Showing papers on "Speckle imaging published in 1989"

Speckle processing method for synthetic-aperture-radar phase correction.

Abstract: Uncompensated phase errors present in synthetic-aperture-radar data can have a disastrous effect on reconstructed image quality. We present a new iterative algorithm that holds promise of being a robust estimator and corrector for arbitrary phase errors. Our algorithm is similar in many respects to speckle processing methods currently used in optical astronomy. We demonstrate its ability to focus scenes containing large amounts of phase error regardless of the phase-error structure or its source. The algorithm works extremely well in both high and low signal-to-clutter conditions without human intervention.

Direct phase estimation from phase differences using fast elliptic partial differential equation solvers.

Abstract: Obtaining robust phase estimates from phase differences is a problem common to several areas of importance to the optics and signal-processing communities. Specific areas of application include speckle imaging and interferometry, adaptive optics, compensated imaging, and coherent imaging such as synthetic-aperture radar. We derive in a concise form the equations describing the phase-estimation problem, relate these equations to the general form of elliptic partial differential equations, and illustrate results of reconstructions on large M by N grids, using existing, published, and readily available FORTRAN subroutines.

Diffraction-limited imaging with very large telescopes

Abstract: I: Basic principles.- to Fourier Optics and Coherence.- Optical Propagation and Image Formation Through the Turbulent Atmosphere.- Radio Telescopes: Basic Concepts.- Continuously Movable Telescopes for Optical Interferometry.- II: Module and phase estimation, image reconstruction.- Amplitude Estimation from Speckle Interferometry.- Notes on Noise Calibration of Speckle Imagery.- Amplitude Estimation from Diluted Array Interferometry.- First Order Imaging Methods: An Introduction.- Speckle Imaging with the Papa Detector and the Knox-Thompson Algorithm.- Phase Closure Imaging - Theory & Practice.- Speckle Masking, Speckle Spectroscopy, and Optical Aperture Synthesis.- Merits of Phase Restoration Methods.- Pupil-Plane Interferometry.- Real-Time Wavefront Sensing and Adaptive Optics.- Differential Interferometry.- Image Restoration.- Deconvolution Ancient and (Very) Modern.- III: Scientific and technical prospects.- Astrophysical Programs for High Angular Resolution Optical Interferometry.- Frontiers of Interferometry.- Perspectives in Optical Interferometry.- IV: Contributed seminars and posters.- Cophasing Telescope Arrays.- Some Thoughts on the Combination of Beams in Interferometers Using Telescopes of Unequal Size.- Real-Time Image Sharpening.- Scasis, Diffraction-Limited Imaging Using a Redundant Pupil Plane-Interferometer.- Aperture Synthesis in Space: Overview and Results from the Esa Study Group.- Estimation and Reconstruction from Aberrated Amplitude Interferometer Measurements.- Lunar Occultations or Milliarcsecond Resolution and Their Relation to Interferometric Techniques at Present and in Future.- List of posters.

Diffraction-Limited Imaging II. Optical Aperture-Synthesis Imaging of Two Binary Stars

Abstract: We have used the technique of nonredundant masking at the Hale 5 m telescope and radio VLBI imaging software are used to make optical aperture-synthesis maps of two binary stars, β Corona Borealis and σ Herculis The dynamic range of the map of β CrB is about 50:1 For σ Her, we find a separation of 70 milliarcsec These maps demonstrate the potential of the nonredundant masking technique for diffraction-limited imaging of astronomical objects with high dynamic range We find that the optimal integration time for measuring the closure phase is found to be longer than that for measuring the fringe amplitude There is not a close relationship between amplitude errors and phase errors, as is found in radio astronomy, and amplitude self-calibration is less effective at optical wavelengths than at radio wavelengths The primary beam sensitivity correlation made in radio astronomy is not necessary in optical interferometry

Manipulation of the Fourier Components of Speckle Fringe Patterns as Part of an Interferometric Analysis Process

Abstract: A Fourier filtration algorithm has been applied to speckled fringe patterns, as obtained with the electronic speckle pattern interferometer. The Fourier power spectrum components found are then conveniently manipulated to extract noise that is inherent to the speckled fringes, thus rendering holographic-quality displays. The use of this technique facilitates the computer interferometric analysis of such fringe patterns. Some examples of the phase fringes and the corresponding object displacements obtained using this method are shown.

High-resolution imaging of the living human fovea: measurement of the intercenter cone distance by speckle interferometry

Abstract: A high-resolution method that allows direct measurements of the intercenter cone distance in the living human fovea is proposed The experimental technique is similar to that used in stellar speckle interferometry It is based on the recording and posterior processing of coherent short-exposure images of a small area of the central fovea By using this optical–digital procedure, we have obtained what are to our knowledge the first objective measurements in vivo of the cone spacing in the human fovea The reconstruction of the whole spatial information of the cone mosaic would also be possible by further improvements of the technique by subsequent application of image-reconstruction algorithms

The rotating envelope of the hot star gamma Cassiopeiae resolved by optical interferometry

Abstract: CERTAIN hot stars, belonging to the Be class, may have an envelope of hydrogen gas, possibly in the form of a rotating disk1 or spheroidal shell2. Other models involve elliptical rings3 or close binary systems where the Roche lobe of the companion is filled with hydrogen2. The angular size of these features is too small for direct detection by conventional telescopes, and attempts to resolve the structure using speckle interferometry (a technique that restores the diffraction-limited resolution otherwise degraded or spoiled by the atmosphere) have failed. The various models of the structure of the hydrogen envelope are based on spectroscopic data, together with polarization and variability measurements. After a century of spectroscopic observations, which showed considerable but little-understood variations, the hydrogen envelope of the star gamma Cassiopeiae was angularly resolved by the prototype interferometer I2T in 1986 (ref. 4). Here we report observations from its successor GI2T, which show further high-resolution details of the hydrogen envelope. The data clearly show the envelope in rotation and approximately fit a disk model. Thus, the GI2T yields optical information capable of constraining astrophysical models on a milliarcsecond scale.

An electro-optic holography system with real-time arithmetic processing

Abstract: This paper reports the use of a real-time arithmetic image processor in an electro-optic holography system. A speckle interferometer is used to combine an image of an object, lit by laser light, with a mutually coherent reference beam. A CCD TV camera detects the interference pattern, and the phase of the reference beam is advanced by 90° between frames. An image is generated from each set of four sequential TV frames by subtracting alternate frames, squaring, and adding the two results. The result is improved picture quality compared with the use of binary pixels and compared with electronic speckle pattern interferometry. Experimental results are shown.

Speckle from a cascade of two thin diffusers

Abstract: Propagation through a generalized thick diffuser and a spaced cascade of two thin diffusers is analyzed using methods of statistical optics. Second- and fourth-order moments are derived for the scalar component of the far-zone field. In the formulation two states a and b are used, permitting one to consider variations of speckle with angle of illumination and wavelength of an input plane wave and with observation direction as well as with in-plane and longitudinal motion between the two thin diffusers. This two-state correlation function of intensity, i.e., fourth-order moment, is evaluated without the assumption of circularity, and explicit calculations of decorrelation with respect to the state variables are presented for a cascade of two paraboloidal diffusers. Decorrelation occurs with slight transverse motion of one of the diffusers that is essentially independent of the separation H. Decorrelation of the on-axis speckle occurs for a change in spacing ΔH that is also independent of H. For single thin diffusers or a closely spaced diffuser pair, decorrelation with changes of angle of illumination is quite slow. However, rapid angular decorrelation is obtained for large spacings. Wavelength decorrelation of the diffuser pair arises from two unrelated effects, i.e., the dependence of the individual diffusers and of the propagation between diffuser planes on wavelength. Plots are given to illustrate the different speckle dependencies. This analysis of speckle decorrelation can be used in various remote-sensing applications.

Computerized fringe analysis in photomechanics

Abstract: A generalized method is presented for analyzing the fringe patterns frequently encountered in experimental mechanics By utilizing digital-image-processing and computer-graphics techniques, a set of menu-driven software is developed for interactively implementing the fringe processing Tests of this software on the images obtained experimentally by photoelasticity, holographic interferometry and speckle interferometry demonstrate its usefulness Good agreement between the experimental and theoretical results is established

Two-dimensional phase correction of synthetic aperture radar imagery

Abstract: A two-dimensional synthetic-aperture-radar (SAR) phase-correction algorithm is described as a natural extension of a one-dimensional technique developed previously. It embodies some similarities of phase-gradient speckle imaging and incorporates improvements in phase estimation. Diffraction-limited performance has been obtained on actual SAR imagery regardless of scene content or phase-error structure. The algorithm is computationally efficient, robust, and easily implemented on a general-purpose computer or special-purpose hardware.

Two-dimensional phase correction of synthetic-aperture-radar imagery.

Abstract: A two-dimensional synthetic-aperture-radar (SAR) phase-correction algorithm is described as a natural extension of a one-dimensional technique developed previously. It embodies some similarities of phase-gradient speckle imaging and incorporates improvements in phase estimation. Diffraction-limited performance has been obtained on actual SAR imagery regardless of scene content or phase-error structure. The algorithm is computationally efficient, robust, and easily implemented on a general-purpose computer or special-purpose hardware.

New Method Of Contouring Using Digital Speckle Pattern Interferometry (DSPI)

Abstract: A new method of contouring 3-D objects in real-time is suggested. The technique makes use of Digital Speckle Pattern Interferometry (DSPI) alongwith an in-plane sensitive optical configuration. The contour interval can be varied in real-time by varying the tilt applied to the object. The variation of the contour interval with the tilt angle and the angle between the illumination beams is discussed. The decorrelation of speckle pattern due to tilt and consequently the disappearance of the fringes is also discussed. Experimental results have been presented for a variety of objects.© (1989) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Technical aspects of the speckle masking phase reconstruction algorithm

Abstract: Les problemes techniques survenant lorsque l'algorithme a masque speckle 4D complet est execute pour reconstruire les images 2D, limitees par la diffraction, d'objets astronomiques sont discutes, en particulier les sources etendues

Fringe positions in double-exposure speckle photography

Abstract: Double-exposure records in speckle photography or particle image velocimetry are often evaluated by analysis of the system of Young's diffraction fringes. Fringe spacing, necessary to calculate the displacement, is determined from the positions of fringe maxima or minima. These, however, are influenced by the diffraction halo function and by fringe visibility. A generalized theory of the effects is presented, including position dependent visibility and fringe phase. Evaluations are given for disk-shaped particle images in particle image velocimetry, and for coherent and incoherent speckle photography. Fringe shifts are determined numerically for commonly encountered values of fringe density and visibility thus presenting a basis for rapid assessment of accuracy in metrological experiments.

Vacuum stressing technique for composite laminates inspection hy optical method

Abstract: The vacuum stressing as a nondestructive method is an effective tool employed in conjunction with holographic interferometry for the detection of flaws and delaminations on or near the surface of composite laminates. Vacuum stressing can also be used in other optical testing techniques such as shearography, speckle interferometry and Moire analysis.

Interferometric Imaging in Optical Astronomy

Abstract: The atmosphere of the earth restricts the resolution of conventional astronomical imaging to about 1″. Much higher resolution can be obtained by speckle methods. Speckle interferometry, the Knox-Thompson method and the speckle masking method (bispectrum or triple correlation processing) yield diffraction-limited resolution in spite of image degradation by the atmosphere and telescope aberrations. For example, with the ESO 3.6-m telescope a resolution of 0.028″ is attained at a wavelength of 400 nm. The limiting magnitude is about 18. We will outline the theory behind the three methods. High-resolution images and simultaneously the spectrum of each resolution element can be obtained by the objective prism speckle spectroscopy and projection speckle spectroscopy methods. Finally, we will discuss the application of speckle masking to coherent arrays of telescopes. A very interesting example is the 4x8-m ESO VLT, which should yield the fantastic angular resolution of about 2 milli-arcsec.

Amplitude Estimation from Speckle Interferometry

Abstract: Speckle interferometry and, most often, more sophisticated developments aiming at image reconstruction rely on the estimation of the object spectrum amplitude. It is therefore essential for optical image plane interferometry to obtain reliable amplitudes by using well understood procedures. While the basic one consists of a calibration of the modulation transfer function, additional steps are used to take care of its variations and of detectors specificities. Some of them are related to correction of artefacts naturally polluting the data; they are introduced here in a way adequate for case by case adaptation.