Characterization of microlenses by digital holographic microscopy

We demonstrate the use of digital holographic microscopy (DHM) as a metrological tool in micro-optics testing. Measurement principles are compared with those performed with Twyman-Green, Mach-Zehnder, and white-light interferometers. Measurements performed on refractive microlenses with reflection DHM are compared with measurements performed with standard interferometers. Key features of DHM such as digital focusing, measurement of shape differences with respect to a perfect model, surface roughness measurements, and optical performance evaluation are discussed. The capability of imaging nonspherical lenses without any modification of the optomechanical setup is a key advantage of DHM compared with conventional measurement tools and is demonstrated on a cylindrical microlens and a square lens array.     

Recognition and classification of red blood cells using digital holographic microscopy and data clustering with discriminant analysis

We propose to apply statistical clustering algorithms on a three-dimensional profile of red blood cells (RBCs) obtained through digital holographic microscopy (DHM). We show that two classes of RBCs stored for 14 and 38 days can be effectively classified. Two-dimensional intensity images of these cells are virtually the same. DHM allows for measurement of the RBCs' biconcave profile, resulting in a discriminative dataset. Two statistical clustering algorithms are compared. A model-based clustering approach classifies the pixels of an RBC and recognizes the RBC as either new or old based. The K-means algorithm is applied to the four-dimensional feature vector extracted from the RBC profile.     

Parallel two-step phase-shifting digital holograph microscopy based on a grating pair

An optical configuration for parallel two-step phase-shifting digital holographic microscopy (DHM) based on a grating pair is proposed for the purpose of real-time phase microscopy. Orthogonally circularly polarized object and reference waves are diffracted twice by a pair of gratings, and two parallel copies for each beams come into being. Combined with polarization elements, parallel two-step phase-shifting holograms are obtained. Based on the proposed configuration, two schemes of DHM, i.e., slightly off-axis and on-axis DHM, have been implemented. The slightly off-axis DHM suppresses the dc term by subtracting the two phase-shifting holograms from each other, thus the requirement on the off-axis angle and sampling power of the CCD camera is reduced greatly. The on-axis DHM has the least requirement on the resolving power of the CCD camera, while it requires that the reference wave is premeasured and its intensity is no less than 2 times the maximal intensity of the object wave.     

Digital holographic microscopy with sequential off-axis illumination directions based on spatial light modulator

Digital holographic microscopy (DHM) with time-sequential off-axis illumination directions can synthesize a large aperture, and thus have a higher spatial resolution than the one with on-axis illumination. In this paper, time-sequential off-axis illumination directions are generated by a spatial light modulator (SLM) in DHM, and the residual phases along different illumination directions are suppressed by using the phase compensation technique, as such the image with resolution enhancement is reconstructed. The usage of SLM enables shifting the illumination for different orientations and phase shifts without mechanical motion. The experiments have been conducted to verify the feasibility of this method on the residual phase suppression.     

Comparative study of various endoscopes for pulsed digital holographic interferometry

A comparison of several endoscopes as object image carriers in pulsed digital holography is presented. Three multicore flexible fiber endoscopes of different spatial resolution and one rigid endoscope are investigated. The four endoscopes are integrated in a setup for the recording of digital holograms on a CCD camera. A double-pulsed ruby laser is used as the light source. A spatial carrier is introduced by an off-axis reference beam, which permits quantitative evaluation of the phase difference between two holograms recorded with a short time separation (5-600 micros). From reported studies it may be inferred that the quality of the phase maps so derived from digital holographic interferometry has a strong correlation to the spatial resolution of the multicore fiber used in these endoscopes. With the endoscopic technique combined with pulsed digital holography a number of useful applications (in areas such as medical endoscopy, micromechanics, and microelectronics) are envisaged for which access to the objects of interest is otherwise difficult.     

Comparative study with double-exposure digital holographic interferometry and a shack-hartmann sensor to characterize transparent materials

We compare wave-front measurements using double-exposure digital holography and a Shack-Hartmann sensor. A voltage-driven liquid-crystal wedge modulates the optical wave front and provides a refractive-index gradient typical of interesting transparent materials. Measurement accuracy and reliability are similar for both methods. In our opinion, digital holographic interferometry has several advantages for both laboratory and field environments. When compared with Shack-Hartmann methods, these advantages include hardware simplicity and robustness, relative insensitivity to sample dynamic range, and less computational demanding and more straightforward data evaluation algorithms. We believe that digital holography provides the methodology of choice for field studies of transparent materials such as microgravity protein crystal growth experiments.     

Fluid surface compensation in digital holographic microscopy for topography measurement

A novel technique is presented for surface compensation and topography measurement of a specimen in fluid medium by digital holographic microscopy (DHM). In the measurement, the specimen is preserved in a culture dish full of liquid culture medium and an environmental vibration induces a series of ripples to create a non-uniform background on the reconstructed phase image. A background surface compensation algorithm is proposed to account for this problem. First, we distinguish the cell image from the non-uniform background and a morphological image operation is used to reduce the noise effect on the background surface areas. Then, an adaptive sampling from the background surface is employed, taking dense samples from the high-variation area while leaving the smooth region mostly untouched. A surface fitting algorithm based on the optimal bi-cubic functional approximation is used to establish a whole background surface for the phase image. Once the background surface is found, the background compensated phase can be obtained by subtracting the estimated background from the original phase image. From the experimental results, the proposed algorithm performs effectively in removing the non-uniform background of the phase image and has the ability to obtain the specimen topography inside fluid medium under environmental vibrations.     

Digital holographic microscopy for live cell applications and technical inspection

Digital holographic microscopy enables a quantitative phase contrast metrology that is suitable for the investigation of reflective surfaces as well as for the marker-free analysis of living cells. The digital holographic feature of (subsequent) numerical focus adjustment makes possible applications for multifocus imaging. An overview of digital holographic microscopy methods is described. Applications of digital holographic microscopy are demonstrated by results obtained from livings cells and engineered surfaces.     

抗几何攻击的强鲁棒性全息水印研究

目的寻找一种能够抵抗常见几何攻击的安全的水印算法。方法通过分析常见的水印算法,发现傅里叶-梅林变换域水印能够较好地满足抵抗常见几何攻击的需求。对水印图像进行双随机相位全息加密,通过密钥保障水印的安全性,通过把加密的水印嵌入到傅里叶-梅林变换域的方法,设计对比试验,研究含有水印的图像在平移、缩放、旋转、中值滤波、高斯噪声、椒盐噪声、压缩、打印扫描、打印拍摄攻击下提取水印质量的影响。结果水印能够抵抗常见的几何攻击,特别对平移、缩放、旋转组合几何攻击、打印扫描和打印拍摄有很好的效果。结论基于傅里叶-梅林变换的全息水印算法具有很好的鲁棒性,能够抵抗常见的几何攻击;具有很好的安全性,没有密钥不能对水印进行检测和去除。     

改进的数字全息光弹方法理论研究

该文通过改变参考光偏振方向,对数字全息光弹法进行改进,从而可以高效、高精度的检测二维应力场.从理论上导出了该方法的实验原理:加载前后在对应的4 个不同参考光偏振位置拍摄数字全息图,对这4 对数字全息图进行傅里叶变换、滤波及反变换,可以获得4 个光强方程,数值求解这4 个光强方程,再结合应力-光学定律就可以精确获得二维应力场的2 个主应力值和主应力方向.与传统的数字全息光弹法相比,新方法具有以下优点:不需要分离等差线与等倾线条纹;不需要确定条纹级数;方法简单,不需要已有技术以外的实验装置或实验模型.     

Border processing in digital holography by extension of the digital hologram and reduction of the higher spatial frequencies

When a digital holographic reconstruction is performed, digital diffraction effects occur at the borders when the hologram amplitudes at the two opposite border points are different on each vertical or horizontal line. We propose a method of digital hologram extension to reduce such diffraction effects. The method consists of extending the size of the digital hologram and of filling the extended part by complex values that minimize, according to a numerical criterion, the highest spatial frequencies. The theoretical aspects of the method are given and the results from a demonstration are provided.     

Three-dimensional identification of stem cells by computational holographic imaging.

We present an optical imaging system and mathematical algorithms for three-dimensional sensing and identification of stem cells. Data acquisition of stem cells is based on holographic microscopy in the Fresnel domain by illuminating the cells with a laser. In this technique, the holograms of stem cells are optically recorded with an image sensor array interfaced with a computer and three-dimensional images of the stem cells are reconstructed from the Gabor-filtered digital holograms. The Gabor wavelet transformation for feature extraction of the digital hologram is performed to improve the process of identification. The inverse Fresnel transformation of the Gabor-filtered digital hologram is performed to reconstruct the multi-scale three-dimensional images of the stem cells at different depths along the longitudinal direction. For recognition and classification of stem cells, a statistical approach using an empirical cumulative density function is introduced. The experiments indicate that the proposed system can be potentially useful for recognizing and classifying stem cells. To the best of our knowledge, this is the first report on using three-dimensional holographic microscopy for automated identification of stem cells.     

Microlens characterization by digital holographic microscopy with physical spherical phase compensation

Microlenses have been characterized by a digital holographic microscopy system, which is immune to the inherent wavefront aberration. The digital holographic microscopy system takes advantage of fiber optics and uses the light emitted directly from a single-mode fiber as the recording reference wave. By using such a reference beam, which is quasi-identical to the object beam, the inherent wavefront aberration of the digital holographic microscope is removed. The alignment of the optical setup can be optimized with the help of numerical reconstruction software to give the system phase with the off-axis tilt removed. There is one, and only one, reference fiber point position to give a reference wavefront that is quasi-identical to the object wavefront where the system is free of wavefront aberration and directly gives the quantitative phase of the test object without the need for complicated numerical compensation.     

Intrinsic speckle noise in off-axis particle holography

In holographic imaging of particle fields, the interference among coherent wave fronts associated with particle scattering gives rise to intrinsic speckle noise, which sets a fundamental limit on the amount of information that particle holography can deliver. It has been established that the intrinsic speckle noise is especially severe in in-line holography because of superposition of virtual image waves, the direct transmitted wave, and the real image. However, at sufficiently high particle number densities, such as those typical in holographic particle image velocimetry (HPIV) applications, intrinsic speckle noise also arises in off-axis particle holography from self-interference among wave fronts that form the real image of particles. To overcome the latter problem we have constructed a mathematical model that relates the first- and second-order statistical properties of the intrinsic speckle noise to relevant holographic system parameters. Consistent with our experimental data, the model provides a direct estimate of the information capacity of particle holography. We show that the noise-limited information capacity can be expressed as the product of particle number density and the extent of the particle field along the optical axis. A large angular aperture of the hologram contributes directly to achievement of high information capacity. We also show that filtering in either digital or optical form is generally ineffective in removing the intrinsic speckle noise from the particle image as a result of the similar spectral properties of the two. These findings emphasize the importance of angular aperture in designing holographic particle imaging systems.     

Partially coherent light-emitting diode illumination for video-rate in-line holographic microscopy

The light of a light-emitting diode or a common thermal source, such as a tungsten filament lamp, is known to be quasi-incoherent. We generated partially coherent light of these sources with a volume of coherence in the micrometer range of 5-100 μm3 by spatial and spectral filtering. The corresponding degree of partial coherence was adapted for microscopic interference setups, such as a digital in-line holographic microscope. The practicability of the sources was determined by the spectral emittance and the resulting signal-to-noise ratio (SNR) of the detector. The microscale coherence in correlation with the SNR and its resolution for microscopy were analyzed. We demonstrate how low-light-level, non-laser sources enable holographic imaging with a video frame rate (25 frames/s), an intermediate SNR of 8 dB, and a volume of coherence of 3.4×10(4) μm3. Holograms of objects with a lateral resolution of 1 μm were achieved using a microscope lens (50×/NA=0.7) and a CCD camera featuring a 4-12 bit dynamic range.     

Digital holographic microscopy with reduced spatial coherence for three-dimensional particle flow analysis

We investigate the use of a digital holographic microscope working in partially coherent illumination to study in three dimensions a micrometer-size particle flow. The phenomenon under investigation rapidly varies in such a way that it is necessary to record, for every camera frame, the complete holographic information for further processing. For this purpose, we implement the Fourier-transform method for optical amplitude extraction. The suspension of particles is flowing in a split-flow lateral-transport thin separation cell that is usually used to separate the species by their sizes. Details of the optical implementation are provided. Examples of reconstructed images of different particle sizes are shown, and a particle-velocity measurement technique that is based on the blurred holographic image is exploited.     

Photoelastic digital holographic polariscope

A new digital holographic polariscope is developed for measuring isochromatic and isopachic fringe systems of stress-induced birefringent and non-birefringent materials. Our experimental results show that conventional circular polariscope could not record isopachic fringes for stress-induced non-birefringent material whereas the proposed digital holographic microscope clearly records such fringes. Detailed theoretical and experimental analysis is reported.     

Homogenized Fourier transform holographic data storage using phase spatial light modulators and methods for recovery of data from the phase image

We present a method for homogenizing the Fourier spectrum for holographic digital data storage by use of a phase spatial light modulator (SLM), and methods for the recovery of data from a phase image are implemented and discussed. Binary digital data are displayed on a phase SLM operating in 0 and pi phase modes to optimally remove the intense dc peak so as to obtain a homogenized Fourier spectrum. Methods based on holographic interferometry have been developed and employed for recovery of the original amplitude data page from the phase-data page. A new edge-detection-based method also has been demonstrated and analyzed for reconstruction of the original data. Experimental results are presented to confirm the feasibility of these novel techniques.     

Reconstruction domain compression of phase-shifting digital holograms

Phase-shifting digital hologram compression has been mainly studied in the recording domain, where data possess a rather randomlike appearance, yielding reduced compression efficiency. We carry out the compression of such data in the reconstruction domain, which benefits from the spatial correlation of the data yielding, increased efficiency. Real holographic data are used to demonstrate the performance of the new approach. It is also shown that the reconstruction is not limited to the initially obtained view, as additional views can still be obtained with appropriate postprocessing.     

Phase-conjugate holographic lithography based on micromirror array recording

We present phase-conjugate holographic lithography with a hologram recorded by a digital micromirror device (DMD) and a telecentric lens. In our lithography system, a phase-conjugate hologram is applied instead of conventional masks or reticles to form patterns. This method has the advantage of increasing focus range, and it is applicable to the formation of patterns on fairly uneven surfaces. The hologram pattern is dynamically generated by the DMD, and its resolution is mainly determined by the demagnification of the telecentric lens. We experimentally demonstrate that our holographic lithographic system has a large focus range, and it is feasible to make a large-area hologram by stitching each pattern generated by the DMD without a falling off in resolution.