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。     

Sampling and processing for compressive holography [Invited

Compressive holography applies sparsity priors to data acquired by digital holography to infer a small number of object features or basis vectors from a slightly larger number of discrete measurements. Compressive holography may be applied to reconstruct three-dimensional (3D) images from two-dimensional (2D) measurements or to reconstruct 2D images from sparse apertures. This paper is a tutorial covering practical compressive holography procedures, including field propagation, reference filtering, and inverse problems in compressive holography. We present as examples 3D tomography from a 2D hologram, 2D image reconstruction from a sparse aperture, and diffuse object estimation from diverse speckle realizations.     

Fast numerical generation and encryption of computer-generated Fresnel holograms

In the past two decades, generation and encryption of holographic images have been identified as two important areas of investigation in digital holography. The integration of these two technologies has enabled images to be encrypted with more dimensions of freedom on top of simply employing the encryption keys. Despite the moderate success attained to date, and the rapid advancement of computing technology in recent years, the heavy computation load involved in these two processes remains a major bottleneck in the evolution of the digital holography technology. To alleviate this problem, we have proposed a fast and economical solution which is capable of generating, and at the same time encrypting, holograms with numerical means. In our method, the hologram formation mechanism is decomposed into a pair of one-dimensional (1D) processes. In the first stage, a given three-dimensional (3D) scene is partitioned into a stack of uniformed spaced horizontal planes and converted into a set of hologram sublines. Next, the sublines are expanded to a hologram by convolving it with a 1D reference signal. To encrypt the hologram, the reference signal is first convolved with a key function in the form of a maximum length sequence (also known as MLS, or M-sequence). The use of a MLS has two advantages. First, an MLS is spectrally flat so that it will not jeopardize the frequency spectrum of the hologram. Second, the autocorrelation function of an MLS is close to a train of Kronecker delta function. As a result, the encrypted hologram can be decoded by correlating it with the same key that is adopted in the encoding process. Experimental results reveal that the proposed method can be applied to generate and encrypt holograms with a small number of computations. In addition, the encrypted hologram can be decoded and reconstructed to the original 3D scene with good fidelity.     

Multiple-image encryption scheme with a single-pixel detector

Abstract

A multiple-image encryption (MIE) scheme with a single-pixel detector has been proposed according to the principle of ghost imaging. In this scheme, each of the spatially coherent laser beams is modified by a set of phase-mask keys and illuminates on a secret image. All of the transmitted lights are recorded together by a single-pixel (bucket) detector to obtain a ciphertext, but anyone of the secret images can be decrypted from the ciphertext independently without any mutually overlapped despite some noise in them. The MIE scheme will bring convenience for data storage and transmission, especially in the case that different secret images need to be distributed to different authorized users, because the ciphertext is a real-valued function and this scheme can effectively avoid the secret images being extracted mutually. The basic principle of the MIE scheme is described theoretically and verified by computer simulations. Finally, the feasibility, robustness and encryption capacity are also tested numerically.     

Digital holographic microscopy with dual-wavelength phase unwrapping

We apply the techniques of digital holography to obtain microscopic three-dimensional images of biological cells. The optical system is capable of microscopic holography with diffraction-limited resolution by projecting a magnified image of a microscopic hologram plane onto a CCD plane. Two-wavelength phase-imaging digital holography is applied to produce unwrapped phase images of biological cells. The method of three-wavelength phase imaging is proposed to extend the axial range and reduce the effect of phase noise. These results demonstrate the effectiveness of digital holography in high-resolution biological microscopy.     

Digitized holography: modern holography for 3D imaging of virtual and real objects

Recent developments in computer algorithms, image sensors, and microfabrication technologies make it possible to digitize the whole process of classical holography. This technique, referred to as digitized holography, allows us to create fine spatial three-dimensional (3D) images composed of virtual and real objects. In the technique, the wave field of real objects is captured in a wide area and at very high resolution using the technique of synthetic aperture digital holography. The captured field is incorporated in virtual 3D scenes including two-dimensional digital images and 3D polygon mesh objects. The synthetic field is optically reconstructed using the technique of computer-generated holograms. The reconstructed 3D images present all depth cues like classical holograms but are digitally editable, archivable, and transmittable unlike classical holograms. The synthetic hologram printed by a laser lithography system has a wide viewing zone in full-parallax and give viewers a strong sensation of depth, which has never been achieved by conventional 3D systems. A real hologram as well as the details of the technique is presented to verify the proposed technique.     

Multiple-viewpoint projection holograms synthesized by spatially incoherent correlation with broadband functions

We present a theoretical framework for recording and reconstructing incoherent correlation holograms of real-existing three-dimensional scenes observed from multiple viewpoints. This framework is demonstrated by generating and reconstructing a modified Fresnel hologram as well as a new correlation hologram called a protected correlation hologram. The reconstructed scene obtained from the protected correlation hologram has a significantly improved transverse resolution for the far objects in the scene compared to the modified Fresnel hologram. Additionally, the three-dimensional information encoded into the protected correlation hologram is scrambled by a secretive point spread function and thus the hologram can be used for encrypting the observed scene. The proposed holography methods are demonstrated by both simulations and experiments.     

Optical sectioning by optical scanning holography and a Wiener filter

I propose a novel digital technique that reduces defocus noise in the reconstruction of the sectional images from the complex hologram of a thick object. In three-dimensional microscopy applications of holography, reducing the defocused light scattered from outside the focused plane is an important issue. In this technique I first extract a complex hologram of a thick object by using optical scanning holography. After that, I separate the power spectra of the focused and defocused planes from the complex hologram. Finally, I construct a Wiener filter by use of the power spectra. The Wiener filter reduces the defocus noise in the reconstruction of the sectional image of the focused plane. Computer simulations show that the proposed Wiener filter reduces the defocus noise and provides the sectional images.     

Three types of computer-generated hologram synthesized from multiple angular viewpoints of a three-dimensional scene

We describe various techniques to synthesize three types of computer-generated hologram (CGH): the Fresnel-Fourier CGH, the Fresnel CGH, and the image CGH. These holograms are synthesized by fusing multiple perspective views of a computer-generated scene. An initial hologram is generated in the computer as a Fourier hologram. Then it can be converted to either a Fresnel or an image hologram by computing the desired wave propagation and imitating an interference process of optical holography. By illuminating the CGH, a 3D image of the objects is constructed. Computer simulations and experimental results underline the performance of the suggested techniques.     

Holography with Light Beams of Finite Spectral Width

The effects of spectral coherence on holographic images have been analysed in terms of the Fresnel-transform formulation of the process of holography. In this paper the case of very rough object or diffuse illumination has not been considered. The results have been obtained as contrast modulation in the image of a spatial frequency component in the object. The effect of polychromaticity in the recording beam is seen to be more severe than that due to the polychromaticity in the reconstruction beam. Fourier-transform holography seems to yield a better image. With the lensless Fourier-transform hologram recorded on monochromatic light, white light reconstruction seems to be possible if the illuminating beam is suitably dispersed before it falls on the hologram.     

Generation of digital textured surface models from hologram recordings

Digital sensors and fast digital image processing facilitate the use of pulsed holography for 3D surface measurement of moving objects. The real image of a hologram is reconstructed optically. A sequence of high-resolution projection images of the real image with a varying distance to the hologram is recorded digitally. Focus detection in this image sequence by digital image processing yields the shape of the recorded object. The image intensity serves as a precise pixel-matching texture. An application of this concept is the generation of a textured 3D computer model of a facial surface from a portrait hologram.     

改进的基于 QR 码的数字全息水印

目的提出改进的基于快速响应矩阵码(QR码)的数字全息水印。方法利用加密傅里叶全息技术对QR码进行加密并生成全息图,在标准颜色空间下,抽取彩色载体图像的L分量进行二级小波分解,将全息图嵌入到其分解后的低频系数中,以实现水印信息的隐藏。为提高QR码译码成功率,对提取后的水印进行十字型中值滤波处理。结论 QR码水印能够保存完整的水印信息,比普通图像水印鲁棒性强,且相比于其他基于QR码的数字全息水印算法具有更强的鲁棒性。     

Simple holographic polarogram

A new technique capable of obtaining quantitative values of the rotation angle of the polarization vector by using holography is presented. This is a two-stage holographic process; during the recording stage a hologram of the object of interest is obtained. The reference beam is composed of two beams that form a small angle between them and keep their polarization states at right angles to each other. In the reconstruction stage of the hologram, two images from the hologram are obtained along two different angles. As a result of the interference between these two images, a set of parallel fringes is formed at the image plane. The fringe contrast on the reconstruction is related to the angle of the polarization vector of the light at each position on the image plane. Measurements of the rotation of the polarization angle of a fraction of a degree were obtained. The main application of this technique is in the study of transient phenomena, where single-shot measurements are the only means of obtaining reliable data.     

Intrinsic aberrations due to Mie scattering in particle holography

Holography of small particles is a newly revived topic because of its importance in holographic particle image velocimetry (HPIV). However, the property of particle images formed through holography remains largely unexplored. This fact undermines the measurement reliability of HPIV techniques and has become one of the obstacles in the full deployment of HPIV. We study the intrinsic aberrations in the holographic particle image introduced by particle light scattering and investigate how accurately holography can deliver information about the particles that are being imaged. Consistent with our experimental observations, simulations based on Mie scattering theory show that even with a perfect hologram the reconstructed particle images demonstrate complex three-dimensional morphologies and bodily shifts. These characteristics, manifested as image aberrations, result from uneven scattering amplitude and phase distributions across the finite aperture of the hologram. Such aberrations degrade the signal-to-noise ratio in the reconstructed image as well as introducing systematic errors in detected particle image positions. We examine the effect of these aberrations on HPIV measurements.     

Synthetic display holography: Line segments through the hologram

Abstract

In synthetic display holography the reconstruction of 3D objects that are intersected by the hologram plane are of interest. Some characteristics and limitations of such a situation are investigated. We concentrate on images assembled of lines. Certain constraints exist for the calculation of distributions to be stored in holograms. Some depend on the properties of the lines and others are of a more general character, e.g. the hologram sampling. The influence of major parameters related to a reconstructed line are analysed.     

Large-viewable-angle rainbow holography

A new, to our knowledge, method for recording rainbow holograms of three-dimensional diffused objects in one step without the use of slits is proposed. No lens is needed for recording in this kind of holography; instead, multiple synthetic slits are created by displacement and multiple exposure of an object to expand the horizontal and the vertical viewing angles of the hologram.     

Simultaneous depth determination of multiple objects by focus analysis in digital holography

Focus analysis techniques from computer vision are applied to digital holography to determine the depth (range) of multiple objects and their surfaces from a single hologram capture. With this method the depths of objects can be determined from a single hologram capture without the need for manual focusing and without prior information on object location. Variance and the Laplacian of Gaussian are analyzed as focus measures, and techniques are proposed for focus plane determination from the focus measure curves. The algorithm is described in detail and demonstrated through simulation and optical experiment.     

Correction of anisoplanatic phase errors in digital holography

The quality of coherent images computed from digital holography or heterodyne array data is sensitive to phase errors of the reference and/or object beams. A number of algorithms exist for correcting phase errors in or very near the hologram plane. In the case of phase errors introduced a nonnegligible distance away from hologram plane, the resulting imagery exhibits anisoplanatism. A feature of coherent imaging is that such phase errors may be corrected by simply propagating the aberrated fields (from the object) from the hologram plane to the plane where the phase errors were introduced and applying the phase-error correction algorithms to the fields in that plane. We present experimental results that demonstrate correction of such anisoplanatic phase errors.     

Pixel-size-maintained image reconstruction of digital holograms on arbitrarily tilted planes by the angular spectrum method

We present an effective method for the pixel-size-maintained reconstruction of images on arbitrarily tilted planes in digital holography. The method is based on the plane wave expansion of the diffraction wave fields and the three-axis rotation of the wave vectors. The images on the tilted planes are reconstructed without loss of the frequency contents of the hologram and have the same pixel sizes. Our method shows good results in the extreme cases of large tilting angles and in the region closer than the paraxial case. The effectiveness of the method is demonstrated by both simulation and experiment.     

Quantitative space-bandwidth product analysis in digital holography

The space-bandwidth product (SBP) is a measure for the information capacity an optical system possesses. The two information processing steps in digital holography, recording, and reconstruction are analyzed with respect to the SBP. The recording setups for a Fresnel hologram, Fourier hologram, and image-plane hologram, which represent the most commonly used setup configurations in digital holography, are investigated. For the recording process, the required SBP to ensure the recording of the entire object information is calculated. This is accomplished by analyzing the recorded interference pattern in the hologram-plane. The paraxial diffraction model is used in order to simulate the light propagation from the object to hologram-plane. The SBP in the reconstruction process is represented by the product of the reconstructed field-of-view and spatial frequency bandwidth. The outcome of this analysis results in the best SBP adapted digital holographic setup.