试光和参考光间光程差的2～5倍。     

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.