Showing papers on "Speckle imaging published in 1992"

Blind deconvolution of speckle images

Abstract: A technique for deconvolving an image from both a single convolution and an ensemble of differently blurred images is presented. The method is more robust than the earlier blind deconvolution algorithms proposed by Ayers and Dainty [ Opt. Lett.13, 547 ( 1988)] and Davey et al. [ Opt. Commun.69, 353 ( 1989)]. The performance of the algorithm in the presence of noise is evaluated. It is also demonstrated how the algorithm can be modified to utilize the much greater amount of information contained in an ensemble of differently blurred pictures of an image. Reconstructions using both computer simulations and infrared astronomical speckle data are presented. The speckle reconstructions are compared with those obtained by both Fourier phase retrieval and bispectral estimation.

Three-dimensional dimension of laser speckle

Abstract: It is well known that the three-dimensional dimension of laser speckle is a critical parameter in laser holographic interferometry, laser speckle photography, and holospeckle interferometry. In this paper a statistical analysis for computing the three-dimensional dimension of laser speckle is presented. Explicit formulas have been derived for laser speckle in a free space from a circular scattering area along with some supporting results from an experimental investigation.

Speckle interferometry in the measurements of biotissue vibrations

Abstract: The mechanism of speckle interferometer output signal forming for biovibration measurements is discussed. Statistical characteristics analysis of output signal amplitude is carried out. The factors creating parasitic amplitude-phase modulation of the output signal are investigated. The possibility of differential homodyne speckle interferometers application in Tibetan pulse diagnostics is shown.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Absolute quadrant determinations from speckle observations of binary stars

Abstract: Reduction of speckle data obtained for binary stars is typically carried out using power spectrum or, equivalently, autocorrelation methods. An especially powerful algorithm from which accurate differential astrometry can be obtained is the vector-autocorrelation technique. While such methods are highly suited to extracting astrometric information from very large volumes of speckle data in near real time, they inherently introduce a 180° ambiguity in the position angle measurement.We here briefly describe why this can be a problem in determining the orbital motions in binaries, present a new algorithm which maintains most of the simplicity of vector autocorrelation while removing the quadrant ambiguity, and provide results of new absolute quadrant determinations for 66 binary star systems first resolved by the long-term GSU/CHARA speckle program carried out at the National Optical Astronomy Observatories

Image recovery from correlations

Abstract: Many image-recovery problems involve the determination of a real-valued, nonnegative image whose nth-order correlation approximates some measured or specified function. Special cases are image recovery from second-order correlations, or phase retrieval, and image recovery from third-order correlations, or triple-correlation recovery. Examples include astronomical speckle imaging, spectroscopy, imaging correlography, and the measurement of ultrashort laser pulses. We propose an iterative technique for recovering images from measurements of nth-order correlations. This technique is shown to preserve nonnegativity, allow for the inclusion of known support constraints, and produce a sequence of images whose nth-order correlations better approximate the measured data as the iterations proceed.

A speckle hologram of the interstellar plasma

Abstract: Observations of a speckle hologram of scattering material along the line of sight to the Vela pulsar indicate that this material is concentrated in the Vela supernova remnant, deep within the Gum Nebula. The speckle hologram is observed through the amplitude and phase variations of the interferometric cross-power spectrum with time and frequency. These variations describe the density fluctuations of the interstellar plasma, in a holographic fashion. The decorrelation due to the phase variations of the speckles yields the angular size of the scattering disk; comparison with the bandwidth of their amplitude variations yields a characteristic distance from earth to the scattering material of 0.81 +/- 0.03 of the distance from earth to the pulsar. This result is consistent with theories of irregularities associated with particle acceleration in shocks in supernova remnants.

Two-Dimensional Measurement of Strain Distribution by Speckle Correlation

Abstract: Electronic speckle pattern correlation is applied to two-dimensional measurement of in-plane displacement and strain distributions. Speckle displacement distribution is determined by the peak of the 2-D cross-correlation of each block of images before and after surface deformation. From the distribution, we can derive the biaxial strain distributions with the sensitivity of millistrains.

Spatiotemporal frequency multiplex heterodyne interferometry

Abstract: To have the advantages of both spatial and temporal heterodyne techniques and to make efficient use of the limited spatiotemporal frequency bandwidth of image detection systems, we propose a technique of spatiotemporal heterodyne interferometry using both spatial and temporal carrier frequencies. By means of spatiotemporal frequency multiplexing, the technique permits the simultaneous recording of multiple-phase objects on a single space–time interferogram.

A real-time digital shearing speckle interferometer

Abstract: Shearography is an interferometric method which measures surface displacement derivatives. Although useful for strain analysis and non-destructive testing, the conventional technique is cumbersome and not very suitable for an industrial environment. The authors present the development of an electronic/digital technique which eliminates most of the shortcomings. It enables real-time shearographic fringes to be obtained.

Linear reconstruction of compensated images: theory and experimental results.

Abstract: Linear image reconstruction techniques are proposed for postprocessing astronomical images measured with compensated imaging systems. Linear techniques use averaging to overcome the effects of noise and deconvolution to remove system effects. Experimental results from compensated image measurements of four single stars and one binary star at visible wavelengths are reported for the first time, to our knowledge, and a previously derived analytic expression relating the statistics of the compensated optical transfer function to the compensated image spectrum signal-to-noise ratio is verified. The performance of deconvolution on a bright binary star with angular subtense previously estimated to be 0.52 arcsec (2.52 μrad) is demonstrated.

Computerized optical fringe pattern analysis in photomechanics: a review

Abstract: Computerized analysis of optical fringe patterns has become an accepted method of reducing the time and tedium associated with extracting quantitative data from phase encoded intensity distributions. The computing power of today's personal computers makes digital image processing of moire, holographic, photoelastic, and speckle interferometric fringe patterns both economical and efficient. We review several automatic and semiautomatic image processing algorithms that enable the reduction of both speckle and full-field fringe patterns to quantitative data. The speckle patterns are analyzed using morphological properties, while the full-field fringe patterns are analyzed using standard thinning and fringe order seeding, sinusoidal fitting, Fourier transform, and phasestepping techniques. The mapping that each fringe pattern experiences when being digitized is also discussed so that proper system calibration can be developed. Analysis of geometric moire, moire interferometric, photoelastic, and speckle interferometric fringe patterns are presented as examples of digital image processing based data reduction.

Extended-image reconstruction through horizontal path turbulence using bispectral speckle interferometry

Abstract: Results are presented from a horizontal path imaging experiment in which a 0.5-m telescope was focused on targets located at a range of 1 .2 km. The targets varied in complexity from simple binary letters to extended representations of satellites with gray scale and size variations. Imaging at a center wavelength of 0.7 μm, we found an atmospheric degradation factor of D / r 0 = 17, on average. We used a slow read-rate bare CCD detector and thus had to deal effectively with additive noise in the speckle measurements. Our image reconstruction algorithms are based on the use of the complex bispectrum, and we have demonstrated diffraction-limited imaging down to light levels approaching a few photons per speckle per resolution area. We have paid careful attention to the effects of additive noise on the reconstruction process and have shown that they can be adequately overcome. These results support the feasibility of high-resolution speckle imaging of high-earth-orbit satellites using CCDs.

Diffraction-limited imaging with partially redundant masks. I. Infrared imaging of bright objects

Abstract: The utility of partially redundant pupil geometries has been studied in the context of near-infrared speckle imaging with ground-based telescopes. Using both numerical simulations and experimental data collected with a 4-m-class telescope, we find that the decrease in redundancy resulting from apodizing the telescope pupil results in an enhancement of the quality of reconstructed images at high light levels. This improvement in imaging fidelity is particularly valuable when short-term variations in the statistics of the atmosphere make the seeing calibration of speckle interferograms difficult. However, the use of an apodizing mask necessarily restricts the faintest source that can be imaged, leading to a loss in sensitivity of one to two magnitudes. For many of the brighter near-infrared astrophysical sources in the sky that have been the subject of previous speckle-imaging studies, the use of a partially redundant pupil is expected to enhance the fidelity of the imaging procedure considerably.

Infrared Speckle Imaging - Data Reduction with Application to Binary Stars

Abstract: The extended Knox-Thompson reduction scheme currently in use at Kitt Peak National Observatory and Steward Observatory for the reduction of infrared speckle interferometry is described in detail. Applications of this algorithm and the less computationally intensive shift-and-add analysis to both simulated and real binary star data is presented demonstrating that the latter may be successfully used for image reconstruction by using a separately measured point source. The performance of both techniques is shown to improve by application of an iterative deconvolution algorithm to apply a priori image constraints without degradation of photometric information.

Speckle imaging of satellites at the U.S. Air Force Maui Optical Station

Abstract: Results are presented from a series of experiments in which the U.S. Air Force Maui Optical Station’s 1.6-m telescope and a bare CCD speckle camera system were used to image satellites at distances of up to 1000 km. A brief overview of the image reconstruction algorithms is presented. The choice of the experiment site and various imaging parameters are described. Power spectra and power spectral signal-to-noise ratio curves that result from imaging several point stars are compared with theory. Reconstructed images of several binary stars are shown as a base-line assessment of our technique. High-quality image reconstructions of an Earth-satellite, the Hubble Space Telescope, are presented. The results confirm that speckle imaging techniques can be used with a bare CCD imaging system to provide a powerful and flexible method for imaging objects of moderate magnitude.

Solar speckle polarimetry

Abstract: The combination of a polarimeter with real-time frame selection and differential speckle imaging results in diffraction-limited magnetograms that provide new insight into the morphology of solar small-scale magnetic fields. The method to record diffraction limited narrow-band filtergrams of solar features is based on two cameras taking simultaneous short exposure images through a broad-band and a narrow-band filter, respectively. Speckle imaging reconstructs the image in the broad-band channel. This reconstruction determines the instantaneous optical transfer function (OTF) for each individual broad-band exposure

Shape measurement using phase-shifting speckle interferometry

Abstract: To determine the in-plane strain components from the 3-D displacement vector data of a loaded object, its shape has to be known. The phase shifting speckle interferometric system, described in this paper, has been used for both the displacement vector measurements and for the acquisition of the essential shape information. The numerical shape data resulting from the application of a phase shifting technique allows further calculations, e.g. the determination of the object surface normal vector. The shape measurement method (a modified two- illumination-source technique) is discussed and the experimental system is described. An overview of the basic image processing algorithms, involved in the evaluation of the speckle interference patterns, is given. Improved algorithms concerning phase unwrapping and intensity averaging in the case of low light levels are presented more extensively. Results of shape measurements on a metal cube and a glass bottle are shown. The shape measurement error is estimated at 0.1 mm r.m.s.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Phase-measuring fiber optic electronic speckle pattern interferometer: phase step calibration and phase drift minimization

Abstract: . A phase-measuring fiber optic electronic speckle pattern in- terferometer (ESPI) was recently constructed and demonstrated. In thisarrangement, phase stepping is introduced by stretching the fiber wrappedaround a piezoelectric transducer. Calibration of the phase step is criticalin phase measurement techniques for obtaining good phase plots. Meth-ods used for calibrating the stretcher are outlined and the drifts in thephase caused by fluctuations in temperature and mechanical stressesare discussed. Ways to minimize this drift are also presented. Subject terms: speckle interferometry; electronic speckle pattern interferometry.Optical Engineering 31(2), 315-321 (February 1992).phase step calibration is important in order to obtain error- free phase maps. Different methods for calibrating the piezoelectric transducer (PZT) in an interferometric arrangement13 as well as in a fiber optic interferometric sensor arrangement14 have been developed. Methods suchas phase-lock, sinusoidal extrema sensing, and employingsoftware routines to evaluate the phase from equations andintensity values are successful with specular interferometricfringes. On the other hand, the ESPI fringes are speckled.Even though good calibration can be obtained with phase-lock and sensing methods ,

Binary Star Orbits From Speckle Interferometry. IV. The Old Disk Population Star HR 1071

Abstract: HR 1071 (F7V, V=+6.36) is an old disk population star of low rotational velocity, moderately low metallicity, and extremely low Li abundance. The star was not recognized as being a binary system until its discovery as such from speckle observations obtained in late 1985. Rapid orbital motion has now permitted us to make a preliminary orbit determination from which we find an orbital period of 13.8 yr. Combining the elements of the apparent orbit with an estimation of the distance to the system and with a qualitative determination of the magnitude difference, we show that the system is a likely candidate for detection as a double-lined spectroscopic binary

Statistical Properties of Phase Variation in Shearing Speckle Interferometry and Its Effect on Fringe Visibility

Abstract: The statistical properties of two interfering speckle fields indicate a reduction in phase variation as the coherent correlation between them increases. In this paper, the impact of this phenomenon on fringe visibility in shearing speckle interferometry, where two laterally shifted speckle fields are super-imposed, is studied. Work is also done to correlate fringe visibility with the amount of lateral shear based on the limitations imposed by phase variance statistics.

Statistical approach to bias effects in the techniques of speckle interferometry and speckle masking

Abstract: We present an approach to the speckle interferometry techniques of Labeyrie and Weigelt that is based on the use of the moment-generating function (MGF). The remarkable properties of the MGF’s of photodetected images make it possible to obtain the expression for the photon-bias terms by a method completely different from the usual Goodman–Belsher approach [ GoodmanJ. W., Statistical Optics ( Wiley, New York, 1985)]. It gives us the mathematical relations to predict the effect of a clipped photodetection of the image on the autocorrelation and triple correlation functions. As a result, clipping ruins the linear relationship between object and reference source for these functions. The effect of photon recentering is also investigated and is directly related to a single-photon clipping effect. Analytical expressions for a normal speckle pattern are given.

Recovery of planetary images by speckle imaging.

Abstract: Limits to the recovery of planetary images by speckle imaging were investigated by means of a numerical simulation of the image-forming process. Laboratory measurements established that the numerical model correctly represented the process. With this numerical model we studied the observing conditions required to obtain useful planetary data for Neptune and Io, and we learned which factors are important for the successful recovery of images. We also showed that our methods of amplitude and phase recovery do not contribute to image degradation, which is essentially caused by photon noise alone. From this conclusion we were led to two simple models of speckle imaging that involve neither details of the recovery procedure nor of the atmospheric behavior (except for the size of the seeing disk). Both models are governed by a dimensionless ratio that involves all parameters relevant to the process of image restoration. The models can be used by any observer, and they have been used by us to predict the performance of speckle imaging for T-tauri stars and for a Mars cloud study.

Automatic heterodyning in fiber optic speckle pattern interferometry using laser velocimetry

Abstract: We combine the operational advantages ofthe complementary techniques of electronic speckle pattern interferometry (ESPI) and laser Doppler velocimetry (LDV) for out-of-plane vibrational analysis. A cw laser was used in a fiber optic system to generate time-averaged ESPI fringes. By mutual phase-locking of the ESPI and LDV signals, automatic heterodyning of the ESPI signal was achieved (i.e., without operator intervention) together with compensation for unwanted nondeterministic out-of-plane whole-body motion of the object under study. Automatic heterodyning extends the operating range of time-averaged ESPI systems to higher vibrational amplitudes and reveals phase relationships in the object vibration. A practical demonstration of the technique is described.

New speckle technique for noncontact measurement of small creep rates

Abstract: A highly sensitive method for measuring thermal expansion, mechanical strain, and creep rates has been developed. This technique is based on Yamaguchi's speckle strain gauge concept, but uses a novel data-processing approach that provides estimates of the time rate of in-plane strain. The approach is appropriate for assessing very small strain rates in hostile environments. It provides simultaneous global estimates of the strain at both small and large gauge sizes. This may be of importance in studying materials with different short- and long-range orders. General advantages of the technique are compact design, modest resolution requirements, insensitivity to surface microstructure changes (as seen with oxidation), and relative insensitivity to zero-mean noise processes such as turbulence and vibration.

Speckle interferometry: noise reduction by correlation fringe averaging

Abstract: A method for noise reduction in double-exposure speckle interferometry is proposed, based on averaging independent spatially filtered correlation fringe patterns.

Contrast of the vibration fringes in time-averaged electronic speckle-pattern interferometry: effect of speckle averaging

Abstract: This paper presents a detailed investigation of the effect of speckle averaging in electronic speckle-pattern interferometric fringes. The theory states that the constrast of the resultant smoothed fringes increases as the number of frames is increased. It is shown that the contrast of the fringes is optimized with a limited number of superpositions, and further addition results in the reduction of speckle noise with the contrast remaining almost the same. The contrast of the fringes obtained with π and π/2 phase-stepping methods with speckle averaging is also discussed. Both theoretical and experimental results are presented in this paper.