Showing papers on "Speckle imaging published in 2003"

Modified laser speckle imaging method with improved spatial resolution

Abstract: A two-dimensional map of blood flow is crucial for physiological studies. We present a modified laser speckle imaging method (LSI) that is based on the temporal statistics of a time-integrated speckle. A model experiment was performed for the validation of this technique. The spatial and temporal resolutions of this method were studied in theory and compared with current laser speckle contrast analysis (LASCA); the comparison indicates that the spatial resolution of the modified LSI is five times higher than that of current LASCA. Cerebral blood flow under different temperatures was investigated by our modified LSI. Compared with the results obtained by LASCA, the blood flow map obtained by the modified LSI possessed higher spatial resolution and provided additional information about changes in blood perfusion in small blood vessels. These results suggest that this is a suitable method for imaging the full field of blood flow without scanning and provides much higher spatial resolution than that of current LASCA and other laser Doppler perfusion imaging methods.

Shot-noise detection of ultrasound-tagged photons in ultrasound-modulated optical imaging

Abstract: We propose a new detection method for ultrasound-modulated optical tomography that allows us to perform parallel speckle detection with optimum shot-noise sensitivity, using a CCD camera. Moreover, we show that making use of a spatial filter system allows us to fully filter out speckle decorrelation noise. This method is confirmed by a test experiment.

Three-dimensional deformation field measurement with digital speckle correlation

Abstract: Digital speckle correlation is based on a detailed analysis of changes in speckle images that are recorded from laser-illuminated rough surfaces. The two in-plane components are obtained by cross-correlation of corresponding subimages, a method also known as digital speckle photography. The local gradient of the hitherto inaccessible out-of-plane component is determined from the characteristic dependence of the speckle correlation on the spatial frequency. A detailed experimental study is carried out to analyze the new technique for systematic and random measuring errors. For moderate decorrelation the accuracy of the out-of-plane measurement is better than λ/10 and thus comparable with interferometric techniques. Yet the extremely simple and robust optical setup is suited to nondestructive-testing applications in harsh environments. The quality of the deformation maps is demonstrated in a practical application.

Impact of speckle on laser range-gated shortwave infrared imaging system target identification performance

Abstract: The U.S. Army and the U.S. Air Force are investigating laser range-gated shortwave IR (LRG-SWIR) imaging systems for use in target identification. When coupled to an electron-bombarded CCD, the imaging system can obtain high-resolution images at long ranges. Speckle, an image artifact inherent in laser-illuminated imaging systems, results from interference patterns caused by the coherent illumination. Laser speckle degrades target identification performance but can be reduced by averaging successive LRG-SWIR images. This research is a first attempt at quantifying target identification performance degradation associated with laser speckle. The research begins with a laboratory experiment to verify a speckle model that includes power spectral density and intensity probability density functions. An LRG-SWIR sensor simulation is developed that includes coherent illumination resulting in speckle target images. A field demonstration is performed to verify the fidelity of the simulation. The simulation is then applied to the U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) target identification set with various levels of image averaging and blur. Observer performance results are analyzed in terms of target identification probability and the effects of various levels of blur and speckle are characterized.

Bispectrum speckle interferometry of the massive protostellar outflow source AFGL 2591

Abstract: We present bispectrum speckle interferometry of the massive protostellar object AFGL 2591 in the near-infrared K-band. Our reconstructed image of the outflow cavity of AFGL 2591 has a resolution of 170 mas, corresponding to physical scales of ∼170 AU at the distance of the object, and shows the loops which extend from the bright, compact source in un- precedented detail. The central source is clearly resolved and has an uniform-disk diameter of ∼40 mas (40 AU). We use 2D radiation transfer simulations to show that the resolved structure probably corresponds to the inner rim of a geometrically thick circumstellar disk or envelope at the dust sublimation radius. Our image also reveals a structure that might represent an edge-on circumstellar disk around one of the other young stellar objects near AFGL 2591.

Overview of optical techniques that measure displacements: Murray lecture

Abstract: Optical techniques that measure displacements play a very important role in current experimental mechanics, material sciences and metrology. This paper presents a survey of developments in these techniques from a personal experience point of view. Three main aspects are considered. Mathematical and numerical models used in the interpretation of fringe information and the corresponding data processing techniques. Optical and electro-optical developments that have taken place to improve the sensitivity, and the efficiency of these methods to make them competitive with purely numerical methods. Applications that have arisen from the synergy between advanced computational capabilities and optics are also presented.

CMOS vs. CCD sensors in speckle interferometry

Abstract: In the field of interferometric metrology the use of high resolution CCD sensors with 1024×1024 to 2048×2048 pixels is predominant. Due to special features (e.g. random pixel access, characteristic curve) CMOS sensors with similar resolution can be an interesting alternative. We compare some characteristics of both sensor types that are important for interferometry and demonstrate two exemplary applications that are only possible by using CMOS cameras.

Anisoplanatic performance of horizontal-path speckle imaging

Abstract: We have previously demonstrated and reported on the use of sub-field speckle processing for the enhancement of both near and far-range surveillance imagery of people and vehicles that have been degraded by atmospheric turbulence. We have obtained near diffraction-limited imagery in many cases and have shown dramatic image quality improvement in other cases. As it is possible to perform only a limited number of experiments in a limited number of conditions, we have developed a computer simulation capability to aid in the prediction of imaging performance in a wider variation of conditions. Our simulation capability includes the ability to model extended scenes in distributed turbulence. Of great interest is the effect of the isoplanatic angle on speckle imaging performance as well as on single deformable mirror and multiconjugate adaptive optics system performance. These angles are typically quite small over horizontal and slant paths. This paper will begin to explore these issues which are important for predicting the performance of both passive and active horizontal and slant-path imaging systems.

Absolute measurement of roughness and lateral-correlation length of random surfaces by use of the simplified model of image-speckle contrast

Abstract: We present a method of image-speckle contrast for the nonprecalibration measurement of the root-mean-square roughness and the lateral-correlation length of random surfaces with Gaussian correlation. We use the simplified model of the speckle fields produced by the weak scattering object in the theoretical analysis. The explicit mathematical relation shows that the saturation value of the image-speckle contrast at a large aperture radius determines the roughness, while the variation of the contrast with the aperture radius determines the lateral-correlation length. In the experimental performance, we specially fabricate the random surface samples with Gaussian correlation. The square of the image-speckle contrast is measured versus the radius of the aperture in the 4f system, and the roughness and the lateral-correlation length are extracted by fitting the theoretical result to the experimental data. Comparison of the measurement with that by an atomic force microscope shows our method has a satisfying accuracy.

Efficient characterization of regional mesenteric blood flow by use of laser speckle imaging

Abstract: We present a noninvasive full-field method--laser speckle imaging (LSI)--for measuring the regional mesenteric blood flow without scanning. A system of LSI was designed and validated in a model experiment. Dynamics of regional blood flow in the rat mesentery under the influence of noradrenaline were monitored by this method. Spatial and temporal characteristics of the mesenteric blood-flow response were achieved with high resolution. These suggested that LSI might provide a new approach to microcirculation studies.

Phase retrieval in digital speckle pattern interferometry by use of a smoothed space–frequency distribution

Abstract: We evaluate the use of a smoothed space-frequency distribution (SSFD) to retrieve optical phase maps in digital speckle pattern interferometry (DSPI). The performance of this method is tested by use of computer-simulated DSPI fringes. Phase gradients are found along a pixel path from a single DSPI image, and the phase map is finally determined by integration. This technique does not need the application of a phase unwrapping algorithm or the introduction of carrier fringes in the interferometer. It is shown that a Wigner-Ville distribution with a smoothing Gaussian kernel gives more-accurate results than methods based on the continuous wavelet transform. We also discuss the influence of filtering on smoothing of the DSPI fringes and some additional limitations that emerge when this technique is applied. The performance of the SSFD method for processing experimental data is then illustrated.

Seeing through biological tissues using the fly eye principle

Abstract: During the past decade, optical imaging through scattering medium has proved to be a powerful technique for many applications. It is especially effective in medical diagnostic, since it is safe, noninvasive and low-cost compared with the conventional radiation techniques. Based on a similar principle of the fly’s visual system, we show a novel method of optical imaging through scattering medium. An image of bones hidden between two biological tissues (chicken breast) is recovered from many noisy speckle pictures obtained on the output of a multi-channeled optical imaging system. The operation of multiple imaging is achieved using a microlens array. Each lens from the array projects a different speckled image on a digital camera. The set of speckled images from the entire array are first shifted to a common center and then accumulated to a single average picture in which the concealed object is exposed.

New Automatic FFT Filtration Method for Phase Maps and its Application in Speckle Interferometry

Abstract: In this article a new automatic phase FFT filtration method is described. It is compared with other well known filtration techniques for noise removal. This automatic FFT filter is applied on interferograms obtained through one dimensional folding shear interferometry for displacements measurement of a loaded glass flask. Different algorithms have been applied for processing of digitally synthesized phase maps and the advantages of the enhanced automatic FFT filtration are shown. The phase filtered maps obtained in this way can be demodulated even with the simplest unwrapping technique.

Fundamentals and applications of speckle

Abstract: Speckle arises from interference of many waves with random phase relationship. In spite of its random appearance its statistical properties are independent of surface roughness of diffusers and depend only on macroscopic parameters of optical systems under usual conditions of observation. In this article we first discuss the size distribution of speckle from physical viewpoints. Then we describe the dynamic behaviors of speckle that are caused by displacement and deformation of diffuse surfaces as well as by change in optical system. Finally we overview applications of these properties of speckle to measurement of displacement and deformation.

Numerical model for dynamic speckle: an approach using the movement of the scatterers

Abstract: Dynamic speckle or biospeckle is a phenomenon generated by laser light scattering in biological tissues and it is also present in some industrial processes where the surfaces exhibit some kind of activity. In this work we present some results of a numerical model to describe the time history of a dynamic speckle pattern considering very simplified situations. In this case we simulated only the movement of the scatterers, and the obtained results are in agreement with some of the expected from the theory and qualitatively with the experimental results outlined in previous works. Also, we tested the first order statistics of the spatial speckle, the Briers contrast of the time varying part and the moment of inertia of the co-occurrence matrix.

Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry

Abstract: Based on 14 data points obtained with near-infrared speckle interferometry and covering an almost entire revolution, we present a first visual orbit for the low-mass binary system Gliese 22 AC. The quality of the orbit is largely improved with respect to previous astrometric solutions. The dynamical system mass is $0.592\pm 0.065\,M_{\odot}$, where the largest part of the error is due to the Hipparcos parallax. A comparison of this dynamical mass with mass-luminosity relations on the lower main sequence and theoretical evolutionary models for low-mass objects shows that both probably underestimate the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates that this object is a very low-mass star with a mass close to the hydrogen burning mass limit.

Optimal re-referencing rate for in-plane dynamic speckle interferometry

Abstract: We investigate experimentally the optimal rate at which the reference speckle pattern should be updated when dynamic speckle interferometry is used to measure transient in-plane displacement fields. Images are captured with a high-speed camera and phase shifting and phase unwrapping are done temporally. For a wide range of in-plane velocities, up to a maximum of 40% of the Nyquist limit, the random errors in the calculated displacement field are minimized by updating the reference speckle pattern after a speckle displacement of 1/10 of the pixel spacing. The technique is applied to measurements of microscale deformation fields within an adhesive joint in a carbon-fiber epoxy composite.

Low-coherence speckle interferometer (LCSI) for characterization of adhesion in adhesive-bonded joints

Abstract: Low Coherence Speckle Interferometry (LCSI) combines the depth-resolved measurement of Low Coherence Interferometry (LCI) with the high-accuracy deformation measurement of Electronic Speckle Pattern Interferometry (ESPI). Depth-resolved deformation measurement enables the characterization of the behavior of interfaces in multi-layer materials or structures while changing the ambient conditions. In this paper LCSI is introduced as a new tool for characterization of adhesion. The experimental set-up and the principle of work are described. A FEM-model of an adhesive bonded aluminum joint is developed to analyze the behavior of the Al-adhesive interface during mechanical testing. Some recent results are shown. This application demand measurements on a microscopic scale (camera field of view down to 500 x 500 μm).

Progress in horizontal and slant-path imaging using speckle imaging

Abstract: The difficulty in terrestrial imaging over long horizontal or slant paths is that atmospheric aberrations and distortions reduce the resolution and contrast in images recorded at high resolution. This paper will describe the problem of horizontal-path imaging, briefly cover various methods for imaging over horizontal paths and then describe the speckle imaging method actively being pursued at LLNL. We will review some closer range (1-3 km range) imagery of people we have already published, as well as show new results of vehicles we have obtained over longer slant-range paths greater than 20 km.

Adaptive phase-shifting algorithm for temporal phase evaluation

Abstract: Most standard temporal-phase-shifting (TPS) algorithms evaluate the phase by computing a windowed Fourier transform (WFT) of the intensity signal at the carrier frequency of the system. However, displacement of the specimen during image acquisition may cause the peak of the transform to shift away from the carrier frequency, leading to phase errors and even unwrapping failure. We present a novel TPS method that searches for the peak of the WFT and evaluates the phase at that frequency instead of at the carrier frequency. The performance of this method is compared with that of standard algorithms by using numerical simulations. Experimental results from high-speed speckle interferometry studies of carbon fiber panels are also presented.

Diverse phase speckle inversion applied to data from the Swedish 1-meter solar telescope

Abstract: We report on the use of a new joint phase diverse speckle code, an implementation of a method where a single object and individual phases are estimated from several pairs of phase diverse data. The code was used on 430.5 nm G-band data collected with the newly installed Swedish 1-meter solar telescope in La Palma, equipped with a low-order adaptive optics system. We describe the algorithm briefly, show wavefront statistics and object estimates from the processing and discuss the results. We demonstrate a resolution of 0.12 arc seconds for a time sequence and a large field of view, which is a break-through for ground based solar telescopes.

Video surveillance with speckle imaging

Abstract: A surveillance system looks through the atmosphere along a horizontal or slant path. Turbulence along the path causes blurring. The blurring is corrected by speckle processing short exposure images recorded with a camera. The exposures are short enough to effectively freeze the atmospheric turbulence. Speckle processing is used to recover a better quality image of the scene.

Use of electronic speckle interferometry for defect detection in fabricated devices

Abstract: Electronic speckle interferometry is used to detect submicron-sized indication in fabricated devices, such as membranes Indications include indentations, deformations or defects For example, disbonds between a membrane surface and a bonded edge surface can be detected An acoustic source can be used to excite the membrane The acoustic source can produce a sine wave to vibrate the membrane An interference image of the membrane is created to show whether submicron-sized defects exist in the membrane