Showing papers on "Speckle imaging published in 2022"

Speckle Interferometric Observations With the Gemini 8-m Telescopes: Signal-to-Noise Calculations and Observational Results

Abstract: Using 3 years of observations with the Zorro and ’Alopeke speckle interferometric instruments at Gemini South and North, respectively, we present an analysis of the sensitivity of the data taken in two narrow-band optical filters centered at 562 and 832 nm (widths of 54 and 40 nm, respectively). In this paper we focus on model calculations of the predicted signal-to-noise values achievable and the results of over 2500 actual observations. We find that S/N values of several 100 are easily achieved, but that the sky background during full moon is a very limiting factor in the observations, especially those performed in the short-wavelength (blue) optical spectral range and for targets fainter than R ∼14. A comparison of our Gemini speckle observations over six observing semesters reveals that red band-pass observations provide more robust results in general, likely due to better atmospheric performance at these wavelengths. Using the identical instruments on Gemini North and South, we find that similar results are obtained, yielding typical contrast limits of 5-9 magnitudes from the diffraction limit out to 1.2″ for a range of target brightness (optical magnitudes from ∼ 3 to > 16 ). Using our S/N model along with the observational results, an estimation of the contrast limits achievable for a given observation can be predicted based on the target brightness, sky illumination and seeing conditions, and the total integration time.

Phase derivative estimation in digital holographic interferometry using a deep learning approach.

Abstract: In digital holographic interferometry, reliable estimation of phase derivatives from the complex interference field signal is an important challenge since these are directly related to the displacement derivatives of a deformed object. In this paper, we propose an approach based on deep learning for direct estimation of phase derivatives in digital holographic interferometry. Using a Y-Net model, our proposed approach allows for simultaneous estimation of phase derivatives along the vertical and horizontal dimensions. The robustness of the proposed approach for phase derivative extraction under both additive white Gaussian noise and speckle noise is shown via numerical simulations. Subsequently, we demonstrate the practical utility of the method for deformation metrology using experimental data obtained from digital holographic interferometry.

The Solar Neighborhood. XLIX. New Discoveries and Orbits of M-dwarf Multiples with Speckle Interferometry at SOAR

Abstract: We present the first results of a multiyear program to map the orbits of M-dwarf multiples within 25 pc. The observations were conducted primarily during 2019–2020 using speckle interferometry at the Southern Astrophysical Research Telescope in Chile, using the High-Resolution Camera mounted on the adaptive optics module (HRCam+SAM). The sample of nearby M dwarfs is drawn from three sources: multiples from the RECONS long-term astrometric monitoring program at the SMARTS 0.9 m; known multiples, for which these new observations will enable or improve orbit fits; and candidate multiples flagged by their astrometric fits in Gaia Data Release 2 (DR2). We surveyed 333 of our 338 M dwarfs via 830 speckle observations, detecting companions for 63% of the stars. Most notably, this includes new companions for 76% of the subset selected from Gaia DR2. In all, we report the first direct detections of 97 new stellar companions to the observed M dwarfs. Here we present the properties of those detections, the limits of each nondetection, and five orbits with periods 0.67–29 yr already observed as part of this program. Companions detected have projected separations of 0.″024–2.″0 (0.25–66 au) from their primaries and have ΔI ≲ 5.0 mag. This multiyear campaign will ultimately map complete orbits for nearby M dwarfs with periods up to 3 yr, and provide key epochs to stretch orbital determinations for binaries to 30 yr.

Direct characterization of tissue dynamics with laser speckle contrast imaging.

Abstract: Laser speckle contrast imaging (LSCI) has gained broad appeal as a technique to monitor tissue dynamics (broadly defined to include blood flow dynamics), in part because of its remarkable simplicity. When laser light is backscattered from a tissue, it produces speckle patterns that vary in time. A measure of the speckle field decorrelation time provides information about the tissue dynamics. In conventional LSCI, this measure requires numerical fitting to a specific theoretical model for the field decorrelation. However, this model may not be known a priori, or it may vary over the image field of view. We describe a method to reconstruct the speckle field decorrelation time that is completely model free, provided that the measured speckle dynamics are ergodic. We also extend our approach to allow for the possibility of non-ergodic measurements caused by the presence of a background static speckle field. In both ergodic and non-ergodic cases, our approach accurately retrieves the correlation time without any recourse to numerical fitting and is largely independent of camera exposure time. We apply our method to tissue phantom and in-vivo mouse brain imaging. Our aim is to facilitate and add robustness to LSCI processing methods for potential clinical or pre-clinical applications.

Model of dynamic speckle evolution for evaluating laser speckle contrast measurements of tissue dynamics.

Abstract: We introduce a dynamic speckle model (DSM) to simulate the temporal evolution of fully developed speckle patterns arising from the interference of scattered light reemitted from dynamic tissue. Using this numerical tool, the performance of laser speckle contrast imaging (LSCI) or speckle contrast optical spectroscopy (SCOS) systems which quantify tissue dynamics using the spatial contrast of the speckle patterns with a certain camera exposure time is evaluated. We have investigated noise sources arising from the fundamental speckle statistics due to the finite sampling of the speckle patterns as well as those induced by experimental measurement conditions including shot noise, camera dark and read noise, and calibrated the parameters of an analytical noise model initially developed in the fundamental or shot noise regime that quantifies the performance of SCOS systems using the number of independent observables (NIO). Our analysis is particularly focused on the low photon flux regime relevant for human brain measurements, where the impact of shot noise and camera read noise can become significant. Our numerical model is also validated experimentally using a novel fiber based SCOS (fb-SCOS) system for a dynamic sample. We have found that the signal-to-noise ratio (SNR) of fb-SCOS measurements plateaus at a camera exposure time, which marks the regime where shot and fundamental noise dominates over camera read noise. For a fixed total measurement time, there exists an optimized camera exposure time if temporal averaging is utilized to improve SNR. For a certain camera exposure time, photon flux value, and camera noise properties, there exists an optimized speckle-to-pixel size ratio (s/p) at which SNR is maximized. Our work provides the design principles for any LSCI or SCOS systems given the detected photon flux and properties of the instruments, which will guide the experimental development of a high-quality, low-cost fb-SCOS system that monitors human brain blood flow and functions.

Speckle Interferometry at SOAR in 2021

Abstract: Abstract The speckle interferometry program at the the 4.1 m Southern Astrophysical Research Telescope (SOAR), which started in 2008, now has accumulated over 30,300 individual observations of 12,700 distinct targets. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and low-mass dwarfs in the solar neighborhood. The results from 2021 are published here, totaling 2623 measurements of 2123 resolved pairs and nonresolutions of 763 targets. The median measured separation is 0.″21, and 75 pairs were closer than 30 mas. The calibration of scale and orientation is based on the observations of 103 wide pairs with well-modeled motion. These calibrators are compared to the latest Gaia data release, and minor (0.5%) systematic errors were rectified, resulting in accurate relative positions with typical errors on the order of 1 mas. Using these new measurements, orbits of 282 binaries are determined here (54 first determinations and 228 corrections). We resolved for the first time 50 new pairs, including subsystems in known binaries. A list of 94 likely spurious pairs unresolved at SOAR (mostly close Hipparcos binaries) is also given.

Synthetic exposure with a CMOS camera for multiple exposure speckle imaging of blood flow

Abstract: Speckle contrast imaging is an established technique to obtain relative blood flow maps over wide fields of view. A major improvement of the method relies on the acquisition of raw speckle images at different exposure times but requires simultaneous modulation of a laser pulse in duration and intensity and precise synchronization with a camera. This complex instrumentation has limited the use of multiple exposure speckle imaging. We evaluate here the use of a CMOS camera for a simplified approach based on synthetic exposure images created from the sum of successive frames acquired at a 1 ms exposure time. Both methods have been applied to evaluate controlled flows in micro-channels. The contribution of noises to the speckle contrast have been quantified and compared. Dark, readout and shot noise contributions to the total contrast remain constant for modulated exposure, while all these contributions decrease with increasing exposure time for synthetic exposure. The relative contribution of noises to speckle contrast depends on the level of illumination and the exposure time. Guidelines for flow measurements and limitations of the use of a CMOS camera with a limited frame rate for synthetic exposure acquisition scheme are discussed. The synthetic exposure method is simple to implement and should facilitate the translation of multiple exposure speckle imaging to clinical set-ups.

Simultaneous 3D measurement for infrared chips with speckle interferometry

Abstract: • A compact device that is able to simultaneously measure full-field three dimensional deformation of an object has been developed. • A real-time computing scheme enabling quantitative characterization of 3D deformation is proposed. • The 3D deformation of the infrared sensor due to variations of temperature is characterized. • The precision and robustness of the developed techniques have been validated via experiments and simulations. Temperature-induced deformation in the infrared detector chip is a key index for evaluating image aberration and stress concentration. Development of an evaluation means for simultaneous characterization of 3D deformation is of great interest to engineering societies. In this study, an integrated device based on speckle interferometry has been developed for evaluation of the three-dimensional (3D) deformation of the infrared detector chip with variations of temperature. With the use of a 3CCD color camera and a group of optical fibers, the 3D displacement components of the detector surface are measured simultaneously during the cooling process. A temporal phase shifting scheme based on GPU parallel computing has been proposed to enable quantitatively real-time 3D measurement in three image layers at a rate of 20 fps. The integrated device has been validated with a benchmark test. The experimental results show a good agreement with the theoretical solution validating the stability and accuracy of the experimental device. The developed speckle interferometry device provides an accurate means in simultaneously evaluating the 3D deformation of the infrared detector chip due to temperature variations.

Correcting sampling bias in speckle contrast imaging.

Abstract: When performing spatial or temporal laser speckle contrast imaging (LSCI), contrast is generally estimated from localized windows containing limited numbers of independent speckle grains NS. This leads to a systematic bias in the estimated speckle contrast. We describe an approach to determine NS and largely correct for this bias, enabling a more accurate estimation of the speckle decorrelation time without recourse to numerical fitting of data. Validation experiments are presented where measurements are ergodic or non-ergodic, including in vivo imaging of mouse brain.

Electronic Speckle Pattern Interferometry for fatigue crack monitoring

Abstract: This study aims to monitor the fatigue crack tip through a contactless optical method. A Middle Tension (MT) specimen is initially prepared by submitted it to a cyclic fatigue loading, in order to generate a crack with a specified length. Then, the cracked specimen is statically loaded under a uniaxial tensile condition, and Electronic Speckle Pattern Interferometry (ESPI) is employed to monitor the crack tip. During specimen loading with incremental force values, the behavior of the cracked area is monitored, with displacement and deformation fields acquired for each load increment. The obtained data is used to calculate fracture parameters, such as the Stress Intensity Factor (SIF). For that purpose, an overdeterministic algorithm was developed to calculate SIFs for each measured crack length. The main contribution of the present work is the adoption of a high-resolution optical contactless technique to monitor the fatigue crack tip. The obtained SIF values are compared to the reference solution proposed by ASTM E647 and an acceptable agreement has been verified.

Optimizing Ghost Imaging via Analysis and Design of Speckle Patterns

Abstract: We study the influence rules of the speckle size of light source on ghost imaging, and propose a new type of speckle patterns to improve the quality of ghost imaging. The results show that the image quality will first increase and then decrease with the increase of the speckle size, and there is an optimal speckle size for a specific object. Moreover, by using the random distribution of speckle positions, a new type of displacement speckle patterns is designed, and the imaging quality is better than that of the random speckle patterns. These results are of great significances for finding the best speckle patterns suitable for detecting targets, which further promotes the practical applications of ghost imaging.

Real-time high sensibility vibration detection based on phase correlation of line speckle patterns

Abstract: A vibration extraction system based on laser speckle with phase correlation method is presented. The speckle pattern sequences are recorded by a linear array CMOS, which allows the system to detect high frequency signals more than 20 kHz. The line speckle patterns are fast Fourier transformed and correlated between the adjacent rows in the frequency domain. The phase angles of correlation calculations are directly used as the displacements between the respective two rows in the object space. The recovered audio signals are evaluated by segmental SNR (SegSNR), normalized sub-band envelope correlation (NSEC), and log-likelihood ratio (LLR). Compared with the centroid and quadratic fitting method, the phase correlation method obtains advantages of high stability, high sensibility, and real-time.

Laser speckle image velocimetry by Fast Fourier Transform technique

Abstract: In this paper, laser image velocimetry, a unique optical method for the velocity measurement of fluid flow has been validated. A laser sheet is illuminated on microscopic seeded particles to produce the speckle pattern at the recording plane. Double frame-single exposure speckle images are captured in such a way that the second speckle image is shifted exactly in a known direction. The auto-correlation method has an ambiguity to identify the direction of flow. To sort out this problem, the partial shift of the second image has been premeditated. Cross-correlation of sub interrogation areas of the speckle patterns is attained by the Fast Fourier Transform technique. Segmented four sub-windows are processed to acquire the velocity information with a vector map precisely.

Correction of overexposure in laser speckle contrast imaging.

Abstract: Laser speckle contrast imaging (LSCI) is a method to visualize and quantify tissue perfusion and blood flow. A common flaw in LSCI variants is their sensitivity to the optical setup parameters and that they operate well only on statistics of undistorted laser speckle patterns. The signal saturation of the sensors makes the contrast calculation misleading; hence the illumination level must be well controlled. We describe the theoretical explanation for the saturation-caused degradation. We introduce a linear extrapolation method to eliminate the overexposure induced error up to an extent of 60-70% saturated pixel count. This, depending on the contrast value and use case, enables to use 3-8 times higher external illumination level with no deterioration of the contrast calculation and thus the measured blood flow index. Our method enables a higher signal-to-noise ratio in darker areas by allowing the use of higher illumination, utilizing a larger portion of the dynamic range of the sensors, and making the illumination level setting less cumbersome.

Factors affecting the measurement resolution of super-resolution techniques based on speckle interferometry

Abstract: ABSTRACT A technique is proposed to observe the shape of an object beyond the diffraction limit by analysing the phase of light based on speckle interferometry. Several conditions must be considered during the measurement process to use this technique to observe detailed microstructure. In this study, the primary factors in the measurement process are investigated, and the influence of each factor on the measurement resolution is evaluated. Moreover, the optimum conditions for each factor are proposed. Under appropriate measurement conditions, this measurement technique enables the observation of microstructure beyond the diffraction limit with a high resolution of approximately 100 nm. By proposing proper measurement conditions for this new method of observing beyond the diffraction limit by detecting the phase of light, an environment is created to extend the fields of use including bio-related technologies.

Correlation properties of a spatially quasi-incoherent imaging interferometer.

Abstract: The depth-gating capacity of a spatially quasi-incoherent imaging interferometer is investigated in relation to the 3D correlation properties of diffraction field laser speckles. The system exploits a phase-stepped imaging Michelson-type interferometer in which spatially quasi-incoherent illumination is generated by passing an unexpanded laser beam through a rotating diffuser. Numerical simulations and optical experiments both verify that the depth-gating capacity of the imaging interferometer scales as λ/2NAp2, where λ is the wavelength of the laser and NAp is the numerical aperture of the illumination. For a set depth gate of 150 µm, the depth-gating capacity of the interferometer is demonstrated by scanning a standard USAF target through the measurement volume. The results obtained show that an imaging tool of this kind is expected to provide useful capabilities for imaging through disturbing media and where a single wavelength is required.

Recursion-driven bispectral imaging for dynamic scattering scenes.

Abstract: Imaging dynamic strongly scattering scenes remains a significant challenge because it is typically believed that moving objects and dynamic media provide huge barriers. Instead, we use the dynamics of objects and media and put forward a recursion-driven bispectral imaging (ReDBI) framework here for the reconstruction of a stationary or moving object hidden behind the dynamic media. ReDBI avoids the errors introduced by speckle modulation and phase-retrieval algorithms in the existing studies. We also quantitatively assess the reconstruction difficulty of character and shape objects with the benchmark of the minimum number of speckle images (MNSI) required to achieve a high-quality reconstruction, which can help to comprehend the media's transfer properties.

Simulation-based verification of the shape measurement mechanism of micro structures beyond the diffraction limit using speckle interferometry

Abstract: A method that can measure three-dimensional shapes beyond the diffraction limit of a lens has been proposed that does not require focusing on an image of the measured object, as in traditional processing. In this method, the shape of the microstructure can be measured by high-resolution detection of the amount of phase change of the speckle pattern with a lateral shift of the object. In this study, the measurement mechanism of the method was investigated using computer simulations. It was found that the measurement is realized by a combination of the perfect optical system and speckle interferometry using the scattered light as the illumination light. In addition, the validity of the simulation results was verified using a real optical system.

High-resolution 3D Blood Flow Tomography in the Sub-diffuse Regime Using Laser Speckle Contrast Imaging

Abstract: We present a novel high-resolution (down to capillary levels) computational optical imaging technique based on laser speckle contrast imaging (LSCI) principles, for 3D reconstruction of blood flow maps in complex tissue over a large FOV.

Assessment of tissue perfusion in the lower limbs using laser speckle

Abstract: Diabetic foot is a well-known problem among patients suffering from peripheral arterial diseases (PAD). This article presents an optical sensor for contactless measurement of the anatomical site based on laser speckle techniques. The sensor illuminates the inspected tissue and analyzes the captured back-reflected light from the time-changing speckle patterns. An occlusion test was implemented to provide a statistical parameter to differentiate between a low perfused and a healthy foot. A clinical study of 15 subjects was conducted. The video was analyzed by two methods: dynamic laser speckle (DLS) and laser speckle contrast analysis (LASCA). Data analysis included several classification models, where the KNN model exhibited maximum performance. These findings suggest that a simple and inexpensive system for PAD monitoring can be designed for home use and/or in community clinics.

Two-dimensional speckle technique for slope error measurements of weakly focusing reflective X-ray optics

Abstract: Speckle-based at-wavelength metrology techniques now play an important role in X-ray wavefront measurements. However, for reflective X-ray optics, the majority of existing speckle-based methods fail to provide reliable 2D information about the optical surface being characterized. Compared with the 1D information typically output from speckled-based methods, a 2D map is more informative for understanding the overall quality of the optic being tested. In this paper, we propose a method for in situ 2D absolute metrology of weakly focusing X-ray mirrors. Importantly, the angular misalignment of the mirror can be easily corrected with the proposed 2D processing procedure. We hope the speckle pattern data processing method presented here will help to extend this technique to wider applications in the synchrotron radiation and X-ray free-electron laser communities.

Ptychographic intensity interferometry imaging under low dynamic ranges

Abstract: Typically, high gray-scale imaging requires a high dynamic range camera. High dynamic range is even more crucial to conventional lensless imaging methods such as coherent diffraction imaging, since the dynamic range highly determines the resolution of recovered images. We here propose that ptychographic intensity interferometry imaging (PIII) can detect a complicated-structure object under 1-bit dynamic range (each pixel outputs zero or one only), and reconstruct a high resolution gray-scale image. PIII ptychographically illuminates an object with random speckle light, generating a speckle-like intensity pattern on a detection plane. The second-order correlation of the speckle pattens reveals the power spectrum of the object. Although the depth information of the speckle patterns will be lost because of low dynamic range detections, a small number of multiple detections with different illuminating fields can effectively recover a high dynamic range power spectrum, resulting in a high resolution gray-scale image. A theoretical analysis and comprehensive simulations for the “cameraman” photo are given in this work, which shows that the image under 1-bit dynamic range deteriorates no more than 0.4 dB (peak-signal-to-noise ratio) in comparison to the 16-bit dynamic range one. This method reduces the cost and complexity of implementing a lensless imaging.

Statistical Properties of Stellar Speckle Patterns

Abstract: AbstractThis chapter examines the statistical properties of speckle patterns—stellar speckle patterns—formed in short-exposure star images. Expressions are developed for the probability density functions of the amplitude, phase, complex amplitude, and intensity in these patterns. Expressions are also developed for the average size and shape of individual speckles; these depend on the imaging wavelength, the atmospheric seeing parameters, and the telescope pupil function. Modified expressions are provided when cores are present in the images. While cores are substantially fixed features in the images, the speckle continuously evolves over time. Average speckle size is determined by the intensity point spread function of a diffraction-limited version of the telescope; speckle size is not affected by aberrations. In polychromatic speckle, individual speckles are chromatically stretched into rainbow-like features by an amount proportional to the feature’s radial distance from image center. Stellar speckle is a form of image noise; various speckle reduction strategies are analyzed.

High-resolution common path quantitative phase interferometer for morphological study of deformed Red Blood cells with high spatial phase sensitivity

Abstract: Speckle-free common path quantitative phase interferometer is utilized for the study of deformed RBCs. Highly resolved phase maps are retrieved with full detector resolution and high spatial phase sensitivity. The experimental results and reconstructed phase maps are presented in this work.