数字全息三维显示关键技术与系统综述

三维全息显示能够表现出与真实物体一样的深度和视差,是一种理想的三维显示方法.但是,三维物体计算全息图计算复杂且计算量巨大,因此,如何快速生成三维物体计算全息图是数字三维全息动态显示中的关键问题之一.本文首先论述了数字全息三维显示的关键技术,包括物点散射法、体视全息法、层析法等三种三维物体计算全息图实现方法,一种RGB分离的真彩色全息显示实现方法和若干提高全息再现像质的方法;然后对几种最新典型的数字三维全息显示系统进行了技术分析;最后总结了数字全息三维显示领域的发展动态,指出三维全息显示技术会朝着实时、动态、更大尺寸、更高分辨率方向发展.     

三维物体空间再现技术中的全息图计算

全息空间再现技术可以记录三维(3D)物体并在空间重构出三维像,是真实地再现三维场景的一种新的可行方法。三维物体全息图的计算是全息空间再现技术中的关键技术之一。本文在简要论述三维物体计算全息技术的理论基础上,对具有代表性的三维物体全息计算方法的原理和技术要点进行了重点论述。采用了层析法、菲涅耳波带法、多视角投影合成全息法等三种方法进行了三维物体全息图的计算,并利用LCR-2500空间光调制器及650 nm的150 mW半导体激光器对层析法计算的三维物体全息图进行了光电再现,对其它两种方法获得的三维物体全息图进行了数值再现,并三种计算方法各自的特点和局限性进行了具体的分析。在对目前三维物体计算全息技术中存在的硬件和算法等关键问题进行分析的基础上,讨论了提高三维物体计算全息图的效率和质量的有效途径。     

多重计算全息水印技术

为了增强水印安全性,用三维物体作为水印信息嵌入载体,提出了一种基于层析法的多重计算全息数字水印技术.该方法用层析技术得到三维物体的菲涅尔(Fresnel)全息图;利用Arnold变换对全息图进行置乱加密,置乱后的图像作为待嵌入水印;选择离散余弦变换(DCT)作为嵌入与提取算法;将Arnold变换次数与三维物体每层的衍射距离作为加密密钥.数字仿真与实验结果显示,当密钥正确时,从水印中提取的全息图的再现结果与直接再现全息图的结果一致,可以再现出三维物体相应层次的信息;密钥错误时,从水印中提取的全息图的再现结果无法识别,水印提取失败.实验结果证明了水印嵌入及提取算法的正确性,表明提出的方法可以实现水印信息的立体化和多重化,具有较高的安全性和易于实现等优点.     

基于双通道三维效果宿主图像半色调全息水印

随着现代社会对信息承载量的需求越来越多,提出了一种基于双通道的三维效果宿主图像的半色调全息水印方法。该方法利用双通道技术将两幅三维效果宿主图像的半色调全息水印图像合成一幅图,接着制作成计算全息图,在重现的时候,一幅计算全息图能够显示出两幅水印图像。制作成的计算全息图可以增加信息传输中的安全性与保密性,双通道技术可以增加传输过程中的信息量,三维效果技术可以在不改变编码效率的情况下增强视觉效果。半色调编码全息水印能更好地提升全息水印的鲁棒性。     

基于全息图放大的数字全息显微结构测量

针对典型预放大数字显微全息光路中存在的二次位相误差,本文设计了基于全息图放大的数字全息显微光路。此设计中,光束首先照射透明显微物体,然后与平行参考光干涉,形成全息图,最后经显微镜获得放大的全息图。这种光路从系统上直接消除了主要由球面波引起的位相畸变,有利于数字处理及实时化。作者以位相光栅(30 lines/mm,槽深约0.3 µm)作为实验样本,对此光路进行了分析研究,并分别用菲涅耳近似法和卷积法再现单幅全息图,同时获取了物体的强度信息和三维位相信息,位相深度的计算结果为0.27 µm。结果表明本文设计的光路对二次位相产生的离焦误差有明显的抑制作用。     

基于数字全息的单透镜表面轮廓检测

为了对光学元件的三维面型进行检测,提出了用数字全息的方法实现单透镜的非接触无损、高效率检测。首先构建了离轴透射式数字全息系统,采用角谱算法对数字全息图进行再现,然后采用HRO相减法消除零级衍射干扰像,提高全息图质量,利用最小二乘法对相位进行解包裹,进而获取透镜的三维轮廓信息。实验结果表明,数字全息技术能有效获取光学元件表面轮廓信息。     

计算全息图的基本理论与制作

与传统光学全息相比,计算全息图因具有极高灵活性,制作简单并且能够记录实际不存在物体的特点而被广泛应用。越来越多的专家和学者致力于研究计算全息图。在介绍计算全息基本理论后,采用四阶迂回相位编码方法,基于MatLab平台分别制作了傅里叶二元计算全息图和菲涅耳二元计算全息图。再现实验中得到的再现像直观明显,对研究和开拓计算全息图更广泛的应用具有参考价值。     

全息扫描镜直线扫描分析及像差校正

介绍了全息扫描镜的原理,给出了等空间频率分布及变空间频率分布的全息扫描镜结构.基于全息图相位及相位传递函数方法,分析了全息扫描镜产生直线扫描条件,推导出记录子全息图各参数之间的关系,得出了利用短波长光记录子全息图,用长波长光再现,实现线性扫描的结果.提出了由于波长移动引起的像差校正方法.分析了摆动与偏心对扫描特性的影响.简介了所设计的变空间频率分布的全息扫描镜.     

数字全息位相拼接实验研究

数字全息拼接技术旨在实现较大物体的检测或在同样的测量面积下获得更高的空间分辨力,实现方法是采集被测物体多个子孔径的数字全息图,且保持相邻子孔径之间有一定的重叠区。分别精密再现各个子孔径的数字全息图,然后将各孔径的位相信息进行拼接,以获得全孔径的位相分布。分别处理了反射型分辨力板(子孔径数为2×4)和透射性微透镜阵列(子孔径数为4×4)两个样本的全息图,并给出了消除测量光束和参考光束非等光程所引入的误差的方法。     

傅里叶变换数字全息图的记录与 再现及实时化研究

为了有效的记录和再现物体的数字全息图,搭建了以液晶光阀(LCD)为透镜的傅里叶数字全息装置。根据全息理论,分析了傅里叶数字全息的记录、再现及实验原理,并进行了相应的数字图像处理,结果表明,傅里叶变换数字全息再现算法简单,缩短了再现周期,可实现准实时化。并成功应用傅里叶变换全息来对字母和数字进行实时相干识别。     

A modified signal channel of page-oriented holographic memory

A modified signal channel of holographic memory is proposed and studied theoretically and experimentally. Compared with a conventional optical scheme, the channel enables to decrease the spread in signal of reconstructed images of information elements, which is caused by their interference in the case of a restricted Fourier hologram aperture, increase the signal-background ratio, and reduce the sensitivity to misalignment of the image and the photodetector array.     

Digital holographic microscopy and focusing methods based on image sharpness

Digital holographic microscope allows imaging of opaque and transparent specimens without staining. A digitally recorded hologram must be reconstructed numerically at the actual depth of the object to obtain a focused image. We have developed a high‐resolution digital holographic microscope for imaging amplitude and phase objects with autofocusing capability. If the actual depth of an object is not known a priori, it is estimated by comparing the sharpness of several reconstructions at different distances, which is very demanding in means of computational power when the recorded hologram is large. In this paper, we present 11 different sharpness metrics for estimating the actual focus depths of objects. The speed performance of focusing is discussed, and a scaling technique is introduced where the speed of autofocusing increases on the order of square of the scale ratio. We measured the performance of scaling on computer‐generated holograms and on recorded holograms of a biological sample. We show that simulations are in good agreement with the experimental results.     

减少液晶显示全息图的再现误差

提出计算机生成全息图的新的算法.全息条纹数据以计算机图像格式存储,并以视频信号传输到液晶板上显示,用该电全息实时三维成像.液晶板的分辨率、对比度及由全息图的灰度决定的调制度会对再现造成影响.因此在算法中考虑这些因素,并通过确定像面分布的情况下逆推求出全息面上的光波的振幅和位相分布再优化编码来减少再现误差.最后给出了实验结果,并对再现进行像质评价.     

Digital double-pulse holographic interferometry using Fresnel and image plane holograms

Different arrangements for digital double-pulse holographic and speckle interferometry for vibration analysis are described. In the case of digital double-pulse holographic interferometry, two separate holograms of an object under test are recorded within a few microseconds using a CCD camera and stored in a frame grabber. The phases of the two reconstructed wave fields are calculated from the complex amplitudes which are obtained by digital reconstruction of the wavefront produced by the hologram. The deformation is obtained from the phase difference. In the case of ESPI (or image plane hologram) the phase can be calculated by using the sinusoid-fitting method or the Fourier method. Using three directions of illumination and one direction of observation, all the information necessary for the reconstruction of the three-dimensional deformation vector can be recorded at the same time. Applications of the method for measuring rotating objects are discussed, together with the derotator needed.     

Holographic methods in microscopy

Holography can be used to record, on a flat photographic plate, information about a three-dimensional object. In conventional microscopy a thin slice of an object is observed in focus and recorded. By combining microscopy and holography it is possible to encode, on the same flat record, all the depth information in a three-dimensional microscopic object, not just a single infocus section. Any section of the three-dimensional object may be subsequently reconstructed and brought into focus by using a suitable viewing system to decode the hologram. Arrangements for doing this are described. It is shown that in order to achieve the highest resolution imagery of a three-dimensional object, reversed wave reconstruction is necessary. As holograms are made and reconstructed using a coherent laser light source, holographic microscopes are easily adapted for interferometry and an example of this is described. The differences between coherent and conventional imagery are briefly considered. The coherence of the illumination gives rise to the problems of coherent noise and speckle. Coherent noise is due to stray reflections in the optical system and can be reduced by using as few surfaces as possible or by using holographic lenses. A speckle reduction technique employing a new type of holographic optical element is described and its application to the stereomicroscopy of fossil ostracods considered.     

Measurements of the characteristics of spray droplets using in-line digital particle holography

In-line digital particle holography is applied to measure the characteristics of spray droplets. Common reconstruction methods were considered and the best one was selected. Several important parameters at the time of hologram recording, such as the object distance and the region of laser beam used, are discussed. The feasibility of the correlation coefficient (CC) method for focal plane determination of 3-D droplets was verified. A double exposure hologram recording system with synchronization system for time control was established, and two digital spray holograms were obtained in a short time interval. For post-processing of reconstruction images, the two-threshold and the image segmentation methods were used in binary image transformation. Using the CC method and some image processing techniques applied to droplets in each double exposure image, the spatial positions of droplets used to evaluate the three dimensional droplet velocities were easily located, which proved the feasibility of in-line digital particle holographic technology as a good measurement tool for spray droplets. This paper was recommended for publication in revised form by Associate Editor Gihun Son Boseon Kang received his B.S. and M.S. degrees in Mechanical Engineering from Seoul National University in 1986 and 1988, respectively. He then went on to receive his Ph.D. degree from University of Illinois, Chicago in 1995. He is currently Professor at School of Mechanical Systems Engineering, Chonnam National University in Gwangju, Korea. His research interests are in the area of sprays, holographic techniques in thermofluid measurements. Yan Yang received his B.S. degree in Mechanical Engineering from Chongqing Insitute of Technology in 1997, and received his M.S. degree in Mechanics from Chongqing University in 2005. He is doctoral student of Department of Mechanical Systems Engineering, Chonnam National University in Gwangju, Korea. He is also currently Associate Professor at Automobile College, Chongqing University of Technology in Chongqing, China. His research direction is digital holographic techniques