Speckle reduction by selective spatial-domain mask in digital holography

An improved method of using a selective spatial-domain mask to reduce speckle noise in digital holography is proposed.The sub-holograms are obtained from the original hologram filtered by the binary masks including a shifting aperture for being reconstructed. Normally, the speckle patterns of these sub-reconstructed images are different. The speckle intensity of the final reconstructed image is suppressed by averaging the favorable sub-reconstructed images which are selected based on the most optimal pixel intensity sub-range in the sub-holograms. Compared with the conventional spatial-domain mask method, the proposed method not only reduces the speckle noise more effectively with fewer sub-reconstructed images,but also reduces the redundant information used in the reconstruction process.     

The identification of fringe positions in speckle patterns

This paper two methods of identifying fringe positions in speckle patterns. The first uses local directional averaging to track fringes, while the second uses standard image processing techniques to connect the patterns into a form where the fringes can be identified.     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

Full-field heterodyne interference microscope with spatially incoherent illumination

A heterodyne interference microscope arrangement for full-field imaging is described. The reference and object beams are formed with highly correlated, time-varying laser speckle patterns. The speckle illumination confers a confocal transfer function to the system, and by temporal averaging, the coherence noise that often degrades coherent full-field microscope images is suppressed. The microscope described is similar to a Linnik-type microscope and allows the use of high-numerical-aperture objective lenses, but the temporal coherence of the illumination permits the use of a low-power achromatic doublet in the reference arm. The use of a doublet simplifies alignment of the microscope and can reduce the cost. Preliminary results are presented that demonstrate full-field surface height precision of 1 nm rms.     

Quality assessment of speckle patterns for digital image correlation

Digital image correlation (DIC) is an optical–numerical full-field displacement measuring technique, which is nowadays widely used in the domain of experimental mechanics. The technique is based on a comparison between pictures taken during loading of an object. For an optimal use of the method, the object of interest has to be covered with painted speckles. In the present paper, a comparison is made between three different speckle patterns originated by the same reference speckle pattern. A method is presented for the determination of the speckle size distribution of the speckle patterns, using image morphology. The images of the speckle patterns are numerically deformed based on a finite element simulation. Subsequently, the displacements are measured with DIC-software and compared to the imposed ones. It is shown that the size of the speckles combined with the size of the used pixel subset clearly influences the accuracy of the measured displacements.     

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.     

激光片光三维传感中降低散斑影响的新方法

提出一种激光片光三维传感中降低散斑影响的新方法,片光面内扫描合成孔径法。通过向被测物体表面投射面内扫描的片状激光束,产生空间变化的动态散斑光场,这样的光场在成像透镜光瞳平面上的移动,其时间平均效果等效于利用了一个大的“合成孔径”,降低了散斑的影响,明显地提高了测量精度。文中给出了合成孔径的理论分析和三维面形测量的实验结果。     

Speckle noise reduction in digital holography by use of multiple polarization holograms

A novel speckle reduction technique for digital holography is proposed. Multiple off-axis holograms are recorded using a circularly polarized illumination beam and a rotating linearly polarized reference beam. The speckle noise in the reconstructed images is suppressed by averaging these fields. We demonstrate the effectiveness of this technique experimentally and conduct additional statistical evaluation.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Anomalous intensity of pinned speckles at high adaptive correction

Ground-based optical searches for faint stellar or planetary companions about other stars may be limited by speckle noise, which is the rapid intensity fluctuations that are due to motions of remnant atmospheric speckles. Adaptive optics (AO) can reduce residual wave-front phase errors to low values, substantially reducing the unwanted power in the speckle halo. At high correction, however, the noise in the halo will be dominated by anomalously bright "pinned" speckles that have a number of unusual properties. They can have negative intensities and will appear in spatially antisymmetric patterns; they are spatially pinned to Airy rings and have zero mean in a sufficiently long integration. Some of these properties may be used to reduce the unanticipated effect of pinned speckles on companion searches, depending on details of the AO system. But, in short exposures, pinned speckles dominate speckle noise over much of the inner halo for Strehl ratios S as low as 0.6 and over much of the outer halo too as Strehl and deformable-mirror actuator densities increase. I show that these anomalously bright pinned speckles are not included in the traditional expression for speckle power in an image, (1 - S), on which sensitivity estimates of future high-performance AO systems have been based.     

数字全息图再现像的像质改善

基于数字图像处理技术,提出了一套对数字全息图及其再现像进行处理的方法,改善了数字全息图再现像的像质.利用频谱匹配滤波的方法可以有效地滤除零级及孪生像干扰.同时,提出了通过均值滤波、中值滤波等方法降低激光散斑噪音的技术.理论和实验证明,综合而又合理地利用数字图像处理技术可以很大程度地改善数字全息图再现像的像质.     

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.     

Effect of the point spread function on the average contrast of image speckle patterns

The effect of the point spread function on the average contrast of image speckle patterns is studied by varying its form depending on the optical imaging system. This effect is actually evaluated on the basis of a computer simulation of various rough surfaces by investigating the average contrast variation of image speckle patterns as a function of the point spread of the optical system. It becomes clear that the form of the point spread function, i.e. the type of the optical imaging system producing image speckle patterns, does not largely affect the average contrast of image speckle patterns.     

Speckle averaging in deformation analysis using video speckle interferometry

We show that speckle averaging in deformation analysis using video speckle interferometry increases the signal to noise ratio and improves the resolution in the resulting image. Speckle averaging in deformation analysis normally demands a very high degree of stability and repeatibility. Averaging phase gradient maps reduces these demands considerably. Experimentally, deformation details down to 2 × 2 pixels in the averaged pictures have been resolved.     

Improvement of speckle noise suppression in digital holography by rotating linear polarization state

An improved polarization recording approach to reduce speckle noise in digital holography is proposed. Multiple off-axis holograms are obtained by rotating the linear polarization state of both illumination and reference wave simultaneously. By averaging the intensity fields, the speckle noise in the reconstructed images is well suppressed. Statistical evaluation of the experimental results shows the effectiveness and improvement of the proposed method.     

Compressed ghost imaging based on differential speckle patterns

We propose a compressed ghost imaging scheme based on differential speckle patterns,named CGI-DSP.In the scheme,a series of bucket detector signals are acquired when a series of random speckle patterns are employed to illuminate an unknown object.Then the differential speckle patterns(differential bucket detector signals) are obtained by taking the difference between present random speckle patterns(present bucket detector signals) and previous random speckle patterns(previous bucket detector signals).Finally,the image of object can be obtained directly by performing the compressed sensing algorithm on the differential speckle patterns and differential bucket detector signals.The experimental and simulated results reveal that CGI-DSP can improve the imaging quality and reduce the number of measurements comparing with the traditional compressed ghost imaging schemes because our scheme can remove the environmental illuminations efficiently.     

三温区溶质传输助熔剂法生长BaTiO_3晶体

ＢａＴｉＯ３晶体新的生长方法，即感应加热三温区溶质传输熔剂法。其要点使高温熔体分为三个温区，坩埚顶部为生长区（Ａ区），温度最低，坩埚底部为溶解区（Ｃ区），温度高于生长区，营养料置于此区，坩埚中部为高温区（Ｂ区），在此区将杂晶熔解。还阐述了此法的机理，调试创造了稳定适宜的三温区温场。用此法生长出了优质ＳＴｉＯ３大单晶，尺寸达３０×３０×１５ｍｍ，并初步观测了其畴结构形貌，实验测定了晶格常数和居里点。     

激光散斑图样的光流场分析研究

提出了一种基于动态图像处理的新的电子散斑计量方法－光流场分析法，从新的角度出发讨论了运用图像序列分析的概念秋解决散斑图样测量的可行性和简便性。并且解决了当散斑图像不满足进行光流场分析的约束条件时的测量问题，即通过多通道的图像处理方法来改变图像连续特性，从而扩大了测量范围。     

数字剪切散班干涉术中的刚体位移补偿的新方法

给出一个数字剪切散斑干涉术中的刚体位移补偿的新方法。当刚体位移大于一个像素时，加载前后的散斑图将会由于位置的变化导致失相关。重新安排每一个像素将会克服此失相关。散斑平均、条纹重构以及迭代方法用来改善条纹质量，消除散斑噪声，最后得到可取结果。     

Encryption and decryption using a sandwich phase diffuser made by using two speckle patterns and placed in the Fourier plane: Simulation results

In the present paper, we describe the encryption and decryption of two-dimensional images. The encryption is done by employing a sandwich phase diffuser made by using two elongated speckle patterns, and placed in the Fourier plane of a double random phase encoding system. After encryption, the two constituent phase diffusers of such a sandwich diffuser are separated. During decryption, if the conjugate of either of the two elongated phase speckle patterns is used, the image cannot be retrieved. Correct decryption is also not possible if a sandwich diffuser with any of the phase speckle patterns is shifted in position with respect to the other. For decryption, the encrypted image is Fourier transformed and multiplied by the conjugate of the sandwich diffuser, and then the product is further Fourier transformed. It is possible to generate the image only if both the elongated phase speckle patterns are matched point-to-point and then the proper conjugate is made. The use of elongated speckle patterns in constituting a sandwich phase diffuser makes the system less complicated as compared to the use of a sandwich diffuser made with normal speckle patterns, enabling an easy alignment of the random phase diffuser at the time of rejoining the constituent diffusers for making the right key. The ease of alignment is due to the reduction of the requirement of 360° scanning at the time of rejoining these diffusers with little reduction in the security of the system. Simulation results are presented in support of the proposed idea. For optical implementation, the decrypted image may be obtained by generating a phase conjugate wave by the phase conjugation technique, and passing through the same sandwich phase diffuser. To evaluate the reliability of the technique, mean square error (MSE) between the decrypted and original image has been calculated.