Showing papers on "Speckle imaging published in 1997"

Wavelength-shift speckle interferometry for absolute profilometry using a mode-hop free external cavity diode laser

Abstract: In this paper we present an absolute surface topography measurement with a tuneable diode laser with an external cavity that has a tuning range of as much as 25 nm without mode hops and a very high tuning speed of less than 1 s for the whole range. With this laser, an absolute wavelength-shift speckle pattern interferometer was realized, capable of measuring optically smooth and rough surfaces. We briefly explain the principles of operation, and the importance of wavelength tuning without mode hops. We discuss the capabilities, possible measuring ranges and some results as well as calibration methods of wavelength-shift speckle profilometry.

New developments in full field strain measurements using speckles

Abstract: Speckle photography is a versatile full field technique for measuring displacement and strain. However, the wet photographic process and tedium of fringe analysis had made it impractical for application to industrial problems. This paper describes a recent development of combining digital recording and image analysis that has rendered the technique a practical tool. In addition, through the development of SIEM (Speckle Interferometry with Electron Microscopy) the technique has been extended into micromechanics domain whereby strain fields within a region only a few micron square can be mapped with confidence. Several examples of this application are presented.

Decorrelation-induced phase errors in phase-shifting speckle interferometry.

Abstract: The purpose of this research is the quantitative investigation of decorrelation-induced phase errors in speckle interferometry. Measurements in speckle interferometry are inherently affected by decorrelation, i.e., by alterations of the speckle fields during measurement. Likewise, the random phases carrying the interferometric information change during decorrelation. Image plane and pupil plane decorrelation are considered for both smooth and speckle reference wave interferometers. Since the decorrelation effect depends on the aperture and the pixel size, the calculations include not only the case of speckles being well resolved by the camera but also the case of unresolved speckles. Different standard deviations of the phase error are obtained from the probability density of the pixel modulation and the phase before and after decorrelation. Most cases (apart from pupil plane decorrelation in speckle reference wave setups) appear to obey exactly the same phase error statistics. In particular, the number of speckles per pixel does not affect the phase error distribution over the whole image. The only important parameters determining the decorrelation-induced phase errors are the amount of decorrelation and the pixel modulation.

Phase-shifting speckle interferometry: a noise reduction filter for phase unwrapping

Abstract: We present a noise reduction filter for the phase unwrapping of data from phase-shifting speckle pattern interferometry. This filter is effective in reducing the noise in wrapped phase maps without affecting the edges of the phase jumps, so that a simple and fast unwrapping algorithm can be used. We also discuss a data analysis strategy, includ- ing a routine for the recovery of unwrapping errors. The filter perfor- mance has been evaluated on real data from a fiber optic phase-shifting speckle interferometer, which we built for the analysis of out-of-plane deformations. The phase shifting is introduced by current modulation of the diode laser source. Some results relative to the analysis of very noisy phase measurements are presented. © 1997 Society of Photo-Optical Instru- mentation Engineers. (S0091-3286(97)01209-9)

Three-dimensional deformation field measurements with simultaneous TV holography and electronic speckle photography.

Abstract: The ordinary recordings of TV holography (phase-stepped electronic speckle interferometry), arranged for out-of-plane sensitivity, are recombined and also analyzed by use of electronic speckle photography for simultaneous measurements of all three components of the deformation vector field. The principles of this technique are presented. Experimental results showing the opening of a slot in a perspex plate are also presented.

Optimization of fringe pattern calculation with direct correlations in speckle interferometry.

Abstract: A direct correlation technique is used to calculate correlation fringe patterns from consecutive speckle patterns acquired with a dual-beam electronic speckle interferometer. Although more calculations are required than in standard image differencing routines, an advantage of the method is that the illumination over the surface of the object need not be uniform. In the method, Pearson’s coefficient of correlation between the intensities within a set of adjacent pixels is calculated. This has the added advantage of being directly related to the theoretical phase-dependent correlation. A mapping of this measure of correlation results in the correlation fringe pattern. Laboratory tests were carried out with in-plane translations ranging from 5 to 45 µm. The correlation calculations were carried out by using cells (square sets of pixels) in the raw speckle images with dimensions ranging from 2 pixels × 2 pixels to 19 pixels × 19 pixels. Both cell dimension and translation magnitude dependent decorrelation effects influence the quality of the correlation fringe patterns.

Laser Speckle Photography Technique Applied for Heat and Mass Transfer Problems

Abstract: Publisher Summary The speckle photography refers to an optical method that measures the refractive index gradients of a phase object based on the optically created and refractively dislocated speckle patterns. The chapter introduces the operating principle of the speckle photography technique and discusses its unique features compared with other competing optical methods. Several different natural heat convection problems are presented for that speckle photography successfully measures the heat transfer coefficients without the need of corrections for the conduction and radiation losses. Speckle results provide data with high spatial resolution and make the experimental processes more reliable. Speckle photography measures the statistical properties for turbulent flows with density and temperature fluctuations. The chapter presents the tomographic reconstruction of a density field from multiple specklegrams. The potential of speckle photography for high-temperature applications such as combusting flames is discussed in the chapter. The viability of the technique emerges from its applications for premixed Bunsen flames of axisymmetric and laminar configuration. The chapter explores an example application of speckle photography for a liquid flow with density variation where the refractive index behaves quite differently from that of air or other gases.

Determination of the foveal cone spacing by ocular speckle interferometry: Limiting factors and acuity predictions

Abstract: We have developed a high-resolution imaging technique, based on speckle interferometry, for the objective determination of the cone spacing in the living human fovea. The spatial resolution attained with this technique is theoretically diffraction limited by the pupil size. However, the highest frequency that we measure varies greatly among subjects, especially for fully dilated pupils. We have conducted several experiments (determination of the cutoff frequency of ocular speckle interferometry, the double-pass modulation transfer function, and the Stiles–Crawford effect) that indicate that, as expected, the resolution is not limited by the incoherent modulation transfer function. We found, though, a high correlation between the cutoff frequency and the width of the eye’s Stiles–Crawford function. This implies that the resolution depends on the structural properties of the cone mosaic itself. In addition, we have compared the Nyquist frequency of the cone mosaic, determined objectively by our technique, with the grating visual acuity measured in the same eyes at the same foveal eccentricities. For our subjects, visual resolution nearly matches the Nyquist frequency within the fovea, except at the foveal center, where the optical transfer function of the eye attenuates the contrast of frequencies close to the Nyquist limit to a value below threshold.

Phase evaluation and speckle averaging in pulsed television holography

Abstract: Double-pulsed (image-plane) TV holograms of transient bending waves in plates are recorded on separate frames in a CCD camera. A small angular offset between the reference and object beams, giving a spatial-frequency bias to the recorded pattern, permits quantitative evaluation of the phase of the interference. The Fourier spectrum of the image exhibits distinct parts that can be filtered out and inverse transformed to yield the phase information. Three different apertures of the imaging system are tested: a single slit, a double slit, and a three-hole aperture. Spatial speckle averaging is possible in the cases of the double-slit and three-hole apertures.

Speckle Interferometry at the US Naval Observatory. I.

Abstract: We present speckle interferometer measurements of 467 binary stars taken at the US Naval Observatory in Washington, DC, using the 66 cm refractor, from 1990 October through 1992 December. The observing program is designed to provide high-quality observations of binaries in the 03-35 range of separations and as faint as 10.0 mag. More than 8000 measurements have been made to date, of which we report the results for 2329. Not only is it our intent to provide accurate data for interesting binary stars, but also, by careful calibration, to firmly relate the "classical" astrometry of binary stars to that being obtained today by speckle and that which will soon be obtained by other modern techniques such as long-baseline optical interferometry.

A compact dual-purpose camera for shearography and electronic speckle-pattern interferometry

Abstract: A dual-purpose camera for both shearography and electronic speckle-pattern interferometry (ESPI) based on the concept of additive - subtractive phase-modulated speckle interferometry has been designed and tested. The compact camera can be operated either in ESPI mode or in shearography mode by simply adjusting a moving mirror in the camera head. This camera has been designed as an optical part of an automatic inspection system for disbond detection in aircraft structures. The primary advantage is the possibility of obtaining information both about an object's displacement and about its gradient using a single low-cost device suitable for field applications.

Advanced Image-Processing Techniques for Automatic Nondestructive Evaluation of Adhesively-Bonded Structures Using Speckle Interferometry

Abstract: In conventional optical nondestructive evaluation (NDE) of structures using shearography or electronic speckle pattern interferometry (ESPI), results are typically provided in the form of fringe patterns or deformation contour plots However, in order to fully automate the process of defect detection, it is desirable to obtain simpler results which are easier to interpret We present here one such optical system based on additive–subtractive shearography/ESPI This system processes additive–subtractive fringe patterns and provides the sizes and locations of defects such as disbonds in adhesively-bonded composite structures This is achieved by exciting the structure under inspection using an acoustic stressing mechanism which sweeps a range of vibration frequencies of the structure Since the defective areas of the structure have different mechanical properties from their neighboring regions, varying and complex fringe patterns are obtained at different stressing frequencies We propose an algorithm which enables the automatic identification and selection of relevant additive–subtractive fringe patterns that pertain only to localized deformations associated with defects, and which excludes images that pertain to any overall modes of the entire structure The algorithm also includes a pixel-by-pixel adjustable thresholding scheme which compensates for intensity variations due to nonuniform reflectivity from unpainted and dirty test objects Morphological processing is then performed to extract the shapes of the defect from the processed fringe clusters Various structures, from simple aluminum specimens with simulated defects to a complex honeycomb-based aviation repair patch specimen, have been successfully evaluated using this system

Comparative measurement by speckle interferometry using holographically reconstructed master object

Abstract: In this paper a new comparative speckle interferometric technique is presented: we call it the difference digital speckle pattern interferometric (DDSPI) method. The technique uses the two holographically reconstructed virtual images of the (unloaded and loaded) master object as references to detect the difference in displacement profiles of two macroscopically similar but microscopically different object surfaces. The two images corresponding to the two different states of the master object are recorded holographically on one holographic plate or on a thermoplastic film. So, the method does not require the real presence of the master object in the comparison process: the master object does not need to be subjected to repeated stresses, and each test object can be compared with the very same two states of the master object, which makes the method more precise and faster than conventional techniques.

Processing of non-stationary interference patterns

Abstract: A new technique is presented, dynamic phase-shifting, which is based on a dedicated phase-shifting algorithm and a wavelet-transform analysis. This technique allows to perform measurements on non-static objects, using whole-field optical methods. These methods include classical, holographic and speckle interferometry as well as fringe projection and moire. They cover a large domain of resolutions and dynamic ranges for the measurement of shape and deformation of rough and smooth objects. However, the lack of efficient solutions to process the fringe patterns obtained in dynamic conditions has hindered the development of high-potential methods such as speckle interferometry outside of the laboratory. The main goal of this thesis work is to find new answers to this problem. The solutions we propose are based on the exploitation of the fringe movement produced by the deformation or displacement of the object. We observe that the corresponding phase variations of the interferogram can be used as a natural phase-shift to perform a quantitative phase evaluation. Moreover, it is shown that by adding a known phase step during image acquisition the sign of the displacement can be known without ambiguity. We demonstrate two particular techniques to process the image series recorded during dynamic phenomena. The first one is a 5-image phase-shifting algorithm, adapted to the problem of phase determination with unknown phase increments. The second solution is based on a wavelet-transform processing of the temporal signal recorded at each pixel of the camera. The goal is to estimate the phase of this sinusoidal signal as a function of time. We demonstrate that it can be obtained directly from the phase of the wavelet transform. The resulting method is highly immune to large signal noise. Moreover, we show that phase errors can be eliminated by combining the estimated phase evolution of neighboring pixels or by combining the corresponding real signals to create complex signals. This last approach is also used to extend the dynamic range of the technique. Application examples in the case of holographic and speckle interferometry are presented and both processing methods compared. Their complementarity and robustness brings forth the possibility to apply optical interferometric techniques in situ, our goal being the development of such methods for civil engineering applications.

Anisotropic speckle patterns in the light scattered by rough cylindrical surfaces.

Abstract: Speckle patterns in the light field scattered from the rough surface of a cylindrical object are experimentally studied. The light speckles are elongated in the direction normal to the cylinder axis. A theoretical model explains the main features of the scattered light field. The dimensions of light speckles depend on both the surface roughness and the surface curvature.

Angular correlation function of speckle patterns scattered from a one-dimensional rough dielectric film on a glass substrate.

Abstract: We present the experimental results of the angular correlation function of far-field speckle patterns scattered by a one-dimensionally random rough surface of a thin dielectric film on a glass substrate when a polarized beam of light is incident upon the rough surface from vacuum. This surface, which separates the vacuum and the dielectric, is rough enough that only diffused speckles are observed. The experiment for the correlation measurement was set up to make use of a CCD camera to obtain the image of the speckle pattern in the specular direction for each given angle of incidence; the cross-correlation function is then calculated from the digitized images. It is found that the intensity correlation functions exhibit two distinct maxima: one arises from the autocorrelation and the other from the reciprocity condition. It is also found that different scattering processes give rise to quite different correlation functions: multiple-scattering processes produce narrow peaks with secondary maxima and single-scattering processes produce relatively broad peaks.

Approach for the evaluation of speckle deformation measurements by application of the wavelet transformation

Abstract: We introduce a new, to our knowledge, method using wavelets and probability theory for the evaluation of speckle interference patterns for quantitative out-of-plane deformation measurements of rough surfaces of nontransparent solids. The experiment uses a conventional Twyman-Green interferometer setup. The speckle interference patterns are obtained by the common method of subtraction of images taken before and after a surface deformation. The data are processed by a wavelet transformation, which analyzes the image structures on different length scales. Thus it is possible to separate the interference fringes from the noise. From the locations of the interference fringes, the deformation of the surface can be reconstructed by means of probability theory.

Space object identification using phase-diverse speckle

Abstract: Space-object identification from ground-based telescopes is challenging because of the degradation in resolution arising from atmospheric turbulence. Phase-diverse speckle is a novel post-detection correction method that can be used to overcome turbulence-induced aberrations for telescopes with or without adaptive optics. We present a simulation study of phase-diverse speckle satellite reconstructions for the Air Force Maui Optical station 1.6-meter telescope. For a given turbulence strength, satellite reconstruction fidelity is evaluated as a function of quality and quantity of data. The credibility of this study is enhanced by reconstructions from actual compensated data collected wit the 1.5-meter telescope at the Starfire Optical Range. Consistent details observed across a time series of reconstructions from a portion of a satellite pass enhance the authenticity of these features. We conclude that phase-diverse speckle can restore fine-resolution features not apparent in the raw aberrated images of space objects.

Dark speckle imaging of colloidal suspensions in multiple light scattering media

Abstract: Quasielastic multiple light scattering experiments have been performed on suspensions of particles in brownian motion embedded inside a solid turbid medium. The photon transport mean free path of the suspension and of its solid environment where adjusted to be identical. We show that by selecting a minimum intensity spot of the speckle pattern generated by the solid medium, it is possible to visualize objects which would be undetectable with common optical techniques. Our results are shown to be in agreement with a diffusion theory for the location of strongly absorbing objects inside multiple scattering media.