Showing papers on "Speckle imaging published in 2017"

Single-shot multispectral imaging with a monochromatic camera

Abstract: Multispectral imaging plays an important role in many applications, from astronomical imaging and earth observation to biomedical imaging. However, current technologies are complex with multiple alignment-sensitive components and spatial and spectral parameters predetermined by manufacturers. Here, we demonstrate a single-shot multispectral imaging technique that gives flexibility to end users with a very simple optical setup, thanks to spatial correlation and spectral decorrelation of speckle patterns. These seemingly random speckle patterns are point spread functions (PSFs) generated by light from point sources propagating through a strongly scattering medium. The spatial correlation of PSFs allows image recovery with deconvolution techniques, while the spectral decorrelation allows them to play the role of tunable spectral filters in the deconvolution process. Our demonstrations utilizing optical physics of strongly scattering media and computational imaging present a cost-effective approach for multispectral imaging with many advantages.

Intraoperative multi-exposure speckle imaging of cerebral blood flow.

Abstract: Multiple studies have demonstrated that laser speckle contrast imaging (LSCI) has high potential to be a valuable cerebral blood flow monitoring technique during neurosurgery. However, the quantita...

Imaging through a thin scattering layer and jointly retrieving the point-spread-function using phase-diversity

Abstract: Recently introduced angular-memory-effect based techniques enable non-invasive imaging of objects hidden behind thin scattering layers. However, both the speckle-correlation and the bispectrum analysis are based on the statistical average of large amounts of speckle grains, which determines that they can hardly access the important information of the point-spread-function (PSF) of a highly scattering imaging system. Here, inspired by notions used in astronomy, we present a phase-diversity speckle imaging scheme, based on recording a sequence of intensity speckle patterns at various imaging planes, and experimentally demonstrate that in addition to being able to retrieve diffraction-limited image of hidden objects, phase-diversity can also simultaneously estimate the pupil function and the PSF of a highly scattering imaging system without any guide-star nor reference.

Imaging through a thin scattering layer and jointly retrieving the point-spread-function using phase-diversity.

Abstract: Recently introduced angular-memory-effect based techniques enable non-invasive imaging of objects hidden behind thin scattering layers. However, both the speckle-correlation and the bispectrum analysis are based on the statistical average of large amounts of speckle grains, which determines that they can hardly access the important information of the point-spread-function (PSF) of a highly scattering imaging system. Here, inspired by notions used in astronomy, we present a phase-diversity speckle imaging scheme, based on recording a sequence of intensity speckle patterns at various imaging planes, and experimentally demonstrate that in addition to being able to retrieve the image of hidden objects, we can also simultaneously estimate the pupil function and the PSF of a highly scattering imaging system without any guide-star nor reference.

Compressive fluorescence imaging using a multi-core fiber and spatially dependent scattering.

Abstract: We demonstrate imaging using a multi-core fiber with a scattering distal tip and compressed sensing signal acquisition. We illuminate objects with randomly structured speckle patterns generated by a coherent light source separately coupled through each fiber core to a ground glass diffuser at the distal end. Using the characterized speckle patterns and the total light collected from the object, we computationally recover pixelation-free object images with up to a seven times higher space-bandwidth product than the number of cores. The proposed imaging system is insensitive to bending of the fiber and extremely compact, making it suitable for minimally invasive endomicroscopy.

Statistical model for speckle pattern optimization

Abstract: Image registration is the key technique of optical metrologies such as digital image correlation (DIC), particle image velocimetry (PIV), and speckle metrology. Its performance depends critically on the quality of image pattern, and thus pattern optimization attracts extensive attention. In this article, a statistical model is built to optimize speckle patterns that are composed of randomly positioned speckles. It is found that the process of speckle pattern generation is essentially a filtered Poisson process. The dependence of measurement errors (including systematic errors, random errors, and overall errors) upon speckle pattern generation parameters is characterized analytically. By minimizing the errors, formulas of the optimal speckle radius are presented. Although the primary motivation is from the field of DIC, we believed that scholars in other optical measurement communities, such as PIV and speckle metrology, will benefit from these discussions.

High-brightness laser imaging with tunable speckle reduction enabled by electroactive micro-optic diffusers.

Abstract: High coherence of lasers is desirable in high-speed, high-resolution, and wide-field imaging. However, it also causes unavoidable background speckle noise thus degrades the image quality in traditional microscopy and more significantly in interferometric quantitative phase imaging (QPI). QPI utilizes optical interference for high-precision measurement of the optical properties where the speckle can severely distort the information. To overcome this, we demonstrated a light source system having a wide tunability in the spatial coherence over 43% by controlling the illumination angle, scatterer's size, and the rotational speed of an electroactive-polymer rotational micro-optic diffuser. Spatially random phase modulation was implemented for the lower speckle imaging with over a 50% speckle reduction without a significant degradation in the temporal coherence. Our coherence control technique will provide a unique solution for a low-speckle, full-field, and coherent imaging in optically scattering media in the fields of healthcare sciences, material sciences and high-precision engineering.

The speckle polarimeter of the 2.5-m telescope: Design and calibration

Abstract: The speckle polarimeter is a facility instrument of the 25-mSAIMSU telescope that combines the features of a speckle interferometer and a polarimeter The speckle polarimeter is designed for observations in several visible bands in the following modes: speckle interferometry, polarimetry, speckle polarimetry, and polaroastrometry In this paper we describe the instrument design and the procedures for determining the angular scale of the camera and the position angle of the camera and the polarimeter Our measurements of the parameters for the binary star HD 9165 are used as an example to demonstrate the technique of speckle interferometry For bright objects the accuracy of astrometry is limited by the error of the correction for the distortion caused by the atmospheric dispersion compensator At zenith distances less than 45◦ the additional relative measurement error of the separation is 07%, while the additional error of the position angle is 03° In the absence of a dispersion compensator the accuracy of astrometry is limited by the uncertainty in the scale and position angle of the camera, which are 015% and 006°, respectively We have performed polarimetric measurements of unpolarized stars and polarization standards The instrumental polarization at the Cassegrain focus in the V band does not exceed 001% The instrumental polarization for the Nasmyth focus varies between 2 and 4% within the visible range; we have constructed its model and give a method for its elimination from the measurements For stars with an intrinsic polarization of less than 02% during observations at the Cassegrain focus the error is determined mainly by the photon and readout noises and can reach 5 × 10−5

Holographic imaging through a scattering layer using speckle interferometry.

Abstract: Optical imaging through complex scattering media is one of the major technical challenges with important applications in many research fields, ranging from biomedical imaging to astronomical telescopy to spatially multiplexed optical communications. Various approaches for imaging through a turbid layer have been recently proposed that exploit the advantage of object information encoded in correlations of the random optical fields. Here we propose and experimentally demonstrate an alternative approach for single-shot imaging of objects hidden behind an opaque scattering layer. The proposed technique relies on retrieving the interference fringes projected behind the scattering medium, which leads to complex field reconstruction, from far-field laser speckle interferometry with two-point intensity correlation measurement. We demonstrate that under suitable conditions, it is possible to perform imaging to reconstruct the complex amplitude of objects situated at different depths.

Measurement of temperature and temperature profile of wick stabilized micro diffusion flame under the effect of magnetic field using digital speckle pattern interferometry

Abstract: The effect of upward decreasing, uniform, and upward increasing magnetic fields on the temperature and temperature profile of a wick stabilized micro diffusion flame is investigated experimentally by using digital speckle pattern interferometry (DSPI). DSPI fringe patterns have inherent speckle noise which leads to inaccuracies in the measurements. To extract data more accurately, the high frequency speckle noise in a DSPI fringe pattern is reduced by using the combination of median filter and Symlet wavelet filter. The optical phase is extracted from the filtered DSPI fringe pattern by using Hilbert transform. The obtained phase is used to calculate the refractive index and temperature distribution in a microflame created by a candle. Temperature in the micro diffusion flame was determined experimentally both in the absence and in the presence of upward decreasing, uniform, and upward increasing magnetic fields. The experimental results reveal that temperature is increased under the effect of uniform and upward decreasing magnetic fields in comparison to the temperature of the microflame without a magnetic field. This is in contrast to the normal diffusion flame, where under a uniform magnetic field, there was no effect on temperature. In the case of an upward increasing magnetic field, the temperature of the microflame decreased.

CoLux: multi-object 3D micro-motion analysis using speckle imaging

Abstract: We present CoLux, a novel system for measuring micro 3D motion of multiple independently moving objects at macroscopic standoff distances. CoLux is based on speckle imaging, where the scene is illuminated with a coherent light source and imaged with a camera. Coherent light, on interacting with optically rough surfaces, creates a high-frequency speckle pattern in the captured images. The motion of objects results in movement of speckle, which can be measured to estimate the object motion. Speckle imaging is widely used for micro-motion estimation in several applications, including industrial inspection, scientific imaging, and user interfaces (e.g., optical mice). However, current speckle imaging methods are largely limited to measuring 2D motion (parallel to the sensor image plane) of a single rigid object. We develop a novel theoretical model for speckle movement due to multi-object motion, and present a simple technique based on global scale-space speckle motion analysis for measuring small (5--50 microns) compound motion of multiple objects, along all three axes. Using these tools, we develop a method for measuring 3D micro-motion histograms of multiple independently moving objects, without tracking the individual motion trajectories. In order to demonstrate the capabilities of CoLux, we develop a hardware prototype and a proof-of-concept subtle hand gesture recognition system with a broad range of potential applications in user interfaces and interactive computer graphics.

Extended bandwidth wavelength swept laser source for high resolution optical frequency domain imaging.

Abstract: Improving the axial resolution by providing wider bandwidth wavelength swept lasers remains an important issue for optical frequency domain imaging (OFDI). Here, we demonstrate a wide tuning range, all-fiber wavelength swept laser at a center wavelength of 1250 nm by combining two ring cavities that share a single Fabry-Perot tunable filter. The two cavities contain semiconductor optical amplifiers with central wavelengths of 1190 nm and 1292 nm, respectively. To avoid disturbing interference effects in the overlapping spectral region, we modulated the amplifiers in order to obtain consecutive wavelength sweeps in the two spectral regions. The two sweeps were fused together in post-processing to achieve a total scanning range of 223 nm, corresponding to 3.3 µm axial resolution in air. We confirm improved image quality and reduced speckle size in tomograms of swine esophagus ex vivo, and human skin and nailbed in vivo.

Improving the estimation of flow speed for laser speckle imaging with single exposure time.

Abstract: Laser speckle contrast imaging is a full-field imaging technique for measuring blood flow by mapping the speckle contrast with high spatial and temporal resolution. However, the statically scattered light from stationary tissues seriously degrades the accuracy of flow speed estimation. In this Letter, we present a simple calibration approach to calculate the proportions of dynamically scattered light and correct the effect of static scattering with single exposure time. Both the phantom and animal experimental results suggest that this calibration approach has the ability to improve the estimation of the relative blood flow in the presence of static scattering.

Speckle reduction in laser projection using microlens-array screens

Abstract: We report on a novel speckle reduction scheme using microlens arrays as screen material for application in laser-based projection systems. The scheme is based on properly adjusting the coherence properties on the screen: when the coherence area on the microlens-array screen is smaller than the microlens footprint, there is no interference between the fields emitted by the different microlenses and as a result no speckle is formed. We measured and modelled the speckle properties of microlens arrays with regular and irregular structure and lens sizes, and also a paper screen for comparison. In the experiments, we tune the laser beam's spatial coherence by sending it through a rotating diffuser. We show the amount of speckle reduction that can be achieved, which mechanisms influence the observed speckle contrast and we discuss the limitations due to increased non-uniformity in the projected image.

Improving the sensitivity of velocity measurements in laser speckle contrast imaging using a noise correction method.

Abstract: We demonstrate that noise is an important factor contributing to the decline of sensitivity and linear response range of velocity measurements for laser speckle contrast imaging. We propose to use a noise correction method to improve the sensitivity of velocity measurements. For a kind of camera in which the mean values of the dark noise have been subtracted and negative counts have been set to zero, we propose a method to estimate the true dark noise based on the maximum likelihood estimation, which expands the application scope of the noise correction method.

Quantitative model of diffuse speckle contrast analysis for flow measurement.

Abstract: Diffuse speckle contrast analysis (DSCA) is a noninvasive optical technique capable of monitoring deep tissue blood flow. However, a detailed study of the speckle contrast model for DSCA has yet to be presented. We deduced the theoretical relationship between speckle contrast and exposure time and further simplified it to a linear approximation model. The feasibility of this linear model was validated by the liquid phantoms which demonstrated that the slope of this linear approximation was able to rapidly determine the Brownian diffusion coefficient of the turbid media at multiple distances using multiexposure speckle imaging. Furthermore, we have theoretically quantified the influence of optical property on the measurements of the Brownian diffusion coefficient which was a consequence of the fact that the slope of this linear approximation was demonstrated to be equal to the inverse of correlation time of the speckle.

Ergodic and non-ergodic regimes in temporal laser speckle imaging

Abstract: The non-ergodicity problem of temporal averaging in the laser speckle contrast imaging (LSCI) is discussed both theoretically and numerically. We demonstrate that temporal speckle statistics assessed within a finite time window might differ from the statistics of the speckle ensemble. The dependence of temporal speckle contrast on sample dynamics is non-monotonic and demonstrates regimes of negative (ergodic), as well as positive correlations with dynamics (non-ergodic). The ergodic regime is similar to an ensemble (spatial) averaging case and is typically assumed for interpretation of LSCI measurements. The positive relation in the non-ergodic regime is an artifact of temporal statistics which we further quantify and describe the transition between two regimes.

Use of kurtosis for locating deep blood vessels in raw speckle imaging using a homogeneity representation.

Abstract: Visualization of deep blood vessels in speckle images is an important task as it is used to analyze the dynamics of the blood flow and the health status of biological tissue. Laser speckle imaging is a wide-field optical technique to measure relative blood flow speed based on the local speckle contrast analysis. However, it has been reported that this technique is limited to certain deep blood vessels (about ρ=300 μm) because of the high scattering of the sample; beyond this depth, the quality of the vessel’s image decreases. The use of a representation based on homogeneity values, computed from the co-occurrence matrix, is proposed as it provides an improved vessel definition and its corresponding diameter. Moreover, a methodology is proposed for automatic blood vessel location based on the kurtosis analysis. Results were obtained from the different skin phantoms, showing that it is possible to identify the vessel region for different morphologies, even up to 900 μm in depth.

Speckle statistics in adaptive optics images at visible wavelengths

Abstract: Residual speckles in adaptive optics (AO) images represent a well-known limitation on the achievement of the contrast needed for faint source detection. Speckles in AO imagery can be the result of either residual atmospheric aberrations, not corrected by the AO, or slowly evolving aberrations induced by the optical system. We take advantage of the high temporal cadence (1 ms) of the data acquired by the System for Coronagraphy with High-order Adaptive Optics from R to K bands-VIS forerunner experiment at the Large Binocular Telescope to characterize the AO residual speckles at visible wavelengths. An accurate knowledge of the speckle pattern and its dynamics is of paramount importance for the application of methods aimed at their mitigation. By means of both an automatic identification software and information theory, we study the main statistical properties of AO residuals and their dynamics. We therefore provide a speckle characterization that can be incorporated into numerical simulations to increase their realism and to optimize the performances of both real-time and postprocessing techniques aimed at the reduction of the speckle noise.

Modeling the depth-sectioning effect in reflection-mode dynamic speckle-field interferometric microscopy.

Abstract: Unlike most optical coherence microscopy (OCM) systems, dynamic speckle-field interferometric microscopy (DSIM) achieves depth sectioning through the spatial-coherence gating effect. Under high numerical aperture (NA) speckle-field illumination, our previous experiments have demonstrated less than 1 μm depth resolution in reflection-mode DSIM, while doubling the diffraction limited resolution as under structured illumination. However, there has not been a physical model to rigorously describe the speckle imaging process, in particular explaining the sectioning effect under high illumination and imaging NA settings in DSIM. In this paper, we develop such a model based on the diffraction tomography theory and the speckle statistics. Using this model, we calculate the system response function, which is used to further obtain the depth resolution limit in reflection-mode DSIM. Theoretically calculated depth resolution limit is in an excellent agreement with experiment results. We envision that our physical model will not only help in understanding the imaging process in DSIM, but also enable better designing such systems for depth-resolved measurements in biological cells and tissues.

Momentum transfer Monte Carlo for the simulation of laser speckle imaging and its application in the skin.

Abstract: Due to its simplicity and low cost, laser speckle imaging (LSI) has achieved widespread use in biomedical applications. However, interpretation of the blood-flow maps remains ambiguous, as LSI enables only limited visualization of vasculature below scattering layers such as the epidermis and skull. Here, we describe a computational model that enables flexible in-silico study of the impact of these factors on LSI measurements. The model uses Monte Carlo methods to simulate light and momentum transport in a heterogeneous tissue geometry. The virtual detectors of the model track several important characteristics of light. This model enables study of LSI aspects that may be difficult or unwieldy to address in an experimental setting, and enables detailed study of the fundamental origins of speckle contrast modulation in tissue-specific geometries. We applied the model to an in-depth exploration of the spectral dependence of speckle contrast signal in the skin, the effects of epidermal melanin content on LSI, and the depth-dependent origins of our signal. We found that LSI of transmitted light allows for a more homogeneous integration of the signal from the entire bulk of the tissue, whereas epi-illumination measurements of contrast are limited to a fraction of the light penetration depth. We quantified the spectral depth dependence of our contrast signal in the skin, and did not observe a statistically significant effect of epidermal melanin on speckle contrast. Finally, we corroborated these simulated results with experimental LSI measurements of flow beneath a thin absorbing layer. The results of this study suggest the use of LSI in the clinic to monitor perfusion in patients with different skin types, or inhomogeneous epidermal melanin distributions.

Very efficient speckle suppression in the entire visible range by one two-sided diffractive optical element

Abstract: The impact of the precision and direction of motion of a two-dimensional (2D) diffractive optical element (DOE) based on binary pseudo-random sequences on speckle suppression efficiency is investigated experimentally. The experimental data show a large speckle suppression efficiency and agree qualitatively with the theoretical results. The experimental results confirm that the wavelength range for the speckle suppression of a two-level 2D DOE based on binary random sequences is narrower than the visible range. The experimental data confirm the theoretical prediction that the wavelength range for the speckle suppression for a double-sided one-dimensional (1D) DOE is wider than the visible range. The large speckle suppression coefficient k>16 and a speckle contrast below 2.6% are obtained in the entire visible range by using one two-sided 1D M-sequence-based DOE with code length N=15. The proposed method has potential applications in laser projection display systems.

Spatial carrier color digital speckle pattern interferometry for absolute three-dimensional deformation measurement

Abstract: It is difficult to measure absolute three-dimensional deformation using traditional digital speckle pattern interferometry (DSPI) when the boundary condition of an object being tested is not exactly given. In practical applications, the boundary condition cannot always be specifically provided, limiting the use of DSPI in real-world applications. To tackle this problem, a DSPI system that is integrated by the spatial carrier method and a color camera has been established. Four phase maps are obtained simultaneously by spatial carrier color-digital speckle pattern interferometry using four speckle interferometers with different illumination directions. One out-of-plane and two in-plane absolute deformations can be acquired simultaneously without knowing the boundary conditions using the absolute deformation extraction algorithm based on four phase maps. Finally, the system is proved by experimental results through measurement of the deformation of a flat aluminum plate with a groove.

Speckle-correlation imaging through scattering media with hybrid bispectrum-iteration algorithm

Abstract: We present an improved iteration algorithm for speckle-correlation imaging through scattering media. We employ an approximate solution obtained from a bispectrum-analysis method as the initial condition of the iterative process. This method avoids several different runs performed with different random initial conditions in the traditional iteration algorithm and reduces the execution time in comparison with the conventional bispectrum-analysis method. Therefore, we obtain a balance between image quality and reconstruction speed. The feasibility of the proposed method is proved by the experimental results.

The measurement of plasma-facing materials’ topography variation by means of temporal phase-shifting speckle interferometry technique

Abstract: Temporal phase-shifting speckle interferometry is proposed to measure plasma-facing materials’ (PFMs) topography variation in tokamak. In this approach, laser ablation method is used to simulate erosion and redeposition, and polished molybdenum/tungsten (Mo/W) samples which are related to PFMs have been tested. The geometric topography of the ablated area was reconstructed successfully via a numerical model developed in this work. The results demonstrate the feasibility of temporal phase-shifting speckle interferometry applied to topography measurement of PFMs.

Near-field speckle imaging of light localization in disordered photonic systems

Abstract: Optical localization in strongly disordered photonic media is an attractive topic for proposing novel cavity-like structures. Light interference can produce random modes confined within small volumes, whose spatial distribution in the near-field is predicted to show hot spots at the nanoscale. However, these near-field speckles have not yet been experimentally investigated due to the lack of a high spatial resolution imaging techniques. Here, we study a system where the disorder is induced by random drilling air holes in a GaAs suspended membrane with internal InAs quantum dots. We perform deep-subwavelength near-field experiments in the telecom window to directly image the spatial distribution of the electric field intensity of disordered-induced localized optical modes. We retrieve the near-field speckle patterns that extend over few micrometers and show several single speckles of the order of λ/10 size. The results are compared with the numerical calculations and with the recent findings in the literature of disordered media. Notably, the hot spots of random modes are found in proximity of the air holes of the disordered system.

Deformation reconstruction by means of surface optimization. Part I: Time-averaged electronic speckle pattern interferometry.

Abstract: Electronic speckle pattern interferometry is a well-known experimental technique for full-field deformation measurements. Although speckle interferometry techniques were developed years ago and are widely used for the visualization of the operating deflection shapes of vibrating surfaces, methods for accurate reconstruction of the observed deflection shape are still an active topic of research. Determination of the relative phase of the motion of vibrating objects is especially difficult and normally phase maps are calculated by direct transformation of the experimentally obtained interferometric images. An alternative method of phase reconstruction is described that involves solving the inverse problem via surface optimization. Compared to previously developed optimization methods, this method offers a higher spatial resolution and is more suitable for analysis of complex vibration patterns.

Neighborhood binary speckle pattern for deformation measurements insensitive to local illumination variation by digital image correlation.

Abstract: Speckle pattern-based characteristics of digital image correlation (DIC) restrict its application in engineering fields and nonlaboratory environments, since serious decorrelation effect occurs due to localized sudden illumination variation A simple and efficient speckle pattern adjusting and optimizing approach presented in this paper is aimed at providing a novel speckle pattern robust enough to resist local illumination variation The new speckle pattern, called neighborhood binary speckle pattern, derived from original speckle pattern, is obtained by means of thresholding the pixels of a neighborhood at its central pixel value and considering the result as a binary number The efficiency of the proposed speckle pattern is evaluated in six experimental scenarios Experiment results indicate that the DIC measurements based on neighborhood binary speckle pattern are able to provide reliable and accurate results, even though local brightness and contrast of the deformed images have been seriously changed It is expected that the new speckle pattern will have more potential value in engineering applications

Compact broadband spectrometer based on upconversion and downconversion luminescence.

Abstract: In this Letter, a compact spectrometer based on upconversion and downconversion luminescence for operation in the infrared, visible, and ultraviolet bands is presented. The proposed spectrometer has three components that are used for dispersion, frequency conversion, and detection. The conversion component converts the incident signal beam into a spectral window appropriate for the detection component. The detection component images the speckle pattern generated by scattering or diffraction in the random structure of the dispersion component. With the two-dimensional intensity data captured from both the speckle pattern and a calibration measurement process, one can reconstruct the spectra of the signal beam by solving a matrix equation. A smoothing simulated annealing algorithm has been implemented to improve the accuracy of the spectral reconstruction. We have analyzed possible sources of error in the algorithm and the corresponding limits of operation. The reported broadband, compact, high-resolution, luminescence-based spectrometer is well suited for portable spectroscopy applications.