In-line Holography Research Articles

Dual-beam interferometric particle imaging for size and shape characterization of irregular coal micro-particle: Validation with digital inline holography

The size and shape characterization of irregular particles is of great significance in the comprehensive understanding of many natural and industrial processes. The dual-beam interferometric particle imaging (DIPI) technique is adapted from a dual-beam backward illumination configuration of interferometric particle imaging to access the size and 2D shape of an irregular particle from its interferogram. An experimental setup combining the digital inline holography (DIH) and DIPI techniques characterizes the same projection of the same particle with a color camera, permitting the accuracy evaluation and validation between these two techniques. The comparison results show a good agreement between the 2D-Fourier transform of DIPI interferogram and the 2D-autocorrelation of DIH reconstructed in-focus image. The DIH gives the reference result for the size measurement of the particles, and we observe a maximum difference as low as 9% with the DIPI measurement. The backward arrangement of the dual-beam interferometric particle imaging technique facilitates its further instrumental development and application to a variety of real circumstances, e.g., solid fuel particles and ice crystals.

Multi-height phase recovery combined with Fast Iterative Shrinkage-Thresholding Algorithm to handle phase microspheres from complex diffracted field in inline holography

ABSTRACTWe describe in this work an approach that individually combines constant initialization, free-space backpropagation, transport of intensity equation (TIE) and multi-height phase recovery with Fast Iterative Shrinkage-Thresholding Algorithm (FISTA) to predict reconstruction of the phase microspheres from diffracted field measurements. We incorporate appropriate initialization and regularization to reconstruct unknown phase microspheres (transparent objects).

Exploration of Bragg in-line electron holography as a possible tool for crystal structure determination

Exploration of Bragg in-line electron holography as a possible tool for crystal structure determination

Automated droplet size distribution measurements using digital inline holography

Droplet generation through spray breakup is an unsteady and non-linear process which produces a relatively dense, highly polydisperse aerosol containing non-spherical droplets with sizes spanning several orders of magnitude. Such variability in size and shape can lead to significant sources of error for conventional measurements based on laser scattering. Although direct imaging of droplets can potentially overcome these limitations, imaging suffers from a shallow depth of field as well as occlusions, which prevents the complete spray from being analyzed. In comparison, digital inline holography (DIH), a low cost coherent imaging technique, can enable high resolution imaging of the sample over an extended depth of field, typically several orders of magnitude larger than traditional imaging. In this study, we showcase an automated DIH imaging system for characterizing monodisperse and polydisperse aerosol droplet size and shape distributions in the 20 μm–3 mm diameter range, over a large sample volume. The high accuracy of the technique is demonstrated by measurements of monodisperse droplets generated by a vibrating orifice aerosol generator (VOAG), achieving a peak resolution of ∼14.2. Measurements of a polydisperse spray from a flat fan nozzle serve to establish the versatility of DIH in extracting a two-dimensional size-eccentricity distribution function, which indicates a strong semilogarithmic scaling between the two parameters, that decays as the droplet migrates away from the nozzle. Due to its low cost, compact setup as well as high density of data obtained, DIH can serve as a promising approach for future aerosol characterization.

Primary Fragmentation Behavior Investigation in Pulverized Coal Combustion with High-Speed Digital Inline Holography

Comprehension on the primary fragmentation of coal particles is important in understanding and optimizing practical coal combustion and creating credible combustion models. High-speed digital inlin...

Cavities Imaged by In-Line Electron Holography in Irradiated Aluminium Alloy

Cavities Imaged by In-Line Electron Holography in Irradiated Aluminium Alloy

Comparison of simulation and experiments for multimode aerodynamic breakup of a liquid metal column in a shock-induced cross-flow

While the mechanisms that drive breakup and aerodynamic dispersion of traditional liquids such as water have been extensively studied, it is not yet clear if models for traditional liquids can be used to accurately describe the behavior of molten metals. In this paper, multiphase simulations with the interface-capturing combined level-set volume-of-fluid approach are used to provide time-resolved morphology and breakup data for a liquid column subject to a shock-induced cross-flow. For the first time, numerical simulation of the behavior of a liquid metal (Galinstan alloy composed of gallium, indium, and tin) is compared to the well-documented behavior of water. Simulations consider a gas cross-flow Weber number between 10 and 12, which produces a multimode breakup morphology consisting of multiple baglike structures. Up to bag breakup, we confirm that the deformation rate of Galinstan follows the same dependence on the gas cross-flow Weber number as ordinary liquids when time is nondimensionalized by including the liquid-gas density ratio. Moreover, we determine that the appearance of a central stem along the column upstream surface in multimode bag breakup is consistent with the occurrence of Rayleigh-Taylor instability. We also resolve bag stretching and fragmentation, to the full extent allowed by our computational resources, and carry out a direct comparison with the measurements of size and velocity of secondary droplets from high-speed digital inline holography. For Galinstan, we illustrate the differences between simulation and experiment that emerge because of the modification of the surface properties of the metal exposed to air.

Dual-beam interferometric particle imaging for size measurement of opaque metal droplet

Size measurement of micron-sized metal droplet is of great demand in industrial applications, e.g., 3D printing and metal propellant. A dual-beam interferometric particle imaging (DIPI) technique is developed to measure the size of opaque metal droplet, which is adapted from the traditional interferometric particle imaging (IPI) for transparent particles. In DIPI, the studied metal droplet is illuminated by two laser beams from different views, and the laser beams are reflected on the surface of the droplet. An interference pattern of the two reflections is yielded in the far-field that was capable to derive the size of aim droplet. The principle of DIPI is formulated in the framework of physical optics, with system configuration, modeling and data processing algorithm presented. A DIPI system is established to characterize the gallium droplet with size ranging from 300 μm to 500 μm experimentally, and the measured sizes with DIPI is compared with those obtained simultaneously by digital inline holography, which is a standard tool for particle measurement. Comparison shows that the measurements with these two methods are highly consistent, with a standard deviation of 1.7%. DIPI has been demonstrated with high measurement accuracy and flexible implementation of the one-sided arrangement.

A hybrid image processing method for measuring 3D bubble distribution using digital inline holography

The paper presents a hybrid bubble hologram processing approach for measuring the size and 3D distribution of bubbles over a wide range of size and shape. The proposed method consists of five major steps, including image enhancement, digital reconstruction, small bubble segmentation, large bubble/cluster segmentation, and post-processing. Two different segmentation approaches are proposed to extract the size and the location of bubbles in different size ranges from the 3D reconstructed optical field. Specifically, a small bubble is segmented based on the presence of the prominent intensity minimum in its longitudinal intensity profile, and its depth is determined by the location of the minimum. In contrast, a large bubble/cluster is segmented using a modified watershed segmentation algorithm and its depth is measured through a wavelet-based focus metric. Our processing approach also determines the inclination angle of a large bubble with respect to the hologram recording plane based on the depth variation along its edge on the plane. The accuracy of our processing approach on the measurements of object size and 3D distributions are assessed through synthetic bubble holograms and oil droplet holograms from an experiment separately. In addition, we evaluate the ability of this algorithm to estimate the bubble inclination with respect to the hologram recording plane through measuring a 3D-printed physical target of pillars with different inclination angles. The holographic measurement technique is further implemented to capture the fluctuation of instantaneous gas leakage rate from a ventilated supercavity generated in a water tunnel experiment. Overall, our paper introduces an inexpensive and compact solution for high resolution characterization of bubbles and other particles in multiphase flows from a broad range of applications.

Twin image reduction method for digital in-line holography by using a digital periphery

Measuring the complex field of objects with a single intensity recording is desirable. Obviously, digital in-line holography can realize it with a compact and less expensive method. However, the existence of the twin image degrades the imaging quality seriously. Here, we propose a method that can eliminate the twin image and improve the performance of digital in-line holography by employing a digital periphery. In the proposed method, the periphery is obtained by introducing a diffuser in the optical system and digital image processing technology. And the twin image is removed by the phase retrieval algorithm which uses the periphery as a boundary condition. The method can realize quantitative imaging complex samples in real-time with a compact optical system. Results of numerical simulations are presented, which validate satisfactory performances of the proposed method.

Holographic particle localization under multiple scattering

We introduce a novel framework that incorporates multiple scattering for large-scale 3D particle-localization using single-shot in-line holography. Traditional holographic techniques rely on single-scattering models which become inaccurate under high particle-density. We demonstrate that by exploiting multiple-scattering, localization is significantly improved. Both forward and back-scattering are computed by our method under a tractable recursive framework, in which each recursion estimates the next higher-order field within the volume. The inverse scattering is presented as a nonlinear optimization that promotes sparsity, and can be implemented efficiently. We experimentally reconstruct 100 million object voxels from a single 1-megapixel hologram. Our work promises utilization of multiple scattering for versatile large-scale applications.

Regularized inverse holographic volume reconstruction for 3D particle tracking.

The key limitations of digital inline holography (DIH) for particle tracking applications are poor longitudinal resolution, particle concentration limits, and case-specific processing. We utilize an inverse problem method with fused lasso regularization to perform full volumetric reconstructions of particle fields. By exploiting data sparsity in the solution and utilizing GPU processing, we dramatically reduce the computational cost usually associated with inverse reconstruction approaches. We demonstrate the accuracy of the proposed method using synthetic and experimental holograms. Finally, we present two practical applications (high concentration microorganism swimming and microfiber rotation) to extend the capabilities of DIH beyond what was possible using prior methods.

Holographic deflection imaging measurement of electric charge on aerosol particles

We propose a simple noninvasive technique called holographic deflection imaging (HDI) to measure electric charge on individual aerosol particles. Particles in many flows such as atmospheric turbulence can carry electric charges, which add forces that compete with turbulence and affect particle dynamics. To measure particle charges, we devised a novel technique for polydispersed particles with bipolar charge distributions. The technique uses common optical hardware and a novel particle sizing algorithm for polydispersed size distributions. A charged particle sample is introduced into a vertical channel air flow and a horizontal electric field. For each particle, In-line digital holography is used to simultaneously track particle positions over successive holograms and decode size information from hologram interference fringes. This enables measurement of the electric-force-induced horizontal component of terminal velocity and the particle size. From velocity and size, particle charge is calculated using a force balance, assuming Stokes drag on spherical particles. Two experiments were conducted, one to validate HDI measurements of a bipolarly charged particle sample against an electrometer, and the other to demonstrate particle size effect on triboelectric charging of aerosols in a fan-driven isotropic turbulence chamber. The HDI setup was configured for each experiment, and measurement uncertainty was estimated. Experiment 1 found good agreement (< 5%) in mean charge between measurements by HDI and the electrometer. Experiment 2 found nearly symmetric bipolar charge distributions, which broadened with increasing particle surface area, as expected for triboelectric charging by the symmetrically installed fans.

Single-shot in-line holographic authentication using phase and amplitude modulation

In this paper, single-shot in-line holographic authentication using phase and amplitude modulation is presented. Amplitude modulation and phase modulation are presented and conducted at reference wave path to obtain noise-like reference wavefront and to generate the parameters (masks) at reference wave path as security keys for single-shot in-line digital holography, and its application, i.e., optical authentication, is presented to show validity of the proposed digital holographic system. A reference optical path using the cascaded random amplitude-only and phase-only masks is applied as an example. Only one in-line digital hologram is used for object reconstruction, and the recovered objects can be effectively authenticated. It is illustrated that parameters (e.g., masks) flexibly designed at reference wave path can facilitate wavefront retrieval in the CCD plane and also can act like security keys for optical information authentication. @ Elsevier, 2019.

Quantitative counting of Zn and O atoms by atomic resolution off-axis and in-line electron holography

We present quantitative counting of Zn and O atoms in zinc oxide epitaxial thin film by different routes; reconstruction of phase from the side and central bands of atomic resolution off-axis and in-line electron holography. Results show that the reconstructed phase from both the side and the central bands and the corresponding number of Zn (Z = 30) and O (Z = 8) atoms are in close agreement with the systematic increase in the number of atoms for a sample area less than the extinction distance. However, complete disagreement is observed for the sample area more than the extinction distance. On the other hand, the reconstructed phase obtained via in-line holography shows no systematic change with thickness for the same sample. Phase detection limits and the atomic model used to count the atoms are discussed.

Implementation of digital optical phase conjugation with embedded calibration and phase rectification

Focused and controllable optical delivery beyond the optical diffusion limit in biological tissue has been desired for long yet considered challenging. Digital optical phase conjugation (DOPC) has been proven promising to tackle this challenge. Its broad applications, however, have been hindered by the system’s complexity and rigorous requirements, such as the optical beam quality, the pixel match between the wavefront sensor and wavefront modulator, as well as the flatness of the modulator’s active region. In this paper, we present a plain yet reliable DOPC setup with an embedded four-phase, non-iterative approach that can rapidly compensate for the wavefront modulator’s surface curvature, together with a non-phase-shifting in-line holography method for optical phase conjugation in the absence of an electro-optic modulator (EOM). In experiment, with the proposed setup the peak-to-background ratio (PBR) of optical focusing through a standard ground glass in experiment can be improved from 460 up to 23,000, while the full width at half maximum (FWHM) of the focal spot can be reduced from 50 down to 10 μm. The focusing efficiency, as measured by the value of PBR, reaches nearly 56.5% of the theoretical value. Such a plain yet efficient implementation, if further engineered, may potentially boost DOPC suitable for broader applications.

SNR enhancement in in-line particle holography with the aid of off-axis illumination.

The digital in-line particle holography suffers from speckle noise, for which considerable efforts have been devoted in order to mitigate it by designing post-processing algorithms. This paper proposes a novel approach, which mitigates the speckle noise by increasing the signal-to-noise ratio (SNR). It involves the joint design of optical systems and post-processing algorithms, and enhances the SNR by combining several holograms captured under different illumination angles. The experimental results show that the proposed scheme performs better than the normal scheme in terms of SNR and false-detection-ratio.

Sparsity-based continuous wave terahertz lens-free on-chip holography with sub-wavelength resolution.

We demonstrate terahertz (THz) lens-free in-line holography on a chip in order to achieve 40 μm spatial resolution corresponding to ~0.7λ with a numerical aperture of ~0.87. We believe that this is the first time that sub-wavelength resolution in THz holography and the 40 μm resolution were both far better than what was already reported. The setup is based on a self-developed high-power continuous wave THz laser at 5.24 THz (λ = 57.25 μm) and a high-resolution microbolometer detector array (640 × 512 pixels) with a pitch of 17 μm. This on-chip in-line holography, however, suffers from the twin-image artifacts which obfuscate the reconstruction. To address this problem, we propose an iterative optimization framework, where the conventional object constraint and the L1 sparsity constraint can be combined to efficiently reconstruct the complex amplitude distribution of the sample. Note that the proposed framework and the sparsity-based algorithm can be applied to holography in other wavebands without limitation of wavelength. We demonstrate the success of this sparsity-based on-chip holography by imaging biological samples (i.e., a dragonfly wing and a bauhinia leaf).

Biology microscopy using well-distributed sphere digital in-line holography

Traditional pinhole spherical wave digital in-line holography has proved to be powerful imaging tools. Image quality is affected by uncertain round of pinhole. Here, we propose a well-distributed sphere wave generation method and it demonstrates wide field of view and high resolution microscopy. The laser focuses into an infinitesimal spot through laser beam expander and microscope objective. Pinhole permutation with different sizes is utilized to match the focal point, and emerges an ideal spherical wave. Interference fringes pattern, formed by reference sphere wave and scattered sphere wave of object, is collected by large area image sensor. The influence of dirty in image sensor and parasitic light is eliminated through subtraction with and without object. Fresnel inverse transformation reconstruction algorithm presents the object information. Biology microscopy experiments demonstrate that the proposed techniques increase the flexibility in producing well-distributed point light source and improve the image quality. Field of view is 3.22 mm×3.22 mm and resolution is 5.09 μm. Furthermore, adjustable field of view with magnification, fast, no-contact make it to be a promising tool in optical element measurement, material identification, biology and medicine.

Compressive holography with resolution improvement and lensless adjustable magnification

In lensless digital holography the resolving power is limited by factors such as the finite size of recorded area, pixel dimensions and sampling rate of digital acquisition system. These factors acts as a filter that discards high-frequency components with useful information about the analyzed object. In the present work, we propose a method to overcome these limitations to proving the resolution recovery in digital in-line holography by utilizing adjustable magnification in a compressive sensing framework. Here, we model the sensed field as low-pass filtered and projected on a Fresnel base with adjustable magnification and use sparse regularization in an inverse problem approach to image reconstruction. Our method enhances image reconstruction resolution from a single recorded hologram with magnification independent of parameters such as propagation distance and wavelength of light, which enables to be used in applications like multi-wavelength digital holography. In addition, this technique offers high noise immunity and eliminates typical problems of holographically reconstructed images such as twin image and speckle. Therefore, we demonstrate by simulations and experimentally results that our approach significantly improves the quality of reconstructed images when compared with conventional techniques.