无透镜傅里叶变换数字全息术中非共面误差的自动补偿算法

无透镜傅里叶变换数字全息术对活细胞进行显微相衬成像测量时,由于细胞处于运动状态导致记录物体与参考点源难以共面,从而引入成像误差.本文提出了一种有效可靠的基于位相分布的自动对焦再现算法,只需通过一幅全息图,即可确定出记录距离的优化数值解,并结合一次位相线性拟合,成功补偿掉非共面误差.将这种算法应用到活体宫颈癌细胞的相衬成像实验中获得了较为理想的实验结果,表明了这种自动补偿非共面误差算法的可行性. 关键词： 数字全息 无透镜傅里叶变换全息 自动对焦 位相畸变校正     

Information reduction using lensless Fourier transform digital composite holography

An optical hologram contains substantially more information than necessary for some specific applications. Practical methodology of handling huge information contents in these holograms for such applications is cumbersome. In this paper, a method of information reduction, which aims at efficient storage and transmission of holograms, is investigated using digital composite holography in lensless Fourier transform configuration. The maximum information reduction factor obtained in our experiment is 105. The advantages/disadvantages of this method over an earlier method used in optical holography [Lin LH. A method of hologram information reduction by spatial frequency sampling. Appl Opt 1968;7:545] are also discussed.     

Elimination of zero-order diffraction in digital off-axis holography

A simple experimental method of eliminating zero-order diffraction in the reconstructed image of off-axis digital holography is presented. Holographic diffraction grating acting as a beam splitter, an off-axis holography system is formed. The holograms of object with different recording parameters are obtained by adjusting the reflecting mirror in the recording optical system to vary the incidence orientation of the object beam in CCD to introduce a phase shift. The zero-order image can be eliminated by numerically processing the holograms of object with different recording parameters. The theoretical analyses have been done in detail and the experimental results are also given. The zero-order image eliminated, the area of reconstructed image increases remarkably, the image quality can then be significantly improved and the better resolution obtained. The experimental results show that the method presented in this paper is feasible, simple in optical system and easy in operation and data processing.     

无零级衍射干扰的彩色数字全息研究

彩色数字全息研究中,用球面波为重建波及角谱衍射公式进行波前重建是一种有效的方法.然而,重建图像上通常伴有强烈的零级衍射干扰.本文通过理论分析,对波前重建过程作了重要改进.用改进后的方法进行彩色数字全息的实验研究表明,重建彩色图像的质量获得显著改善. 关键词： 彩色数字全息 波前重建 零级衍射干扰     

一种实现单步相移无透镜傅里叶同轴数字全息的倒频谱技术

在数字全息技术中无透镜傅里叶同轴数字全息具有高分辨,再现速度快等优点,因此有着广泛的应用前景,但它也存在再现像与直透光场及其共轭像重叠,较难分离的缺点。针对这一问题,在分析了无透镜傅里叶同轴数字全息图记录和再现原理的基础上,提出了单步相移倒频谱技术,并在理论上分析了该技术对分离出清晰再现像的可能性和该技术适用的关键条件。通过仿真实验进行了验证,结果表明:单步相移倒频谱技术能够较好地解决无透镜傅里叶同轴数字全息技术中再现像与直透光场及其共轭像重合这一困难,得到清晰的再现像;同时给出了实际实验中单步相移倒频谱技术能够适用的条件参数,这为后面的进一步实验提供了参考依据。     

无透镜傅里叶变换显微数字全息成像系统的焦深

基于透镜相干光学成像系统的斯特列尔(Strehl)判据, 对无透镜傅里叶变换显微数字全息成像系统的焦深进行了推导, 得到了参考点源对称偏置和非对称偏置两种情况下的焦深表达式. 结果表明, 无透镜傅里叶变换显微数字全息成像系统的焦深决定因素与透镜相干成像系统的焦深决定因素不同, 其焦深不仅与照明光源的波长, 成像系统的孔径及记录距离有关, 还与参考光源的配置有关. 计算机模拟和实验结果均证明了理论分析的正确性.     

High-resolution digital transmission microscopy—a Fourier holography approach

A spherical reference field is used to construct a digital holographic system with a demonstrated resolution up to 228 line pairs per mm. The reference field originates from a GRIN lens placed 1 mm from the illuminated object. This allows the use of a standard sensor to record the hologram with the required numerical aperture. The image is determined by evaluation of the Rayleigh–Sommerfeld diffraction integral that relates the object field in the image plane to the object field in the sensor plane. Experimental results are given for two charge coupled device sensors and one complementary metal-oxide-semiconductor active pixel sensor.     

Monitoring of drying process and cracking/disbonding of paints using lensless Fourier transform digital holography

We have proposed and demonstrated a new application of lensless Fourier transform digital holographic interferometry to study/monitor the drying process and detection of cracking/disbonding of the painted surface by observing the dynamics of interference phase maps. The technique can monitor the rate of drying of paint and visualize the state of dryness and crack/disbond. The technique is simple and easy to implement.     

Contouring of diffused objects by using digital holography

A simple method for contouring of diffused objects by using lensless Fourier transform digital holography (LFTDH) and dual-index immersion method is presented. Depth contour interval up to minimum of 0.12 mm could be achieved without making the interference phase fringes over crowded.     

Image encryption based on extended fractional Fourier transform and digital holography technique

We present a new optical image encryption algorithm that is based on extended fractional Fourier transform (FRT) and digital holography technique. We can perform the encryption and decryption with more parameters compared with earlier similar methods in FRT domain. In the extended FRT encryption system, the input data to be encrypted is extended fractional Fourier transformed two times and random phase mask is placed at the output plane of the first extended FRT. By use of an interference with a wave from another random phase mask, the encrypted data is stored as a digital hologram. The data retrieval is operated by all-digital means. Computer simulations are presented to verify its validity and efficiency.     

Phase reconstruction in lensless digital in-line holographic microscopy

The phase reconstruction in a digital in-line holographic microscopy is compared using two numerical reconstruction methods. The first method uses one Fourier transform and second one uses three Fourier transforms. It is shown that the latter method gives improved object phase reconstruction as compared to the former.     

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.     

Joint approach of the sub-holograms in on-axis lensless Fourier phase-shifting synthetic aperture digital holography

In on-axis lensless Fourier phase-shifting synthetic aperture digital holography, to compose all of the phase-shifting sub-holograms to a large synthetic aperture digital hologram effectively, firstly, the cross-correlation algorithm of the object waves is presented to correct the joint misplacement of the sub-holograms. Secondly, to make the phase-shifting synchronization matching of different sequence phase-shifting holograms, the cross-correlation algorithm of the phase-shifting holograms is employed. Compared with the traditional cross-correlation algorithm of the sub-holograms, the proposed approach makes the joint precision of the sub-holograms reach sub-pixel accuracy, and the resolution of the reconstructed image is improved significantly. In general, the proposed approach is effective in restraining the quality degradation of the synthetic reconstructed image that comes from the joint misplacement of the sub-holograms and the phase-shifting non-synchronization of the phase-shifting holograms.     

Lensless digital holography with short-coherence light source for three-dimensional surface contouring of reflecting micro-object

In this paper, a new lensless digital holography system with short-coherence light source is reported for recording three-dimensional surface contouring of reflecting micro-objects. In the experiment, each of the layers on the inwall of a conical pore is respectively recorded by changing the path length of the object beam, instead of changing that of the reference beam, which can reduce the recording complexity and errors. In addition, the least-square-polynomial-fitting is used for the first time to carry out three-dimensional reconstruction with a series of two dimensional intensity images of a micro-object, which can be used not only to reduce obviously the complication of the three-dimensional reconstruction, but also to carry out three-dimensional reconstruction of a micro-object with strong laser speckle noise, of which the phase images can not be obtained from the conventional phase unwrapping process.     

Improvement of signal-to-noise ratio in digital holography using wavelet transform

The basic problem in optical and digital holography is the presence of speckle noise in the reconstruction process, which reduces the signal-to-noise ratio (SNR). The presence of speckle noise is serious drawback in optical and digital holography since it substantially reduces the SNR in the reconstructed image. In this paper, we present wavelet filtering to improve SNR in the reconstructed images from digital holograms. Experimental results are presented.     

Method for eliminating zero spectrum in Fourier transform profilometry

Windowed Fourier transform (FT) is introduced into Fourier transform profilometry (FTP) to extract zero component included in deformed fringe pattern without capturing two fringe patterns with π phase difference. First, we extract zero component of deformed fringe image using windowed FT and normalize it, then calculate the normalized Fourier spectra of deformed fringe image, finally subtract the zero component from Fourier spectra of deformed fringe image. We give the theoretical analysis of the proposed method. The experiment is carried out to demonstrate that the method can eliminate zero component and extend the measurable slope of height variation to nearly three times of the conventional FTP.     

数字全息用于光波衍射传播理论教学研究

数字全息技术是基于传统光学全息原理,借助于光电探测和数字处理技术,通过单次曝光记录和再现物光场的振幅和相位信息,近年来被广泛研究和应用.将数字全息应用于光波衍射传播理论教学中,不仅能够使学生在学习过程中形象、直观的理解光波衍射传播理论,而且能够通过编程计算完成数字全息图的数值再现,锻炼他们的知识运用能力,从而拓宽专业知识面、了解科学的发展前沿.     

显微数字全息中物光波前重建方法研究和比较

根据全息理论和线性系统理论,采用离轴无透镜傅里叶变换全息记录光路,对利用菲涅耳近似法、基于瑞利—索末菲衍射积分的卷积法以及角谱理论方法数值重建全息图进行了比较研究,并做了计算机模拟验证.结果表明：菲涅耳近似法和角谱方法重建像质比较好,且菲涅耳方法重建速度快;在记录距离极小的情况下,尽管记录距离不满足通常的菲涅耳近似条件,菲涅耳近似公式仍然成立;自由空间脉冲响应的快速傅里叶变换的性质与距离有关,由卷积方法得到的再现像只在某一特定距离下比较理想;对于极小物场、大孔径显微数字全息来说,菲涅耳近似重建方法是较为有效的方法.     

数字显微全息重建图像的景深扩展研究

显微物镜的景深问题限制数字显微全息在大纵深视场中的应用. 本文充分利用数值重建的特点, 采取低频和高频系数子图上的最大亮度梯度的局部方差作为聚焦判据, 在小波分解域内对显微全息重建图像的景深扩展问题进行了研究. 对倾斜的连续物体碳纤维进行三维重建, 分析了重建距离与直径测量误差的关系. 以超声波雾化器生成的微液滴颗粒场为例, 对离散颗粒场的重建图像进行了景深扩展. 利用基于广义洛伦兹-米散射理论的模型分别模拟1-15 μm 的非透明与透明离散颗粒的显微全息图, 分析了该方法重建的颗粒场的纵深定位误差与夫琅禾费系数的关系, 对比了非透明与透明颗粒纵深定位误差的异同点. 实验和模拟结果显示出该方法对于连续物体和离散颗粒场的显微全息重建图像的景深扩展能力, 且能由此准确重建物体信息.