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.     

Digital shearing method for three-dimensional data extraction in holographic particle image velocimetry

We report a new digital shearing method for extracting the three-dimensional displacement vector data from double-exposure holograms. With this method we can manipulate both the phase and the amplitude of the recorded signal, which, like optical correlation analysis, is inherently immune to imaging aberration. However, digital shearing is not a direct digital implementation of optical correlation, and a considerable saving in computation time results. We demonstrate the power of the method by MATLAB simulation and discuss its performance with reference to optical analysis.     

Digital holographic particle validation via complex wave

The inability to distinguish between particle images and noise in holographic reconstruction of dense particle fields hampers the advancement of holographic particle diagnostic techniques including holographic particle image velocimetry. We developed a method to separate particles from the noise by unlocking a unique particle signature in the complex reconstructed field. This complex-wave signature is present in digital particle holograms recorded at any scattering angle. Simulations of single and multiple particle holograms, as well as preliminary laboratory particle-field experiments, not only demonstrated the existence of the particle signature but also evaluated its ability to remove noise. Regardless of particle seeding density, scattering angle of hologram recording and particle size range, the particle identification/validation routine consistently provides >50% removal of "bad" particles and <8% of good particles.     

Hybrid digital holographic imaging system for three-dimensional dense particle field measurement

To apply digital holography to the measurement of three-dimensional dense particle fields in large facilities, we have developed a hybrid digital holographic particle-imaging system. The technique combines the advantages of off-axis (side) scattering in suppressing speckle noise and on-axis (in-line) recording in lowering the digital sensor resolution requirement. A camera lens is attached to the digital sensor to compensate for the weak object wave from side scattering over a large recording distance. A simple numerical reconstruction algorithm is developed for holograms recorded with a lens without requiring complex and impractical mathematical corrections. We analyze the effect of image sensor resolution and off-axis angle on system performance and quantify the particle positioning accuracy of the system. The holographic system is successfully applied to the study of inertial particle clustering in isotropic turbulence.     

Digital color management in full-color holographic three-dimensional printer

We propose a new method of color management for a full-color holographic, three-dimensional (3D) printer, which produces a volume reflection holographic stereogram using red, green, and blue three-color lasers. For natural color management in the holographic 3D printer, we characterize its color reproduction characteristics based on the spectral measurement of reproduced light. Then the color conversion formula, which comprises a one-dimensional lookup table and a 3×3 matrix, was derived from the measurement data. The color reproducibility was evaluated by printing a color chart hologram, and the average CIELAB ΔE=13.19 is fairly small.     

Theoretical analysis of three-dimensional imaging and recognition of micro-organisms with a single-exposure on-line holographic microscope

Single-exposure on-line (SEOL) digital holography is a recently proposed technique for monitoring, visualization, and recognition of three-dimensional (3D) objects. In contrast to traditional multi-exposure on-line digital holography, it uses only one exposure, which makes it particularly suitable for imaging and recognizing moving micro-organisms. However, the cost of using only one exposure is the superposition of a conjugate image on the desired reconstructed image. The influence of the conjugate image on the visualization and recognition performance is investigated. The conditions for which the cross-talk noise induced by the conjugate image is negligible are derived. It is demonstrated that with conditions common in imaging of microscopic 3D biological objects, SEOL digital holography is highly tolerant of cross-talk noise induced by the conjugate image.     

Parameter-optimized digital holographic microscope for high-resolution living-cell analysis

A parameter-optimized off-axis setup for digital holographic microscopy is presented for simultaneous, high-resolution, full-field quantitative amplitude and quantitative phase-contrast microscopy and the detection of changes in optical path length in transparent objects, such as undyed living cells. Numerical reconstruction with the described nondiffractive reconstruction method, which suppresses the zero order and the twin image, requires a mathematical model of the phase-difference distribution between the object wave and the reference wave in the hologram plane. Therefore an automated algorithm is explained that determines the parameters of the mathematical model by carrying out the discrete Fresnel transform. Furthermore the relationship between the axial position of the object and the reconstruction distance, which is required for optimization of the lateral resolution of the holographic images, is derived. The lateral and the axial resolutions of the system are discussed and quantified by application to technical objects and to living cells.     

Multichannel holographic recording method for three-dimensional displays

A multichannel holographic recording method is presented for three-dimensional (3D) displays, utilizing pixel-based recording instead of image-based recording in order to realize parallel processing. The proposed approach is composed of two main stages. In the first stage, each two-dimensional (2D) image acquired from multiple viewpoints is partitioned by holographic recording channels (HRC) into nonoverlapping subimages. In the second stage, the corresponding pixels of the subimages are rearranged to constitute an encoding image. The encoding images are recorded simultaneously by each HRC, respectively, so the recording speed is improved significantly. The experimental results have demonstrated that the three-channel system is feasible and the full-parallax hologram reconstructed with white light is acceptable in quality. The three-channel system saves approximately 60% of the recording time in comparison with the single-channel system. More importantly, the proposed method can accomplish a large-scale final hologram composed of multichannel holograms without sacrificing the hologram quality. Several 3D imaging applications such as medical diagnosis and advertisements could benefit from this research.     

Airborne digital holographic system for cloud particle measurements

An in-line holographic system for in situ detection of atmospheric cloud particles [Holographic Detector for Clouds (HOLODEC)] has been developed and flown on the National Center for Atmospheric Research C-130 research aircraft. Clear holograms are obtained in daylight conditions at typical aircraft speeds of 100 m s(-1). The instrument is fully digital and is interfaced to a control and data-acquisition system in the aircraft via optical fiber. It is operable at temperatures of less than -30 degrees C and at typical cloud humidities. Preliminary data from the experiment show its utility for studies of the three-dimensional spatial distribution of cloud particles and ice crystal shapes.     

In-line holographic particle image velocimetry for turbulent flows

A holographic system has been developed to measure the velocity field in three-dimensional flow regions. The system records the position of small tracer particles on two in-line holograms of the flow obtained simultaneously. Two exposures are recorded on each hologram. The flow velocity is derived from the displacement of the particles between exposures. A general design procedure is described for selecting the particle diameter and the concentration on the basis of the configuration of the flow facility and the resolution characteristics of the holographic imaging system. The system was implemented in a 2 ft x 2 ft (1 ft = 30.48 cm) water channel to measure the velocity field in a turbulent free-surface jet. The spatial resolution of the system is 1 mm, and the field of view is 100 mm, approximately. Measurements performed with this system are compared with results reported in the literature and are found to be in good agreement.     

Automatic threshold technique for holographic particle field characterization

The 3D distribution of a particle field by digital holography is obtained by 3D numerical reconstruction of a 2D hologram. The proper identification of particles from the background during numerical reconstruction influences the overall effectiveness of the technique. The selection of a suitable threshold value to segment particles from the background of reconstructed images during 3D holographic reconstruction process is a critical issue, which influences the accuracy of particle size and number density of reconstructed particles. The object particle field parameters, such as depth of sample volume and density of object particles, influence the optimal threshold value. The present study proposes a novel technique for the determination of the optimal threshold value of a reconstructed image. The effectiveness of the proposed technique is demonstrated using both simulated and experimental data. The proposed technique is robust to variation in optical properties of particle and background, depth of sample volume, and number density of object particle field. The particle diameter obtained from the proposed threshold technique is within 5% of that obtained from the particle size analyzer. There is a maximum ten times increase in reconstruction effectiveness by using the proposed automatic threshold technique in comparison with the fixed manual threshold technique.     

粒子群算法在超声三维坐标测量中的应用

提出一种基于粒子群算法的三维坐标反演模型,实现了超声波三坐标测量中三维坐标的高精度反演,提高了坐标反演迭代过程的鲁棒性.同时介绍了超声波测距系统的原理,推导了三维坐标反演的目标函数,建立了基于粒子群算法的反演模型,并进行了实验研究.通过软件仿真,对粒子群算法在超声波三维测量上的可行性和鲁棒性进行了验证,并搭建超声波三维定位平台,在1m×1m×2m空间中进行了测量实验.结果表明,测量的绝对误差小于10mm,具有较高的反演精度.     

In-line recording and off-axis viewing technique for holographic particle velocimetry

Prior approaches (e.g., off-axis holography) to overcoming the limitations of in-line holography for particle fields, namely, intrinsic speckle noise and depth resolution, involved an increased complexity of the optical system. The in-line recording and off-axis viewing (IROV) technique employs a single laser beam to record an in-line hologram, which is then viewed off axis during reconstruction. The signal-to-noise ratio and depth resolution of IROV are higher than conventional in-line holography by an order of magnitude and are comparable with off-axis holography. IROV is a much simpler approach than off-axis holography and is highly promising for holographic particle velocimetry. Measurements of the three dimensional flow velocity field of a vortex ring obtained by an IROV-based holographic particle velocimetry system are presented.     

Characterization of the DuPont photopolymer for three-dimensional holographic storage

DuPont's HRF-150 photopolymer film is investigated for use in three-dimensional holographic memories. Measurements of sensitivity, hologram persistence, the lateral spread of the photoinitiated reaction, and the variation of diffraction efficiency with modulation depth, spatial frequency and tilt angle, and intensity are reported. We observed that the diffraction efficiency of the HRF-150 photopolymer for a given exposure decreases with increases in intensity and grating tilt angle. The holograms were nondestructively reconstructed for long periods of time at room temperature. The photoinitiated reaction spread less than 100 μm over a period of 16 h.     

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.     

A simple method for measuring thoron spatial distributions

A simple but effective method that allows the measurement of the 220Rn spatial distribution in working or living environments using a solid-state detector is presented in this paper. The method is based on measurements of the alpha particles emitted by 216Po (the first 220Rn progeny) directly deposited on the detector surface at different distances from a 220Rn exhalation source. The validity of the method is shown by comparing the results of an experiment, where the 220Rn activity concentration is measured under conditions of diffusion at constant temperature, with finite-element calculations.     

Two-dimensional wavelet transform and application to holographic particle velocimetry

The goal of holographic particle velocimetry is to infer fluid velocity patterns from images reconstructed from doubly exposed holograms of fluid volumes seeded with small particles. The advantages offered by in-line holography in this context usually make it the method of choice, but seeding densities sufficient to achieve high spatial resolution in the sampling of the velocity fields cause serious degradation, through speckle, of the signal-to-noise ratio in the reconstructed images. The in-line method also leads to a great depth of field in paraxial viewing of reconstructed images, making it essentially impossible to estimate particle depth with useful accuracy. We present here an analysis showing that these limitations can be circumvented by variably scaled correlation, or wavelet transformation. The shift variables of the wavelet transform are provided automatically by the optical correlation methodology. The variable scaling of the wavelet transform derives, in this case, directly from the need to accommodate varying particle depths. To provide such scaling, we use a special optical system incorporating prescribed variability in spacings and focal length of lenses to scan through the range of particle depths.

Calculation shows, among other benefits, improvement by approximately two orders of magnitude in depth resolution. A much higher signal-to-noise ratio together with faster data extraction and processing should be attainable.

     

Microdefocusing method for measuring acoustic properties using acoustic microscope

Many papers have been reporting on measuring acoustic properties of materials by acoustic microscopy. In a conventional method of V (z) curve analysis, the phase velocity and the propagation attenuation of a leaky surface acoustic wave (LSAW) are determined from the interference period Deltaz and the slope of the V(z) curve, respectively. For this method it is necessary to measure the V(z) curve for a period several times as long as the interference period Deltaz. Therefore, it is difficult to measure the acoustic properties of a sample with high resolution by the method. In order to overcome these problems, a method called the microdefocusing method is proposed. The method determines the acoustic properties of a sample by analyzing V (z) values measured in the microdefocusing region within an interference period Deltaz near a focal plane. An ultrasonic transducer called the butterfly transducer is proposed to be applied to this microdefocusing method and a digital signal processing procedure is developed to analyze the output of the ultrasonic transducer. Basic experiments are performed to confirm the principles of the new method.