Digital holography particle image velocimetry for the measurement of 3Dt-3c flows

In this paper a digital in-line holographic recording and reconstruction system was set up and used in the particle image velocimetry for the 3Dt-3c (the three-component (3c), velocity vector field measurements in a three-dimensional (3D), space field with time history (t)) flow measurements that made up of the new full-flow field experimental technique—digital holographic particle image velocimetry (DHPIV). The traditional holographic film was replaced by a CCD chip that records instantaneously the interference fringes directly without the darkroom processing, and the virtual image slices in different positions were reconstructed by computation using Fresnel–Kirchhoff integral method from the digital holographic image. Also a complex field signal filter (analyzing image calculated by its intensity and phase from real and image parts in fast fourier transform (FFT)) was applied in image reconstruction to achieve the thin focus depth of image field that has a strong effect with the vertical velocity component resolution. Using the frame-straddle CCD device techniques, the 3c velocity vector was computed by 3D cross-correlation through space interrogation block matching through the reconstructed image slices with the digital complex field signal filter. Then the 3D-3c-velocity field (about 20 000 vectors), 3D-streamline and 3D-vorticiry fields, and the time evolution movies (30 field/s) for the 3Dt-3c flows were displayed by the experimental measurement using this DHPIV method and techniques.     

Recording of PIV images on photothermoplastic plates

In this paper we explore the possibility of utilizing commercially available photothermoplastic plates as recording support in Particle Image Velocimetry (PIV), to allow real time analysis of recorded data. An experimental apparatus has been developed which allows holographic recording and reconstruction of simulated PIV images. We show that double exposed photothermoplastic plates can successfully be analysed provided that a suitable recording/reconstruction scheme is used.     

二维及三维流场的光学测量方法

对于复杂的非定常流动 ,流场的测量往往要求无干扰、非接触 ,并且能够瞬时记录流场的二维甚至三维信息。对近年来流场测量领域发展快速、应用广泛的几种光学测量方法 ,如 PIV技术及其由此发展而来的 DPIV和 HPV技术 ,做一些介绍和比较。     

Reconstruction resolution enhancement of EPISM based holographic stereogram with hogel spatial multiplexing

We investigate how the splicing mode of a holographic element (hogel) affects the reconstruction of a 3D scene to improve the reconstruction resolution of a holographic stereogram fabricated using the effective perspective image segmentation and mosaicking method (EPISM). First, the effect of hogel spatial multiplexing on holographic recording and reconstruction is studied based on the mechanism of recording interference fringes in the holographic recording medium. Second, combined with the influence of multiple exposures on the hologram's diffraction efficiency, the diffraction efficiency of the holographic stereogram is analyzed in the spatial multiplexing mode. The holographic stereogram is then regarded as a special optical imaging system. The theory of spatial bandwidth product is adopted to describe the comprehensive resolution of the holographic stereogram, which explains why hogel spatial multiplexing can significantly improve the reconstruction resolution of a holographic stereogram. Compared with the traditional printing method under the same parameters in optical experiments, hogel spatial multiplexing has a lower diffraction efficiency but a higher quality of reconstructed image, consistent with the theoretical analysis.     

Correct-image reconstruction in the presence of severe anamorphism by means of digital holography

It is demonstrated for the first time to the authors' knowledge that digital holographic image reconstruction can be applied to correction of image reconstruction in the presence of severe anamorphism and aberrations. It is shown that anamorphism and aberrations can be taken into account and compensated for separately to yield a correct-image reconstruction. Anamorphism and aberrations are introduced by a holographic recording system based on a reflective grating interferometer (RGI). The strong anamorphic behavior of the RGI prevents reconstruction of a correct image unless that behavior is compensated for in the process of numerical holographic reconstruction.     

基于解码技术的全息显微术研究

提出了一种利用解码技术对全息显微畸变图像进行校正的方法。把全息记录中的显微镜预放大过程作为物光波产生畸变的编码过程;在波前再现过程中,用全息记录中的预放大显微镜对再现的赝实像再次放大进行解码,从而获得清晰的全息显微图像。实验验证了该方法的有效性。     

用阈值函数分析粒子场同轴全息非线性记录对再现像的影响

用阈值函数分析了粒子场同轴全息非线性记录对再现像的影响,从理论上得到了非线性记录影响粒子场同轴全息再现像的模型,并数值模拟了不同形状粒子在非线性记录下的再现像,得到再现像出现了边缘剪切效应.实验验证了数值模拟的结果,完善了粒子场同轴全息理论,并为粒子场同轴全息图像处理提供了光学产生边缘剪切效果,节约了图像处理的时间.     

An image-shifting technique based on grey-scale classification for particle image velocimetry

The image-shifting techniques are used to overcome the directional ambiguity of particle image displacement in the measurement of particle image velocimetry (PIV). This paper proposes an image-shifting technique based on grey-scale classification for PIV. By calculating the unified grey-scale statistical frequency of each interrogated unit, the directional ambiguity is resolved without any special requirement of the camera, and the particle image displacement is calculated synchronously. This image-shifting technique can be realized by controlling the difference in the light intensity of two lasers. Using this new technique, a PIV system was developed and used to measure the diesel spray flow. The displacement vector map of fuel particle in the spray flow was obtained, and the structure of the spray flow was investigated. The application confirmed that the image-shifting technique is viable and effective.     

Particle digital in-line holography with spherical wave recording

In this paper, we propose a method of digital in-line holography of particle. A diverging spherical beam is used for illumination in recording hologram, the complex amplitude distribution generated by particle field at a single plane located in the Fresnel diffraction region is recorded by CCD, and a plane beam for reconstructing hologram, then, the magnified image can be obtained by numerical reconstruction in computer. This procedure can be interpreted by Fourier optical theory and the theoretical analysis have been done in detail, the experimental results, the air freshener being subject, are also given.     

Digital color holographic recording and reconstruction using synthetic aperture and multiple reference waves

This paper presents a method for recording and reconstructing high-quality digital color holographic images. The synthetic aperture technique is used to improve the resolution of the reconstructed image and also reduce the speckle size in the reconstruction plane. Subsequently, the multiple reference wave technique is used to suppress speckle noise in the reconstructed color holographic image by superposing speckle fields with different distributions. The final high-quality color holographic image is obtained due to the resolution improvement and speckle noise reduction achieved by the two techniques. Our method has potential applications in the digital color holographic displays.     

数字显微全息中记录参数对颗粒测量影响的数值模拟

数字显微全息技术由于具有三维、非接触和实时测量微小空间内流场的能力, 已引起了国内外学者的广泛关注. 利用数字显微全息方法测量微通道流场时, 记录距离、颗粒尺寸、颗粒浓度、入射光波长、CCD分辨率等参数会对颗粒重建结果产生重要影响. 为了评估颗粒浓度和样本空间深度对重建结果的影响, 本文开展了数值模拟研究. 采用基于洛伦兹-米散射理论的程序产生不同浓度的颗粒全息图, 用小波变换重建算法对其进行重建. 结果表明: 在样本空间深度为24 μm 时, 颗粒浓度n_s在3.44×10⁵ mm^-3–13.77×10⁵ mm^-3 范围内时, 颗粒重建率E_p随着颗粒浓度n_s 的增大而迅速减小, 在13.77×10⁵ mm^-3–55.08×10⁵ mm^-3范围内时, 颗粒重建率E_p 随颗粒浓度n_s增大而缓慢减少. 在颗粒浓度n_s (13.77×10⁵ mm^-3) 保持不变时, 颗粒重建率E_p与样本空间深度满足单调递减的线性关系. 当阴影密度不变时, 重建率的变化呈现一定的规律性:当深度L较小时, 样本空间深度对颗粒重建的影响要比颗粒浓度的影响大; 当深度L较大时, 颗粒浓度对颗粒重建的影响较大. 关键词： 数字显微全息 颗粒浓度 粒径误差 位置误差     

Holography, speckle, and computers

Surveys are presented on automatic and quantitative deformation measurements using CCDs and computers in holographic interferometry and speckle metrology for diffusely reflecting surfaces. For delivering relationships between object deformation and observed quantities, we discuss formations of fringes and signals observed in these methods in terms of correlation functions of spatially randomly varying complex amplitude of light. Dependencies of the observed patterns on object deformation and optical systems are discussed. Physical meanings of the derived relationships are explained in terms of the dynamic behaviors of speckles resulting from surface deformation. Automatic measurements are described in chronological orders. They include analysis of fringe patterns resulting from photographic recording of specklegrams, video recording and analysis of speckle patterns used in electronic speckle pattern interferometry as well as direct digital correlation techniques, and digital holography that uses both digital recording and reconstruction of holograms.     

Three-dimensional solid particle positions in a flow via multiangle off-axis digital holography

We present a new development of digital off-axis (OA) holography for determining the instantaneous solid particle positions in a flow. This holographic imaging method uses a CCD camera for the simultaneous digital recording of two views of digital Fresnel OA holograms on the same support. The reconstruction is obtained numerically. The method provides two orthogonal views of the same flow area of interest at the same instant. It helps to overcome the depth of focus problem existing for the particle image reconstructions and that is inherent to the method. This method has the advantage of being simpler than the methods presently available, and it does not suffer from the flaws of in-line holographic configuration. Furthermore it is completely digital and thus avoids the cumbersome analysis following hologram recording. Digital holograms and digital reconstructions are obtained for solid particles of 200 mum moving into a stirred flow cell of 5 cm(3).     

High efficient generation of holographic stereograms based on wavefront recording plane

A computer generated holographic stereogram based on the wavefront recording plane(WRP) is presented. A WRP closed to the parallax image plane is introduced to record the complex amplitude in a small region for each point in the parallax image. By using three times of fast Fourier transform(FFT) to execute the Fresnel diffraction calculation between the WRP and the holographic stereogram plane, the object wave contributing to the hologram pattern can be achieved. The computation complexity of the proposed approach is dramatically reduced. The results show that the calculation time can be decreased by more than one order of magnitude.     

Improvement of holographic recording property of a Ce: KNSBN crystal by the moving grating technique

The moving grating technique is applied to improve the holographic recording property of the Ce: KNSBN crystal. In the case of extraordinarily polarized recording, the diffraction efficiency at large fringe modulations is enhanced by a factor of up to 35% at the optimum fringe velocity and the fringe modulation dynamic range is improved from m = 0.2 to m = 0.6. In the case of ordinarily polarized recording, a linear holographic reconstruction in the range of m ≤ 0.8 can be achieved by the moving grating at a fringe velocity of 9.6 nm/s. These results are significant in holography where a grey-level object needs to be recorded and reconstructed with high fidelity.     

含铝推进剂燃烧场全息粒子图象处理系统的研究

本文主要围绕微粒场全息术的记录,再现和图象分析这三个过程,对含铝复合推进剂燃烧场的粒子分布进行了深入研究.根据全息图象再现场的三维特点,我们采用4F光学全息记录系统并结合层析分幅和特殊的图象处理技术,建立了一套独特的粒子场采集和自动图象分析体系.在实践中取得了较满意的实验结果.     

粒子场数字全息诊断中的再现算法研究

 粒子场的数字全息诊断中,良好的再现算法能够在较短的时间内给出高质量的再现像。利用标准粒子板模拟单层面的粒子场,使用大面阵CCD实现同轴数字全息记录,得到了大尺寸的数字全息图。针对4种数字全息再现算法,本文从再现图像的质量、再现全息图的大小和计算速度3个主要方面进行了比较研究,结果表明角谱算法(FFT-AS)具有再现图像背景均匀,再现结果中无物理图像压缩,可以再现大尺寸的全息图且具有较快计算速度的优点,适合于粒子场同轴数字全息图的再现计算。     

全息再现的分析

从全息记录与再现技术的特点出发,对平面全息中的原始像、镜像、共轭像和共轭像的镜像的再现进行了分析,讨论了每个像再现的方法.     

A novel three-dimensional holographic display system based on LC-R2500 spatial light modulator

A novel holographic display system is proposed in this paper. The system takes LC-R2500, a kind of reflective liquid crystal spatial light modulator, (LC-SLM) as the core display unit, which can meet the requirement of real-time reconstruction of three-dimensional (3D) objects from holograms in free space. The relationship between hologram recording and image reconstruction is discussed, and the parameters associated with the magnification of reconstructed image over original object are determined. Experimental results of holographic display using the system are also given in the end.     

Synthèse d'impulsions subpicosecondes et visualisation holographique

Subpicosecond pulses have been synthesized with a dye laser. The 8000 GHz broadband source corresponds to a temporal resolution of Δt = 1/Δv = 0.12 ps. The holographic recording set-up proposed by N. Abramson has been used to visualize the light in flight, but the wavelength dependence of the hologram introduces a dispersion in the reconstructed image. Compensation techniques have been applied and an other recording system has been developed. Experimental results of the reconstructed holographic wavefront are presented. The interesting possibility to obtain a reconstruction in white light is compared with the coherent case. The signal-to-noise ratio of the image is discussed. Metrological applications have been outlined.