Showing papers on "Speckle imaging published in 2010"

Speckle Interferometry at the Blanco and SOAR Telescopes in 2008 and 2009

Abstract: The results of speckle interferometric measurements of binary and multiple stars conducted in 2008 and 2009 at the Blanco and SOAR 4 m telescopes in Chile are presented A total of 1898 measurements of 1189 resolved pairs or sub-systems and 394 observations of 285 un-resolved targets are listed We resolved for the first time 48 new pairs, 21 of which are new sub-systems in close visual multiple stars Typical internal measurement precision is 03 mas in both coordinates, typical companion detection capability is Δm ~ 42 at 015 separation These data were obtained with a new electron-multiplication CCD camera; data processing is described in detail, including estimation of magnitude difference, observational errors, detection limits, and analysis of artifacts We comment on some newly discovered pairs and objects of special interest

Quantitative imaging of ischemic stroke through thinned skull in mice with Multi Exposure Speckle Imaging

Abstract: Laser Speckle Contrast Imaging (LSCI) has become a widely used technique to image cerebral blood flow in vivo. However, the quantitative accuracy of blood flow changes measured through the thin skull has not been investigated thoroughly. We recently developed a new Multi Exposure Speckle Imaging (MESI) technique to image blood flow while accounting for the effect of scattering from static tissue elements. In this paper we present the first in vivo demonstration of the MESI technique. The MESI technique was used to image the blood flow changes in a mouse cortex following photothrombotic occlusion of the middle cerebral artery. The Multi Exposure Speckle Imaging technique was found to accurately estimate flow changes due to ischemia in mice brains in vivo. These estimates of these flow changes were found to be unaffected by scattering from thinned skull.

Three-dimensional speckle suppression in optical coherence tomography based on the curvelet transform

Abstract: Optical coherence tomography is an emerging non-invasive technology that provides high resolution, cross-sectional tomographic images of internal structures of specimens OCT images, however, are usually degraded by significant speckle noise Here we introduce to our knowledge the first 3D approach to attenuating speckle noise in OCT images Unlike 2D approaches which only consider information in individual images, 3D processing, by analyzing all images in a volume simultaneously, has the advantage of also taking the information between images into account This, coupled with the curvelet transform’s nearly optimal sparse representation of curved edges that are common in OCT images, provides a simple yet powerful platform for speckle attenuation We show the approach suppresses a significant amount of speckle noise, while in the mean time preserves and thus reveals many subtle features that could get attenuated in other approaches

Speckle noise reduction in digital holography by use of multiple polarization holograms

Abstract: A novel speckle reduction technique for digital holography is proposed. Multiple off-axis holograms are recorded using a circularly polarized illumination beam and a rotating linearly polarized reference beam. The speckle noise in the reconstructed images is suppressed by averaging these fields. We demonstrate the effectiveness of this technique experimentally and conduct additional statistical evaluation.

Laser speckle reduction due to spatial and angular diversity introduced by fast scanning micromirror.

Abstract: We utilize spatial and angular diversity to achieve speckle reduction in laser illumination. Both free-space and imaging geometry configurations are considered. A fast two-dimensional scanning micromirror is employed to steer the laser beam. A simple experimental setup is built to demonstrate the application of our technique in a two-dimensional laser picture projection. Experimental results show that the speckle contrast factor can be reduced down to 5% within the integration time of the detector.

Spatiotemporal laser speckle contrast analysis for blood flow imaging with maximized speckle contrast.

Abstract: Laser speckle contrast imaging is a technique used for imaging blood flow without scanning. Though several studies have attempted to combine spatial and temporal statistics of laser speckle images for reducing image noise as well as preserving acceptable spatiotemporal resolution, the statistical accuracy of these spatiotemporal methods has not been thoroughly compared. Through numerical simulation and animal experiments, this study investigates the changes in the mean speckle contrast values and the relative noise of the speckle contrast images computed by these methods with various numbers of frames and spatial windows. The simulation results show that the maximum relative error of the mean speckle contrast computed by the spatiotemporal laser speckle contrast analysis (STLASCA) method, in which the speckle contrast images are computed by analyzing the 3-D spatiotemporal speckle image cube, is approximately 5%, while it is higher than 13% for other methods. Changes in the mean speckle contrast values and the relative noise computed by these methods for animal experiment data are consistent with the simulation results. Our results demonstrate that STLASCA achieves more accurate speckle contrast, and suggest that STLASCA most effectively utilizes the number of pixels, thus achieving maximized speckle contrast, and thereby maximizing the variation of the laser speckle contrast image.

Speckle reduction in line-scan laser projectors using binary phase codes.

Abstract: A Barker binary phase code of maximum length 13 has previously been used for speckle reduction in line-scan laser projectors, and a speckle contrast factor decrease down to 13% has been achieved. In this Letter, Barker-like binary phase codes of lengths longer than 13 are used at an intermediate image plane. It is shown by theoretical calculation that a much better speckle reduction with a speckle contrast factor up to 6% can be achieved by using longer binary phase codes other than the Barker code.

Noise in laser speckle correlation and imaging techniques

Abstract: We study the noise of the intensity variance and of the intensity correlation and structure functions measured in light scattering from a random medium in the case when these quantities are obtained by averaging over a finite number N of pixels of a digital camera. We show that the noise scales as 1/N in all cases and that it is sensitive to correlations of signals corresponding to adjacent pixels as well as to the effective time averaging (due to the finite integration time) and spatial averaging (due to the finite pixel size). Our results provide a guide to estimation of noise levels in such applications as multi-speckle dynamic light scattering, time-resolved correlation spectroscopy, speckle visibility spectroscopy, laser speckle imaging etc.

Bispectral analysis and recovery of images distorted by a moving water surface

Abstract: We propose a new algorithm to recover a geometrically correct image of an object or scene from a set of images distorted by the wave motion of a water surface. Under mild conditions where the wavy surface normals weakly satisfy a Gaussian distribution, we demonstrate that the geometric distortion can be removed and a corrected image can be recovered. Our method is based on higher-order spectra analysis-in particular, the bispectrum, similar to its use in astronomical speckle imaging. In adapting this technique to imaging through or over a moving water surface, special care must be taken, and specifically tailored techniques are discussed in this paper. Our algorithm has been tested under two different scenarios: the refraction of light through a water surface (the underwater case) and the reflection of light from a water surface (the reflection case). Results in both cases have been encouraging.

Speckle reduction using a motionless diffractive optical element

Abstract: Speckle reduction by moving diffuser has been previously studied in display systems with coherent light sources, such as lasers. In this Letter, we propose a motionless diffractive optical element (DOE) for speckle reduction. The DOE was designed based on finite-element method simulations, fabricated using micromachining technology, and characterized for despeckle efficiency. Experiments using a DOE with two gratings have indicated that the speckle was suppressed to 50%, which shows fair agreement with theoretical analysis. With some modification of this DOE, the speckle noise can be reduced to 10% according to the theory.

SEPARATED FRINGE PACKET OBSERVATIONS WITH THE CHARA ARRAY. I. METHODS AND NEW ORBITS FOR χ DRACONIS, HD 184467, AND HD 198084

Abstract: We present the modification of the orbits of χ Draconis and HD 184467, and a completely new orbit for HD 198084, including data taken at the Center for High Angular Resolution Astronomy (CHARA) Array. These data were obtained using a modification of the technique of separated fringe packets (SFPs). The accuracy of the SFP data surpasses that of data taken by speckle, but the technique is much more time and labor intensive. Additionally, using SFPs with the CHARA Array, it is possible to obtain separations below the detection range of speckle interferometry (30 mas) above the range in “classic” long-baseline interferometry where fringes from a binary overlap are no longer separated (10 mas). Using spectroscopic binary systems with published speckle orbits, we are able to test our new measurements against their ephemerides to calibrate the method as well as produce entirely new orbits for systems with no current astrometric observations.

High-Resolution Imaging at the SOAR Telescope

Abstract: Bright single and binary stars were observed at the 4.1 m telescope with a fast electron-multiplication camera in the regime of partial turbulence correction by the visible-light adaptive optics system. We compare the angular resolution achieved by simple averaging of AO-corrected images (long-exposure), selection and recentering (shift-and-add or "lucky" imaging), and speckle interferometry. The effect of partial AO correction, vibrations, and image postprocessing on the attained resolution is shown. Potential usefulness of these techniques is evaluated for reaching the diffraction limit in ground-based optical imaging. Measurements of 75 binary stars obtained during these tests are given and objects of special interest are discussed. We report tentative resolution of the astrometric companion to ζ Aqr B. A concept of advanced high-resolution camera is outlined.

The oriented spatial filter masks for electronic speckle pattern interferometry phase patterns.

Abstract: we propose the oriented spatial filter masks for filtering in electronic speckle pattern interferometry (ESPI) phase fringe patterns. We establish the oriented derivative operator that only highlights noise without edges of an image. The noise in the image can be removed while still preserving the edges simply by subtracting the oriented derivative image from original image, which can be implemented with one pass of the oriented spatial filter mask. Further, we make an improvement on the oriented spatial filter mask for enhancing the smoothness. The performance of the oriented spatial filter masks is demonstrated via application to a simulated speckle phase fringe pattern and an experimentally obtained phase fringe pattern and comparison with other directional filtering methods.

Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 1: Single and dual pass cells.

Abstract: New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance–limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651 nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1σ) of 2×10−5, but longer term drift of up to 3×10−4 over 22 hours.

Electronic speckle-pattern interferometer using holographic optical elements for vibration measurements

Abstract: We report a simple, compact electronic speckle-pattern interferometer (ESPI) incorporating holographic optical elements (HOEs) for the study of out-of-plane vibration. Reflection and transmission HOEs provide reference and object beams in the interferometer. The alignment difficulties with conventional ESPI systems are minimized using HOEs. The time-average ESPI subtraction method is used to generate the fringe pattern and remove background speckle noise by introducing a phase shift between consecutive images. The amplitude and phase maps are obtained using path-difference modulation.

Electronic speckle pattern interferometry and digital holographic interferometry with microbolometer arrays at 10.6 μm

Abstract: Electronic speckle pattern interferometry and digital holographic interferometry are investigated at long infrared wavelengths. Using such wavelengths allows one to extend the measurement range and decrease the sensitivity of the techniques to external perturbations. We discuss the behavior of reflection by the object surfaces due to the long wavelength. We have developed different experimental configurations associating a CO2 laser emitting at 10.6μm and microbolometer arrays. Phase-shifting in-plane and out-of-plane electronic speckle pattern interferometry and lensless digital holographic interferometry are demonstrated on rotation measurements of a solid object.

Three-dimensional speckle suppression in optical coherence tomography based on the curvelet transform

Abstract: Optical coherence tomography is an emerging non-invasive technology that provides high resolution, cross-sectional tomographic images of internal structures of specimens. It holds great potentials for a wide variety of applications, especially in the field of biomedical imaging. OCT images, however, are usually degraded by significant speckle noise. Here we report a 3D approach to attenuating speckle noise in OCT images. This approach is based on the 3D curvelet transform, and is conveniently controlled by a single parameter that determines the threshold in the curvelet domain. Unlike 2D approaches which only consider information in individual images, 3D processing, by analyzing all images in a volume simultaneously, has the advantage of also taking the information between images into account. This, coupled with the curvelet transform's nearly optimal sparse representation of curved edges that are common in OCT images, provides a simple yet powerful platform for speckle attenuation. We show the approach suppresses a significant amount of speckle noise, and in the mean time preserves and thus reveals many subtle features that could get attenuated in other approaches.

Quantitative imaging with multi-exposure speckle imaging (mesi)

Abstract: Methods and systems relating to multi-exposure laser speckle contrast imaging are provided. One such system comprises a laser light source, a light modulator, and a detector for the measurement of reflected light comprising at least one camera, at least one magnification objective, and at least one microprocessor or data acquisition unit.

Observing Deformation and Fracture of Rock with Speckle Patterns

Abstract: The high resolution technique of electronic speckle pattern interferometry (ESPI) can be very useful in determining deformation of laboratory specimens and identifying initiation of failure. The in-plane ESPI theory is described and the fringe pattern of the processed ESPI image is analyzed to determine deformation and crack opening displacement. Fringes on the ESPI image represent lines of equal intensity, which relate to surface displacement. An ESPI system was constructed and calibrated for measuring uni-directional displacements. Several types of the experiments, such as uniaxial compression and fracture testing, were conducted to demonstrate the utility of ESPI.

Speckle suppression in projection displays by using a motionless changing diffuser.

Abstract: Speckle suppression in projection displays with a laser light source can be achieved by imaging a changing diffuser with random phase cells onto the screen. Theoretical expressions for the speckle contrast in this method have been earlier obtained in the case when different realizations of the phase diffuser produced statistically independent patterns of the light field on the screen. In the present paper, these expressions are generalized in the case when different realizations of the phase diffuser produce partly correlated speckle patterns. The possible structure of a motionless changing diffuser is presented. It includes a dynamic diffractive optical element (DDOE) and a light homogenizer. The DDOE can be based on the electrically controlled spatial light modulator (SLM) with a deformable polymer layer. This type of SLM can handle high light power and, therefore, can be used in projection displays with powerful laser beams.

Phase Extraction in Dynamic Speckle Interferometry with Empirical Mode Decomposition and Hilbert Transform

Abstract: In many respects, speckle interferometry (SI) techniques are being considered as mature tools in the experimental mechanics circles. These techniques have enlarged considerably the field of optical metrology, featuring nanometric sensitivities in whole-field measurements of profile, shape and deformation of mechanical rough surfaces. Nonetheless, when we consider classical fringe processing techniques, e.g. phase-shifting methods, the deformation range is intrinsically limited to the correlation volume of the speckle field. In addition, the phase evaluation from such patterns is still computationally intensive, especially in the characterisation of dynamic regimes, for which there is a growing interest in a wide range of research and engineering activities. A promising approach lies in the pixel history analysis. We propose in this paper to implement the empirical mode decomposition (EMD) algorithm in a fast way, to put the pixel signal in an appropriate shape for accurate phase computation with the Hilbert transform. © 2008 Blackwell Publishing Ltd.

Observation of Piezoelectrically Induced Lamb Wave Propagation in Thin Plates by Use of Speckle Interferometry

Abstract: Lamb waves have shown to be favorably applicable in the development of health monitoring systems in structural mechanics since they show reflections, refractions, as well as mode conversions at fault locations. For the observation of the wave propagation, the double pulsed electronic speckle pattern interferometry (ESPI) as a high precision, full-field, and non-contact measurement technique is under investigation in this work. First, it is shown that symmetric and antisymmetric Lamb waves can be captured in a faultless plate. Moreover, mode conversions are observed at stiffness discontinuities in a specially prepared plate. Finally, a riveted lightweight structure is under investigation. In these experiments, mode conversions indicate the location of the rivets.

High contrast optical imaging of companions: the case of the brown dwarf binary HD-130948BC

Abstract: High contrast imaging at optical wavelengths is limited by the modest correction of conventional near-IR optimized AO systems.We take advantage of new fast and low-readout-noise detectors to explore the potential of fast imaging coupled to post-processing techniques to detect faint companions to stars at small separations. We have focused on I-band direct imaging of the previously detected brown dwarf binary HD130948BC,attempting to spatially resolve the L2+L2 benchmark system. We used the Lucky-Imaging instrument FastCam at the 2.5-m Nordic Telescope to obtain quasi diffraction-limited images of HD130948 with ~0.1" resolution.In order to improve the detectability of the faint binary in the vicinity of a bright (I=5.19 \pm 0.03) solar-type star,we implemented a post-processing technique based on wavelet transform filtering of the image which allows us to strongly enhance the presence of point-like sources in regions where the primary halo dominates. We detect for the first time the BD binary HD130948BC in the optical band I with a SNR~9 at 2.561"\pm 0.007" (46.5 AU) from HD130948A and confirm in two independent dataset that the object is real,as opposed to time-varying residual speckles.We do not resolve the binary, which can be explained by astrometric results posterior to our observations that predict a separation below the NOT resolution.We reach at this distance a contrast of dI = 11.30 \pm 0.11, and estimate a combined magnitude for this binary to I = 16.49 \pm 0.11 and a I-J colour 3.29 \pm 0.13. At 1", we reach a detectability 10.5 mag fainter than the primary after image post-processing. We obtain on-sky validation of a technique based on speckle imaging and wavelet-transform processing,which improves the high contrast capabilities of speckle imaging.The I-J colour measured for the BD companion is slightly bluer, but still consistent with what typically found for L2 dwarfs(~3.4-3.6).

Magnetomotive laser speckle imaging

Abstract: Laser speckle imaging (LSI) involves analysis of reflectance images collected during coherent optical excitation of an object to compute wide-field maps of tissue blood flow. An intrinsic limitation of LSI for resolving microvascular architecture is that its signal depends on relative motion of interrogated red blood cells. Hence, with LSI, small-diameter arterioles, venules, and capillaries are difficult to resolve due to the slow flow speeds associated with such vasculature. Furthermore, LSI characterization of subsurface blood flow is subject to blurring due to scattering, further limiting the ability of LSI to resolve or quantify blood flow in small vessels. Here, we show that magnetic activation of superparamagnetic iron oxide (SPIO) nanoparticles modulate the speckle flow index (SFI) values estimated from speckle contrast analysis of collected images. With application of an ac magnetic field to a solution of stagnant SPIO particles, an apparent increase in SFI is induced. Furthermore, with application of a focused dc magnetic field, a focal decrease in SFI values is induced. Magnetomotive LSI may enable wide-field mapping of suspicious tissue regions, enabling subsequent high-resolution optical interrogation of these regions. Similarly, subsequent photoactivation of intravascular SPIO nanoparticles could then be performed to induce selective photothermal destruction of unwanted vasculature.

Measurement of three-dimensional deformations using digital holography with radial sensitivity

Abstract: A measurement system based on digital holography for the simultaneous measurement of out-of-plane and radial in-plane displacement fields for the assessment of residual stress is presented. Two holograms are recorded at the same time with a single image taken by a digital camera, allowing the separate evaluation of in-plane and out-of-plane movement. An axis-symmetrical diffractive optical element is used for the illumination of the object, which causes radial sensitivity vectors. By the addition and, respectively, the subtraction, of the four phase maps calculated from two camera frames, the in-plane and out-of-plane deformation of an object can be calculated separately. The device presented is suitable for high-speed, high-resolution measurement of residual stress. In addition to the setup, first measurement results and a short comparison to a mature digital speckle pattern interferometry setup are shown.

Vector tomographic X-ray phase contrast velocimetry utilizing dynamic blood speckle

Abstract: We present a time-resolved tomographic reconstruction of the velocity field associated with pulsatile blood flow through a rotationally-symmetric stenotic vessel model. The in-vitro sample was imaged using propagation-based phase contrast with monochromated X-rays from a synchrotron undulator source, and a fast shutter-synchronized detector with high-resolution used to acquire frames of the resulting dynamic speckle pattern. Having used phase retrieval to decode the phase contrast from the speckle patterns, the resulting projected-density maps were analysed using the statistical correlation methods of particle image velocimetry (PIV). This yields the probability density functions of blood-cell displacement within the vessel. The axial velocity-field component of the rotationally-symmetric flow was reconstructed using an inverse-Abel transform. A modified inverse-Abel transform was used to reconstruct the radial component. This vector tomographic phase-retrieval velocimetry was performed over the full pumping cycle, to completely characterize the velocity field of the pulsatile blood flow in both space and time.

Time-resolved quantitative multiphase interferometric imaging of a highly focused ultrasound pulse

Abstract: Interferometric imaging is a well-established method to image phase objects by mixing the image wavefront with a reference one on a CCD camera. It has also been applied to fast transient phenomena, mostly through the analysis of single interferograms. It is shown that, for repetitive phenomena, multiphase acquisition brings significant advantages. A 1MHz focused sound field emitted by a hemispherical piezotransducer in water is imaged as an example. Quantitative image analysis provides high resolution sound field profiles. Pressure at focus determined by this method agrees with measurements from a fiber-optic probe hydrophone. This confirms that multiphase interferometric imaging can indeed provide quantitative measurements.

Textured pattern sensing using partial-coherence speckle interferometry

Abstract: A system for imaging a textured surface includes a light source that is configured to project an electromagnetic radiation beam onto the textured surface, wherein the projected beam generates first radiation reflected from a first portion of the textured surface to form a speckle pattern, and second radiation reflected from a second portion of the textured surface which is substantially uniform in intensity. The reflected first and second reflected radiation is received by an optical detector, and may be processed to generate an image of the textured surface from the first and second reflected radiation. Methods for textured surface sensing are also disclosed.

The ATST Visible Broadband Imager: A Case Study For Real-time Image Reconstruction and Optimal Data Handling

Abstract: At future telescopes, adaptive optics systems will play a role beyond the correction of Earth's atmosphere. These systems are capable of delivering information that is useful for instrumentation, e.g. if reconstruction algorithms are employed to increase the spatial resolution of the scientific data. For the 4m aperture Advanced Technology Solar Telescope (ATST), a new generation of state-of-the-art instrumentation is developed that will deliver observations of the solar surface at unsurpassed high spatial resolution. The planned Visual Broadband Imager (VBI) is one of those instruments. It will be able to record images at an extremely high rate and compute reconstructed images close to the telescope's theoretical diffraction limit using a speckle interferometry algorithm in near real-time. This algorithm has been refined to take data delivered by the adaptive optics system into account during reconstruction. The acquisition and reconstruction process requires the use of a high-speed data handling infrastructure to retrieve the necessary data from both adaptive optics system and instrument cameras. We present the current design of this infrastructure for the ATST together with a feasibility analysis of the underlying algorithms.