同轴数字全息用于粒径测量的影响因素分析

采用同轴数字全息技术测量研究粒子场,其高分辨力的再现像和粒径高精度测量受到诸多因素的影响.本文简要介绍了同轴数字全息的基本概念、原理、方法及其在粒子直径测量中的应用,主要通过计算机仿真研究了记录距离和粒子浓度这两个因素对粒径测量精度的影响,给出了记录距离的范围,同时提出了基于灰度梯度提取粒子边缘进行粒径测量的方法,并通过实验进行了验证.     

单次相移数字全息中相移值的提取

为了消除相移误差对数字全息中再现像的像质的影响,本文时单次相移数字全息进行了研究.基于相位统计特性,提出了一种有效消除相移误差的新方法,该方法能够对任意未知相移量进行提取,并利用数字全息图所有抽样点的强度偏差之和作为评价标准,通过逐步改变计算得到的初始相移值来寻找正确的实际相移角.计算机模拟得到了很好的再现结果,证明了该方法的可行性和有效性.     

基于数字全息和离散余弦变换的数字水印技术

提出了一种基于数字菲涅耳全息和离散余弦变换的数字水印技术.利用全息技术得到原始水印图像的菲涅耳全息图,之后将其嵌入宿主图像离散余弦变换域的中频系数,将水印添加位置和光学系统的几何结构参数作为恢复水印信息的密钥,并且算法属"盲数字水印技术",从而该算法具有较好的安全性和实用性.对算法的测试结果也表明,该算法对噪声,滤波、剪切,旋转、有损压缩等常见的图像处理操作具有较高的稳健性.因此,该算法具有一定的实用价值.     

同轴数字全息中粒子轴向的精确定位

为了减小由于同轴数字全息系统的较大焦深而导致粒子的轴向深度定位具有的不确定性,本文提出了一种用于粒子轴向精确定位的基于最大梯度的自动聚焦算法.自动聚焦算法对包含粒子的对焦窗口中求出任意可能方向的梯度,比较后得到最大的梯度,利用最大的梯度作为最后的结果,其它的则丢弃.同时,考虑到自动聚焦算法的精确度和算法的稳定性,再引入阈值参数,利用这个阈值可以很好地滤除粒子再现像中的背号信息和噪声带来的干扰.经实验表明,该算法具有较好的单峰性和稳定性.     

Identification of Specimen Position and Orientation Using Standard Deviation of Intensity in Phase‐Shifting Digital Holography1

Abstract: Phase‐shifting digital holography is a useful method to measure the displacement distribution and the strain distribution of an object surface. The complex amplitude distribution of an object surface is obtained as the complex amplitude distribution at a reconstruction distance. It is, however, difficult to measure the reconstruction distance by actual measurement. We discovered that the standard deviation of the intensity on the reconstructed image becomes the maximum value when the reconstruction distance is the same as the actual optical path length. The displacement distributions are obtained for the x‐, y‐ and z‐directions. When the normal direction of an object surface inclines from the z‐direction, the displacements defined on the xyz‐coordinate system should be transformed into the object coordinate system. It is, therefore, required to develop a measurement method of the orientation of the object to obtain the parameters for transforming from the xyz‐coordinate system into the object coordinate system. In this paper, the method to identify the position and the orientation of a specimen using the standard deviation of the intensity distribution is proposed.     

一种去除离焦粒子像的简便方法

记录着不同深度位置的粒子全息图在数值重构过程中由于直透光、孪生像的影响以及离焦粒子像的存在,导致了聚焦粒子再现像质量的下降.针对这一问题,本文提出一种数值处理方法来减小上述三因素对聚焦粒子再现像的影响.该方法通过对数值重构出的两个聚焦与非聚焦面上粒子复振幅相减,将直透光、孪生像和离焦粒子像对聚焦粒子的影响同时减小,因此提高和改善了聚焦粒子再现像的对比度.在同轴数字全息层析再现粒子场过程中,该方法适用于在某一聚焦面仅显示聚焦粒子.此外,该过程仅需要一张全息图,而且不需要对全息图做预前和后期处理.给出了简要理论以及仿真、实验结果.     

Measurement of Surface Shape and Position by Phase‐Shifting Digital Holography

Abstract: Phase‐shifting digital holography is applied to measure the shapes and positions of rough surfaces from the averaged conjugate product of the reconstructed complex amplitudes, named complex coherence factor, corresponding to dual wavelengths. The phase of the averaged product at the object plane provides the shape of the surface, whereas the peak position of the modulus with respect to reconstruction distance provides the position of the surface. The phase is almost free from speckle noise and easy to be unwrapped. The resultant resolution of surface shape amounts to a few tens of micrometres. The sensitivity of the position measurement that requires no marking on the object is several per cent of object distance from a charge‐coupled device (CCD). The method corresponds to the detection of the position of maximum contrast of the fringes to be observed in conventional holographic interferometry. It makes use of fringe phase for shape measurement and fringe contrast for position measurement. Both the theory explaining the principle and experimental results are presented.     

声场中倏逝波特性及改进全息重建方法的研究

近场声全息是通过在紧靠被测声源物体的近场测量面上记录全息数据,不但可以记录传播波成分,还可以获得反映声场高空间特性的倏逝波成分。通过分析传播波和倏逝波在声场中的分布情况与声场空间位置、声源大小及频率的关系,得出了传播波和倏逝波在空间声场中的变化规律、影响因素以及二者之间的对应关系。通过对不同背景噪声下倏逝波的传播情况确定了可利用倏逝波传播距离。在此基础上,提出了一种改进的声全息方法,通过用这种方法,可得出声源面上传播波与倏逝波成分,获取等效于近场声全息的重建结果。最后,通过仿真验证了这一结果。     

Contouring of Diffused Objects by Using Lensless Fourier Transform Digital Holography and Dual‐Index Immersion Method

Abstract: A simple method for contouring of diffused objects by using lensless Fourier transform digital holography and dual‐index immersion method is presented. It is noticed that to get more accurate results speckle noise should be eliminated/reduced from the reconstructed phase map from digital holograms. Speckle noise is handled by 5 × 5 median filtering. Depth contour interval up to a maximum of 0.12 mm could be achieved without making the interference phase fringes overcrowded.     

Study of the Mechanical Behaviour of a Clarinet Reed Under Forced and Auto-oscillations With Digital Fresnel Holography

Abstract:  This paper presents methods for vibration analysis using digital Fresnel holography. Methods are based on time averaging for forced oscillations and pulsed recording devoted to auto-oscillations. The two methods are applied to a clarinet reed. In the forced oscillation regime, the reed is excited by an acoustic wave with controlled frequency and amplitude such that the reconstructed holograms exhibit resolvable Bessel fringes. In the case of the auto-oscillation regime, the reed is placed in an artificial mouth. Deformation of the clarinet reed can be extracted with the recording of 3150 pulsed digital holograms. Experimental results show the vibration behaviour of the clarinet reed under forced and auto-oscillation regimes, exhibiting the modal structures and high amplitude shocks.     

Computational terahertz imaging with dispersive objects

We present measurements of optical characteristics of sodium polyacrylate in the terahertz frequency range and discuss how material dispersion affects object visualisation in terahertz imaging. We demonstrate how important is to take into account the material dispersion for phase and relief reconstruction of the investigated object.     

医学层析图像的计算全息三维可视化

提出了一种医学层析图像的计算全息三维可视化技术。首先研究了层析图像序列的三维信息融合,将层析图像序列的二维信息融合成三维信息,用计算全息图的方法进行三维信息记录。然后结合空间光调制器的结构特性,对全息系统的空间频率、参物光夹角、取样间隔以及全息再现像的再现区域和视角等进行了讨论和分析,并设置了相关参数,使计算全息系统与电子显示系统相匹配。最后用液晶空间光调制器作为全息图显示载体,用计算机控制全息图的实时输出,用雾屏承载三维空间再现像,建立三维图像光电再现与实时显示系统,实现层析图像序列的三维可视化,给出了理论分析与实验结果。     

俄歇电子全息的计算机模拟研究和数值重现

原子的芯能级电子吸收能量后被激发 ,经过退激发过程发射出的俄歇电子波在传播过程中被周围的原子散射 ,带有周围原子结构信息的散射波 (物波 )与未被散射的电子波 (参考波 )相干涉形成的衍射图即俄歇电子全息图 ,通过全息图的重现 ,可获得原子级分辨率的三维结构信息。本文在合理的物理模型下 ,以Cu单晶以及高温超导YBCO中的Y原子近邻的一些原子簇为计算实例 ,首次就受激原子周围单层近邻原子散射和多层原子散射产生的俄歇电子全息进行了计算机模拟和数值重现 ,获得了三维晶格结构参量 ,并讨论了不同原子序数的原子在全息图形成及重现中的作用。本工作有助于材料微结构的研究     

低能电子投射全息的计算机模拟和数值重现研究

点源发出的低能量电子波经过样品的散射后,带有物体原子结构信息的散射波(物波)和未被散射的电子波(参考波)在收集平面上相互干涉形成的电子全息图即为低能电子投射全息图.本文在合理的物理模型下,以石墨的晶体结构为试样,通过计算机模拟的方法,首次对低能电子投射全息进行了研究,讨论了一些关键参数对晶体结构电子全息的重现像和分辨率的影响,并且实现了晶体结构三维数值重现,讨论了三维重现的效果.     

Multi-plane imaging for arbitrary reconstruction positions in digital holography

A novel method for multi-plane imaging in digital holography is proposed: holograms of objects located at various places are recorded and then reconstructed simultaneously through one-step Fresnel diffraction using the quadratic distorted phase factor (QDPF). The theory of one-step Fresnel diffraction with the QDPF is deduced and experimental results support the theoretical investigation.     

Electron Holography of Organic and Biological Materials

Electron holography offers the advantage of high resolution imaging of electric and magnetic fields, which is rarely achieved by other microscopic techniques. The variety of scientific tasks investigated by electron holography extends from subatomic‐resolution imaging of inorganic crystal structures up to morphological studies of polymers or biological samples by making use of subtle differences in their chemical composition or density. In this paper, we discuss basic prospects and applications of holography in the field of organic semiconductor devices and biological samples, as well as in polymer science.     

同轴压缩全息再现多层样本

为了研究3层物体的压缩全息分层再现效果,进行了层析再现多层样本的实验,并且将压缩全息再现与传统反衍射重建方法进行了对比分析。采用了频域减采样的方法来实现全息图的稀疏化,将数据压缩到原始全息图的25%。通过仿真实验,发现采用压缩全息再现的物像和模拟样本的相关系数达到0.64。基于4f光学系统建立了实验系统,实现了3层物体的压缩全息再现,可以使所重建的物像更加清晰以及有效去除背景噪声信号。     

Digital holographic scanning of large objects using a rotating optical slab

Simple optical glass is used in digital holographic set up to scan and record holograms of a large object. Dimension of the object is such that it does not satisfy sampling theorem. Experimental results and calculations illustrate that an optical slab can be used to scan the surface of the large object by this method. © 2007 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 16, 258–261, 2006     

数码照相机的三大技术剖析

本文阐述了支撑现代数码照相机的几项关键的技术，即：光学技术、传感器技术和图像处理技术，包括其基本工作原理和在数码成像过程中的作用和功能，并例举了一些最新典型的数码照相机和关键部件，以说明其发展趋势和在目前所达到的成就。     

数字影像输出催生数字相纸

本文在对数字影像相纸化优势以及数字影像相纸化曝光特性分析的基础上，探讨了数字相纸的特点和在存放使用数字相纸方面应特别注意的一些问题。