Parallel-quadrature phase-shifting digital holographic microscopy using polarization beam splitter

We present a digital holography microscopy technique based on a parallel-quadrature phase-shifting method. Two π/2 phase-shifted holograms are recorded simultaneously using polarization phase-shifting principle, slightly off-axis recording geometry, and two identical CCD sensors. The parallel phase-shifting is realized by combining circularly polarized object beam with a 45° degree polarized reference beam through a polarizing beam splitter. DC term is eliminated by subtracting the two holograms from each other and the object information is reconstructed after selecting the frequency spectrum of the real image. Both amplitude and phase object reconstruction results are presented. Simultaneous recording eliminates phase errors caused by mechanical vibrations and air turbulences. The slightly off-axis recording geometry with phase-shifting allows a much larger dimension of the spatial filter for reconstruction of the object information. This leads to better reconstruction capability than traditional off-axis holography.     

An error detection and recovery algorithm for the transmission of digital holography over noisy channels

Digitalization enables and expertise storage and transmission of holographic data. However, the reconstructed object image is vulnerable to transmission errors over the noisy channel. We present a novel algorithm of what we believe is the first approach to reduction of noise effects on digital holograms corrupted by transmission errors. To accomplish this work, we exploit new interdependencies on the interference holograms and develop a detection-based algorithm to detect error points and to identify the corresponding interference holograms with the largest errors. In the reconstruction process, the conventional four-step algorithm is used for the recovery of error-free points. In contrast, a three-step algorithm is employed to reconstruct error points by removing the largest error holograms. With accurate detection, the proposed algorithm is capable of recovering the object image from digital holograms with severe transmission errors. It is shown that the proposed algorithm provides much better reconstruction performances than the conventional four-step reconstruction algorithm in an error-prone environment.     

基于旋转照明光的数字全息相干噪声抑制

提出一种数字全息相干噪声抑制方法。首先在照明光方位角给定的前提下,通过在极向平面内连续旋转照明光,实现在多角度照明条件下记录离轴数字全息图。然后对所获得的全息图分别重构,同时利用倾斜相差补偿算法和数字图像自相关配准方法分别补偿再现像的相位倾斜及横向偏移。最后将重构像进行非相干叠加,从而有效抑制再现像的相干噪声。实验结果及相关统计评价数据皆验证了该方法的有效性。     

一种抖动与孤立像素点分离相结合的灰度级增强方法

为了使由等离子体显示器(PDP)反γ校正和有限灰度级显示方法引起的灰度损失得到改善,在分析灰度损失原因及有序抖动和随机抖动特点的基础上,提出了一种抖动与孤立像素点分离相结合的灰度级增强方法。该方法首先采用有序抖动和孤立像素点分离对γ校正后的图像进行灰度增强;然后把图像数据映射到有限灰度级,计算有限灰度级与需要显示的灰度级之间的误差;最后采用随机抖动和孤立像素点分离对有限灰度级图像进行灰度增强。实验结果表明,反γ校正和有限灰度级图像的灰度均得到了显著增强,低灰度级和中高灰度级过渡平滑,没有周期性抖动噪声,孤立像素点的亮度显著降低,图像显示效果明显优于仅采用随机抖动或有序抖动的图像。     

基于两步正交相移干涉的振幅图像光学加密技术

提出一种基于两步正交相移干涉的光学图像加密技术.这种相移干涉数字全息只要记录两幅干涉图,不需要记录物光波和参考光波的强度信息,就可以再现没有零级像和共轭像的再现像.物光波对应的光路经过两次菲涅尔变换,并结合双随机相位编码.参考光分别引入0和π/2相位,用数字化记录介质记录两幅数字全息图作为加密图像.解密时只要获得正确的密钥,经过简单的计算就可以重建清晰的原始图像.模拟实验验证了它的可行性和有效性,分析了抗裁剪和噪音的鲁棒性.     

四步相移数字全息中随机误差的抑制方法

提出一种在四步相移数字全息中消除随机相移误差及CCD噪声的方法。通过多次重复四步相移数字全息的实验过程,使得每一步的相移都有多幅全息图像与之对应,把每一步对应的多幅全息图像进行数学平均就会得到4幅全息均值图像。理论分析表明,使用这4幅全息均值图像进行数字全息的重建,可以得到精确的物光波的分布,有效地消除随机相移误差及CCD噪声的影响。计算机模拟结果表明:该方法有助于提高数字全息重建光场质量。     

显微数字全息中的3维信息重建

通过理论分析和模拟验证,研究了基于横向剪切的数字全息相位重建方法,分析并指明了详细的重建过程,提出了利用平坦区域相位数据进行线性拟合,从而获得线性相位畸变系数的方法,并指出对原始包裹相位图进行1维相位展开是横向剪切法重建数字全息相位信息的前提。对无噪声及含有噪声的全息图进行了数值重建,结果表明:对于弱噪声干扰的全息图,该方法很有效;而对于较强噪声干扰的全息图,采用中值滤波方法对原始相位图进行滤波后再重建,并对重建的相位图再次进行中值滤波,可以得到高质量的再现像;减小再现像平面抽样间隔,使剪切相位图中相邻的两个像元之间相位差的最大值小于2π,才可以获得正确的相位重建。     

Reduction of speckle noise in digital holography by using digital image processing

A fundamental problem in optical and digital holography is the presence of speckle noise in the reconstruction process. Many approaches have been carried out in order to overcome such a problem ranging from modifying the spatial coherence of the illumination (optical techniques) to image processing techniques (digital techniques). This work shows the merged use of digital image processing techniques in order to reduce the speckle noise in digital reconstruction of optically recorded Fresnel's holograms. The proposed filtering techniques are illustrated with experimental results.     

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.     

Synthetic Aperture Radar Image Despeckling Based on Multi-Weighted Sparse Coding

Synthetic aperture radar (SAR) images are inherently degraded by speckle noise caused by coherent imaging, which may affect the performance of the subsequent image analysis task. To resolve this problem, this article proposes an integrated SAR image despeckling model based on dictionary learning and multi-weighted sparse coding. First, the dictionary is trained by groups composed of similar image patches, which have the same structural features. An effective orthogonal dictionary with high sparse representation ability is realized by introducing a properly tight frame. Furthermore, the data-fidelity term and regularization terms are constrained by weighting factors. The weighted sparse representation model not only fully utilizes the interblock relevance but also reflects the importance of various structural groups in despeckling processing. The proposed model is implemented with fast and effective solving steps that simultaneously perform orthogonal dictionary learning, weight parameter updating, sparse coding, and image reconstruction. The solving steps are designed using the alternative minimization method. Finally, the speckles are further suppressed by iterative regularization methods. In a comparison study with existing methods, our method demonstrated state-of-the-art performance in suppressing speckle noise and protecting the image texture details.     

Multiple-exposure high-sensitivity holographic interferometry for studying moving objects

A method for increasing the sensitivity of measurements through aberration compensation upon reconstruction of interferograms from two multiple-exposure holograms is proposed. At the early stage of object investigation, the holographic structures recorded at certain time instants are rerecorded by two coherent beams on new image carriers. In this case, the interference moiré method is employed to monitor the equality of the vectors of the holographic structures rerecorded. At the final stage, the new nonlinear holograms are processed in an optical analyzer of conjugate holograms with the use of incoherent light. The hologram thus reconstructed offers a high sensitivity of measurements and is free of aberrations. The method is tested by visualizing the temperature-field variations in a glass substrate with a conducting coating.     

彩色全息图的计算机产生和数字再现

提出了一种基于色彩合成以及菲涅尔衍射原理,用计算机产生全息图并用数字方法再现彩色全息图的方法。该方法的第一步是将物体的彩色RGB图像分离为三基色强度图,再利用博奇编码的方法制成修正离轴参考光分色计算机全息图;第二步是分别对分色全息图在频域进行调制以实现用原参考光真实再现原始物光波。通过滤波消除零级衍射及共轭像的影响,获得了所需要的实像并提高了像质。提供的实验是选用一幅RGB图像作为原始物体,给出了用博奇编码法制成的全息图以及最后经色彩合成获得的再现像。结果表明,该方法能使各分色全息图的再现像准确重合,解决了在色彩合成时容易出现的色串扰问题。     

Controlling image size as a function of distance and wavelength in Fresnel-transform reconstruction of digital holograms

A method for controlling the size of amplitude and phase images reconstructed from digital holograms by the Fresnel-transform method is proposed and demonstrated. The method can provide a constant reconstruction pixel width in the reconstructed image plane, independent of the recording and reconstruction distance. The proposed method makes it possible to maintain the size of an object for a sequence of digital holograms recorded at different distances and, therefore, to subtract phase maps for an object recorded at different distances. Furthermore, the method solves the problem of superimposition in multiwavelength digital holography for color display and holographic interferometry applications.     

Optical retrieval of encrypted digital holograms for secure real-time display

Secure data transmission by use of encrypted digital holograms and an optical retrieval system for secure real-time display are proposed. Original images are encrypted by a double-random phase encryption technique and then are recorded as digital holograms in a CCD, together with a reference plane wave. This digital hologram of the encrypted image can be transmitted to receivers via any conventional electronic or digital communication channels. The decryption key is also recorded as a digital hologram. At the receiver, the original image can be retrieved by an optical correlation-based reconstruction process. Both encryption and retrieval can be performed by all-optical means, and thus a real-time secure display can be implemented. We demonstrate the proposed system numerically and experimentally.     

长工作距离显微成像数字全息合成孔径方法(英文)

提出了一种用于长工作距离下显微成像的数字全息合成孔径方法。首先,在不同斜入射照明条件下记录多幅包含不同物光频谱范围的数字全息图。然后,利用每一幅全息图通过数字方法重构并放大物体强度像。最后,通过非相干叠加这些强度像得到分辨力提高且散斑噪声减小的合成物体强度像。理论分析和实验结果表明,此方法可应用于长工作距离下的原位显微观测。     

Recovering correct phase information in multiwavelength digital holographic microscopy by compensation for chromatic aberrations

We demonstrate experimentally that correct phase imaging without 2pi ambiguity is obtainable in digital holography by using a multiwavelength approach in the microscope configuration. We describe a general approach for removing chromatic aberrations and for controlling the pixel size of the reconstructed phase image in multiwavelength digital holography when the Fourier transform method is adopted for the numerical reconstruction of digital holograms. The retrieved phase is affected by the unavoidable, unwanted chromatic aberration. The correct phase can be obtained by evaluating the phase from the reference holograms reconstructed at different wavelengths to compensate for the chromatic aberration.     

Holographic Subsurface Radar Technique for Nondestructive Testing of Dielectric Structures

Holographic subsurface radar method is compared with the conventional technology of impulse radars. Basic relationships needed for the reconstruction of complex microwave holograms are presented. Possible applications of the proposed technology are discussed. Diagnostics of polyurethane foam coatings of spacecrafts is used as an example of the efficiency of holographic subsurface radars. Results of reconstruction of complex and amplitude microwave holograms are compared. It is demonstrated that the image quality that results from reconstruction of complex microwave holograms is higher than the image quality obtained with the aid of amplitude holograms.     

Quality of images decrypted from in-line holographic encryption of images

We investigate quality of images decrypted from in-line holograms recorded with a random phase mask placed behind an input image. High- and low-contrast fingerprint images are used as test scenes. The simulation results show that due to multiple convolution processes and a limitation of the CCD sensor resolution, quality of decrypted images is sensitive to spatial separations between the input image, the phase mask and the hologram. We propose a new method for improving quality of decrypted images by using background noise removal. It is found that in comparison with high-contrast input, the low-contrast image is more robust to distortion caused by the encryption–decryption process.     

Double-exposure variable shear holographic interferometry with controlled sensitivity

A method of simultaneous recording of double-exposure holograms with different lateral shears is proposed. This method makes it possible to reconstruct a series of interference patterns representing the phase shift caused by the object under study with different sensitivities. Aberration-free interference patterns in fringes of infinite and finite width with different visualization sensitivities for melting zones of a flat polyme-thylmethacrylate sample are presented.     

Method for eliminating zero-order diffraction in lensless Fourier transform digital holography

A method to induce phase shifting in lensless Fourier of digital holography system is presented. In this method, by computer simulation and theoretical analysis on the technology to eliminate the influence of zero-order diffraction in force, it can be found that, a reference light induced in a random non-2π integral number of phase shifting shooting to get the second hologram, and wave reconstruction can be got by the difference value image of the two holograms. And, the method of different digital holography record system with different phase shifting should be used. In this paper, theoretical analyses have been done in detail to discuss the problems that exist in the unsuitable phase-shifting methods. Furthermore, some experiments have been done to prove the reliability of this method. This method can significantly improve the image quality and give better resolution of the reconstructed image.