Single-random-phase holographic encryption of images

In this paper, a method is proposed for encrypting an optical image onto a phase-only hologram, utilizing a single random phase mask as the private encryption key. The encryption process can be divided into 3 stages. First the source image to be encrypted is scaled in size, and pasted onto an arbitrary position in a larger global image. The remaining areas of the global image that are not occupied by the source image could be filled with randomly generated contents. As such, the global image as a whole is very different from the source image, but at the same time the visual quality of the source image is preserved. Second, a digital Fresnel hologram is generated from the new image, and converted into a phase-only hologram based on bi-directional error diffusion. In the final stage, a fixed random phase mask is added to the phase-only hologram as the private encryption key. In the decryption process, the global image together with the source image it contained, can be reconstructed from the phase-only hologram if it is overlaid with the correct decryption key. The proposed method is highly resistant to different forms of Plain-Text-Attacks, which are commonly used to deduce the encryption key in existing holographic encryption process. In addition, both the encryption and the decryption processes are simple and easy to implement.     

Objective evaluation of images reconstructed from digital in-line Fresnel holograms by using coherent background elimination

Quality of images reconstructed from in-line Fresnel holograms using elimination of a coherent background is quantitatively studied through computer simulation and experiment. The coherent background is digitally calculated by averaging an intensity of a recorded hologram. The results show that in spite of the fact that virtual image is still intact, the background elimination can resolve images with quality comparable to the phase-shifting technique.     

Resolution of holographic images of underwater objects

Interest is growing in the use of holographic visual inspection using real-image reconstruction as a means of making high resolution dimensional measurements on subsea components and structures. In this paper we report on the resolution obtained from holographic real images of targets in water filled observation tanks. These results indicate that measurements achieved in such circumstances can be superior to those obtained by stereo photogrammetry. A figure as high as 18 lp mm^-1 (line pairs per millimetre) has been obtained for a resolution target imaged through 300 mm of turbid water underlining the potential of the technique.     

单光路共轴数字全息去背景新方法及应用

针对数字全息图像背景去除的问题,提出两种全息图像重建去背景的方法:背景减弱法和自适应滤波法.搭建了共轴数字全息显微成像实验系统,利用该系统分别对洋葱表皮、植物根茎横切标本、叶片气孔标本和血细胞标本进行背景去除和图像重建.通过光强分布曲线、对比度计算等证明了自适应滤波方法在共轴数字全息结构中去除背景的有效性.实验结果表明,根据环境、样品等不同,全息图的背景也会发生变化.基于此结果,将全息数字显微图像分为三类,并针对不同的背景特点,得出了获取最佳重建图像应采用的有效去除背景方法.研究结果有助于共轴光路数字全息显微术中的重建成像.     

Extraction of object position from in-line holograms by using single wavelet coefficient

A novel digital method for tracking position of objects from in-line holograms by using single wavelet coefficient is proposed. In the proposed method, a wavelet transform is used to analyze the holograms. An axial position of the object being studied is determined by using a real value of a resultant wavelet coefficient appears at a center position of interference fringes. A feasibility of this method is experimentally verified by analyzing holograms of an optical fiber.     

Image encryption based on pure intensity random coding and digital holography technique

We propose a novel image encryption method that combines the pure intensity random encoding and the digital holography technique. A phase-shifting interferometer records both phase and amplitude information of a complex object with a CCD sensor array. The encryption is performed by placing two pure intensity random masks between the image to be encrypted and an intensity recording device. Electronic decryption can be performed with fast Fresnel reconstruction procedure. Numerical simulation results show the validity of the algorithm and an optoelectronic implementation setup is also presented.     

A method for the determination of three-dimensional velocity fields from holographic images

A simple general method is reported for particle tracking and velocity field determination from the reconstruction of holographic images. A preliminary experimental application is reported.     

Three-dimensional holographic reconstruction of two-dimensional image information from serial sections and its applications in biomedicine

An experimental study to synthesize holographically two-dimensional projections of serial sections at various depths into three-dimensional images is described. The effects caused by the position, size and resolution of virtual images during reconstruction are analysed, and the advantages and limitations of this method are discussed. An application of this experimental method — the three-dimensional reconstruction of serial sections of a newt embryo — is presented.     

Self-aligned imaging system using phase conjugate readout from elementary holographic grating

We propose a self-aligned imaging system (SAIS) using phase conjugate readout from an elementary holographic grating. The SAIS provides a huge amount of interconnections between a lot of points on two corresponding imaging planes without troublesome mechanical alignment. In the SAIS, a hologram for one-to-one interconnection is used for many-to-many interconnections. In addition, the SAIS has the capability of correcting aberration. Preliminary experiments verify the principle and the capability of the SAIS.     

基于数学形态学的数字全息再现像融合方法

针对数字全息中不同再现距离获得的携带不同聚焦信息的再现像, 提出了一种基于数学形态学的多聚焦再现像融合方法, 以有效扩展成像景深。首先通过小波-Controulet变换获得源图像的高频和低频分量;然后, 针对数字全息中含散斑噪声的特点, 对高频分量采用基于数学形态学区域能量的方法进行融合, 对低频分量采用加权对比度法进行融合;最后, 将融合系数反变换得到融合图像。通过对算法的有效性分析和实验验证, 将本文提出的方法与不加入数学形态学的融合方法进行了对比研究。结果表明, 基于数学形态学的融合方法能充分抑制散斑噪声的影响, 保留更多细节信息, 有效扩展了成像景深范围达11.5 cm。其中, 对于表面粗糙且信息量较少的骰子, 基于数学形态学方法的空间梯度算子提高了11.8%, 熵值提高了2.7%;对于表面光滑且信息量较多的硬币, 其空间梯度算子提高了13.6%, 熵值提高了2.8%。     

Improvement of images generated reconstructed from computer- holograms using an iterative method

An iterative method is presented which can uniformly distribute the Fourier transformed amplitudes of an original object. The method is effective in making phase quantized holograms whose amplitudes have constant values, and further, it is effective for such imaging systems where the decreased spectral information is desirable for transmission and storage. The hologram is produced by iterations of the Fourier transform and inverse Fourier transform operations. The image quality depends upon the number of iterations, however, a great many iterations will not necessarily lead to a marked improvement in the image quality because a rapid improvement is often achieved by only a few iterative operations. From our experiments, the iterative operations have made remarkable improvements in reconstructed images. The proposed method is also suitable for other types of holograms and for applications in imaging systems.     

Determination of depth-of-focus in lensless in-line digital particle holography

Lensless in-line digital particle holography (DPH) can be thought of as a special case of photography, and its depth of focus (DoF) characteristics combine those of both photography and holography. The effect of important parameters such as the pixel size and the number of pixels in a charge-coupled device (CCD), the object's distance, the wavelength of the laser, and the particle diameter on the DoF of a reconstruction image in lensless in-line DPH was investigated using simulation holograms. The DoF is directly proportional to the object distance and the particle size. As the wavelength of the laser increases, the DoF decreases. The DoF is inversely proportional to the pixel size and to the number of pixels in a CCD. Instead of expressing nonspecific proportional relationships for DoF, the DoF prediction equations for typical CCD cameras were obtained using DoF data from many simulation holograms and efficient data-fitting software. Finally, the DoF prediction equations were verified using real holograms from a calibration target for several cases. Good agreement between measured and predicted values was confirmed.     

Correction of quadratic phase error in two-wavelength digital holographic interferometry using laser diodes

We present the correction of a quadratic phase error in two-wavelength digital holographic interferometry using laser diodes. This phase error arises from numerical reconstructions of wavefronts from digital holograms based on the Fresnel diffraction integral. To correct the quadratic phase error, it is numerically produced by computer on the basis of the theoretical prediction and is subtracted from the phase difference map in two-wavelength digital holographic interferometry. Experimental results show that the method of correction in this paper is useful for two-wavelength digital holographic interferometry using laser diodes.     

基于干涉原理的双图像加密系统

提出一种新的虚拟光学加密方法,该方法分3个步骤将两幅图像隐藏于3个随机相位板中。将两幅图像分别作为相位和幅度信息隐藏于一个复数场中,利用随机相位板作为参考光,并采用数字全息术将该复数场转换成平稳随机白噪声,利用干涉加密原理进一步将此白噪声储存于2个随机相位板中,从而将两幅图像信息隐藏于3个纯随机相位板中。该方法原理简单,无需进行迭代操作。计算机模拟结果显示:任何一个密钥错误,均无法得到原始图像的任何信息。同时,解密过程对系统中距离及波长等参数非常敏感,较大地提升了系统的安全性。     

单幅同轴全息图两步迭代收缩重建

利用压缩传感理论中的两步迭代收缩重建算法,开展单幅同轴全息图重建实验研究,实现单幅同轴全息图共轭重建像的消除并克服数字全息技术在轴向聚焦平面识别能力的不足。以数字图像和标准分辨率板为记录物体,比较分析了基于两步迭代收缩算法和菲涅尔近似衍射重建算法的重建质量;以两根裸光纤为实验样本,分析了两步迭代收缩重建算法对记录物体轴向不同焦平面的识别能力。实验结果表明两步迭代收缩重建算法可得到清晰度高于68.73%的重建信息,同时对直径为125 m的两根光纤在9 mm的轴向间距条件下,显示出了比全息菲涅尔近似算法更好的聚焦平面识别能力。这一轴向聚焦识别能力有助于数字全息技术应用于功能材料梯度参数或功能涂层光学器件涂层厚度检测。     

Viewing effects of 3-D images synthesized from a series of 2-D tomograms by VAP and HAP approaches

We report, for the first time, the experimental result and its analysis of synthesizing a series of simulating 2-D tomograms into a 3-D monochromatic image. Our result shows clearly the advantage in monochromaticity of a vertical area-partition (VAP) approach over a horizontal area-partition (HAP) approach during the final white-light reconstruction. This monochromaticity will ensure a 3-D image synthesis without any distortion in gray level or positional recovery.     

Viewing effects of 3-D images synthesized from a series of 2-D tomograms by VAP and HAP approaches

We report, for the first time, the experimental result and its analysis of synthesizing a series of simulating 2-D tomograms into a 3-D monochromatic image. Our result shows clearly the advantage in monochromaticity of a vertical area-partition (VAP) approach over a horizontal area-partition (HAP) approach during the final white-light reconstruction. This monochromaticity will ensure a 3-D image synthesis without any distortion in gray level or positional recovery.     

Cryptanalysis and improvement of a digital image encryption method with chaotic map lattices

A digital image encryption scheme using chaotic map lattices has been proposed recently. In this paper, two fatal flaws of the cryptosystem are pointed out. According to these two drawbacks, cryptanalysts could recover the plaintext by applying the chosen plaintext attack. Therefore, the proposed cryptosystem is not secure enough to be used in the image transmission system. Experimental results show the feasibility of the attack. As a result, we make some improvements to the encryption scheme, which can completely resist our chosen plaintext attack.     

An image encryption algorithm based on spatiotemporal chaos and middle order traversal of a binary tree

This paper proposes an image encryption algorithm based on spatiotemporal chaos and middle order traversal of a binary tree. Firstly, other programming software is used to perform the middle order traversal, and the plaintext image is sorted according to the middle order traversal sequence on the permutation. Secondly, the chaotic sequence is generated using the coupled map lattice to set the chaotic interference value. Finally, the XOR operation between the adjacent pixel values of the replacement image is completed to generate the ciphertext matrix. The simulation and experimental results show that the proposed algorithm can resist typical attacks and has good robustness.     

月球表面遥感图像阴影消除及其信息恢复研究

由于月球表面的地形起伏引起月面遥感图像像素与太阳相对位置和几何取向不一致,导致月面遥感数字图像上阴影像素的产生。为了解决这一问题,利用与遥感图像匹配的DEM和光照方位参数进行遥感图像的阴影判断,基于DEM数据,利用邻坡反射辐射,进行自然地形条件下的月表遥感图像阴影像素的阴影消除,恢复成太阳光谱照度相等（入射角,反射角和距离相同）时的像素遥感值。仿真实验结果表明：该方法较好地消除了月表影像的阴影,充分恢复了月表影像的反射/光谱特征。