Average speckle size as a function of intensity threshold level: comparisonof experimental measurements with theory

Smartphone-based holographic measurement of polydisperse suspended particulate matter with various mass concentration ratios

Real-time monitoring of suspended particulate matter (PM) has become essential in daily life due to the adverse effects of long-term exposure to PMs on human health and ecosystems. However, conventional techniques for measuring micro-scale particulates commonly require expensive instruments. In this study, a smartphone-based device is developed for real-time monitoring of suspended PMs by integrating a smartphone-based digital holographic microscopy (S-DHM) and deep learning algorithms. The proposed S-DHM-based PM monitoring device is composed of affordable commercial optical components and a smartphone. Overall procedures including digital image processing, deep learning training, and correction process are optimized to minimize the prediction error and computational cost. The proposed device can rapidly measure the mass concentrations of coarse and fine PMs from holographic speckle patterns of suspended polydisperse PMs in water with measurement errors of 22.8 ± 18.1% and 13.5 ± 9.8%, respectively. With further advances in data acquisition and deep learning training, this study would contribute to the development of hand-held devices for monitoring polydisperse non-spherical pollutants suspended in various media.

Ordering of binary colloidal crystals by random potentials

Structural defects are ubiquitous in condensed matter, and not always a nuisance. For example, they underlie phenomena such as Anderson localization and hyperuniformity, and they are now being exploited to engineer novel materials. Here, we show experimentally that the density of structural defects in a 2D binary colloidal crystal can be engineered with a random potential. We generate the random potential using an optical speckle pattern, whose induced forces act strongly on one species of particles (strong particles) and weakly on the other (weak particles). Thus, the strong particles are more attracted to the randomly distributed local minima of the optical potential, leaving a trail of defects in the crystalline structure of the colloidal crystal. While, as expected, the crystalline ordering initially decreases with an increasing fraction of strong particles, the crystalline order is surprisingly recovered for sufficiently large fractions. We confirm our experimental results with particle-based simulations, which permit us to elucidate how this non-monotonic behavior results from the competition between the particle-potential and particle-particle interactions.

Determination of laser beam focus position based on secondary speckles pattern analysis

Proper positioning of a laser beam focus is a universal problem for various applications that does not have a universal solution. Quite often the taken approach relies on some sort of a calibration and temporal stability of the laser and the optical train. While such an approach can be suitable for a large number of applications its applicability becomes limited in the cases where the laser beam properties uncontrollably change with time. The latter can occur due to the thermal effects, for example. In those cases, the laser focus positioning method should include direct analysis of the laser beam properties. In this contribution we present a simple optical method based on the secondary speckles pattern analysis suitable for determination of the absolute focal spot position. The method does not require any a priori knowledge of the laser beam properties and is suitable for various diffuse or partially diffuse surfaces of interest.

Assessment of incident intensity on laser speckle contrast imaging using a nematic liquid crystal spatial light modulator

Before laser speckle contrast imaging (LSCI) can be used reliably and quantitatively in a clinical setting, there are several theoretical and practical issues that still must be addressed. In order to address some of these issues, an electro-optical system that utilizes a nematic liquid crystal spatial light modulator (SLM) to mimic LSCI experiments was assembled. The focus of this paper is to address the issue of how incident intensity affects LSCI results. Using the SLM-based system, we systematically adjusted incident intensity on the SLM and assessed the resulting first- and second-order statistics of the imaged speckle to explain the corresponding spatial contrast values in both frozen and time-integrated speckle patterns. The SLM-based system was used to generate speckle patterns with a controlled minimum speckle size, probability intensity distribution, and temporal decorrelation behavior. By eliminating many experimental parameters, this system is capable of serving as a useful intermediary tool between computer simulation and physical experimentation for further developing LSCI as a quantitative imaging modality.

Performances of different subset shapes and control points in subset-based digital image correlation and their applications in boundary deformation measurement

Digital image correlation (DIC) is an effective and popular tool for displacement and strain measurements. In the standard subset-based algorithms, the center point of a subset is considered by default as the control point for calculation, and it is difficult to obtain the deformation information at the boundary. Proper selection of the subset shape and the location of control points are vital to the displacement calculation at the boundary. In this paper, registration accuracies of several typical types of subset shapes and different locations of control points are investigated. The results illustrate that different choices of subset shapes can greatly affect the registration accuracy, while different choices for the locations of control points have little impact on it. Based on these results, the noncentral algorithm is developed for the whole-field deformation measurement.

A simulation analysis of an extension of one-dimensional speckle correlation method for detection of general in-plane translation

The purpose of the study is to show a proposal of an extension of a one-dimensional speckle correlation method, which is primarily intended for determination of one-dimensional object's translation, for detection of general in-plane object's translation. In that view, a numerical simulation of a displacement of the speckle field as a consequence of general in-plane object's translation is presented. The translation components a_x and a_y representing the projections of a vector a of the object's displacement onto both x- and y-axes in the object plane (x, y) are evaluated separately by means of the extended one-dimensional speckle correlation method. Moreover, one can perform a distinct optimization of the method by reduction of intensity values representing detected speckle patterns. The theoretical relations between the translation components a_x and a_y of the object and the displacement of the speckle pattern for selected geometrical arrangement are mentioned and used for the testifying of the proposed method's rightness.

Retrieving controlled motion parameters using two speckle pattern analysis techniques: spatiotemporal correlation and the temporal history speckle pattern

This paper presents simulation of speckle activity through controlling a moving plate. We present two procedures to extract the initial movement frequency and amplitude, either through correlation calculus or through processing the temporal history of the speckle pattern. We compare and discuss these two methods in terms of efficiency and the ability to retrieve motion parameters. The correlation technique seems to be more suitable for monitoring biospeckle activity as it provides more reliable parameter estimation than the temporal history of the speckle pattern. The evolution of temporal history of the speckle pattern parameters and their response sensibility with amplitude and frequency variations have been studied and quantified. Briers contrast appears to depend only on movement amplitude, whereas inertia moment varies with amplitude and frequency.

Single pulse coherence measurements in the water window at the free-electron laser FLASH

The spatial coherence of free-electron laser radiation in the water window spectral range was studied, using the third harmonic (λ<(3rd) = 2.66 nm) of DESY's Free-electron LASer in Hamburg (FLASH). Coherent single pulse diffraction patterns of 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) multilamellar lipid stacks have been recorded. The intensity histogram of the speckle pattern around the first lamellar Bragg peak, corresponding to the d = 5 nm periodicity of the stack, reveals an average number of transverse modes of M¯ = 3.0 of the 3rd harmonic. Using the lipid stack as a 'monochromator', pulse-to-pulse fluctuations in the third harmonic λ(3rd) have been determined to be 0.033 nm.

Effect of signal intensity and camera quantization on laser speckle contrast analysis

Laser speckle contrast analysis (LASCA) is limited to being a qualitative method for the measurement of blood flow and tissue perfusion as it is sensitive to the measurement configuration. The signal intensity is one of the parameters that can affect the contrast values due to the quantization of the signals by the camera and analog-to-digital converter (ADC). In this paper we deduce the theoretical relationship between signal intensity and contrast values based on the probability density function (PDF) of the speckle pattern and simplify it to a rational function. A simple method to correct this contrast error is suggested. The experimental results demonstrate that this relationship can effectively compensate the bias in contrast values induced by the quantized signal intensity and correct for bias induced by signal intensity variations across the field of view.

Quantitative laser speckle flowmetry of the in vivo microcirculation using sidestream dark field microscopy

We present integrated Laser Speckle Contrast Imaging (LSCI) and Sidestream Dark Field (SDF) flowmetry to provide real-time, non-invasive and quantitative measurements of speckle decorrelation times related to microcirculatory flow. Using a multi exposure acquisition scheme, precise speckle decorrelation times were obtained. Applying SDF-LSCI in vitro and in vivo allows direct comparison between speckle contrast decorrelation and flow velocities, while imaging the phantom and microcirculation architecture. This resulted in a novel analysis approach that distinguishes decorrelation due to flow from other additive decorrelation sources.

Noninvasive radiation burn diagnosis using speckle phenomenon with a fractal approach to processing

Radiation burns account for the vast majority of damage by accidental radiation exposure. They are characterized by successive and unpredictable inflammatory bursts that are preceded by a clinically latent postirradiation period. Diagnosis and prognosis of the clinical course of radiation burns have proven to be a difficult task. In a classical clinical setting, no technique can distinguish irradiated versus healthy skin during the clinically latent period, hence development of new tools is required. This work describes a noninvasive technique based on speckle phenomenon, designed to support radiation burn diagnosis and prognosis. Speckle produced by strongly scattering media contains information about their optical properties. The difficulty is to extract significant information from speckle patterns to discriminate between strongly scattering media and to characterize any change. Speckle patterns from irradiated and nonirradiated porcine skins are recorded in vivo several times after radiation exposure. A fractal approach is used in the treatment of speckle patterns. The results show that this technique allows discrimination between healthy and irradiated skin, in particular during the clinically latent period (p<0.01). Parameters extracted from speckle patterns discriminate and vary differently with radiation, which means they represent different information about skin changes.

Influence of atmospheric turbulence on the uplink propagation in an optical time transfer

The time transfer by laser link experiment T2L2 aims for a precision of 60 ps, which could be degraded by the atmospheric turbulence because of the strong variations of the photon number received by the satellite detector, from a measurement to another. The light intensity fluctuations in the satellite plane are estimated for the planned situation for which the beam radius at the atmosphere exit is significantly larger than the coherence length. Such speckle-type fluctuations are experimentally studied.