The technique of laser speckle photography has been developed with the aim of measuring the strain field around the tip of a fast crack. An image processing system to allow automatic point-by-point analysis of a speckle photograph is described. The Young's fringes diffraction pattern, produced by directing a narrow laser beam through the photograph, is digitised and processed by computer. Two algorithms have been developed based on Fourier and Walsh spectral analysis. The system can measure speckle displacements with an accuracy of better than 0.1 μm. A new technique for measuring time-varying displacement fields by multiple exposure of a single photograph is presented. Results are shown from a five-exposure speckle photograph of a cantilever deformed under quasi-static conditions. Extension of the technique to dynamic problems requires a stroboscopic laser light source; one chapter deals with the development of a Q-switched ruby laser for this purpose. Modulation of the cavity losses with a Pockels cell at up to 1 MHz results in a train of short (~50 ns) light pulses at the modulation frequency. Results are presented from the analysis of speckle photographs of cracks in polymethylmethacrylate (PMMA) under quasi-static loading. Three different data analysis methods for estimating the stress intensity factor from the displacement field are proposed and evaluated. Preliminary results from dynamic crack propagation studies are described. The dynamic stress intensity factor is estimated from double exposure speckle photographs of fast cracks in PMMA, recorded with the ruby laser in double pulse mode. The application of the techniques of moire and high speed photography to the study of dynamic fracture is also discussed; results are presented from a sequence of moire photographs recorded at 5 x 10<SUP>5</SUP> frames s<SUP>-1</SUP> . Further topics covered in the dissertation include a theoretical analysis of viscoelastic losses as a function of crack velocity, and strain-to-failure measurements on plastic bonded explosives in the Brazilian test using laser speckle photography.

Thesis (Ph. D.)--Oregon Graduate Center, 1982.

Ph.D. / Applied Physics / Using the extended Huygens Fresnel principle, the effect of the atmospheric turbulence on the statistical properties of a polychromatic speckle field, generated by a diffuse target, is studied in detail. The results, substantiated by experimental data, indicate that the atmospheric perturbation increases the variance of the received intensity substantially and is sensitive to the wavelength, beam size and beam geometry. The results for the covariance of the received intensity, normalized to the variance, indicate that, at low turbulence levels, reduction in vacuum speckle contrast ratio (VSCR) also reduces the normalized covariance but, with further increase in the turbulence level, reduction in the vacuum speckle contrast ratio increases the normalized covariance. Also it is found that for small detector spacings, the normalized covariance remains approximately constant even with substantial increase in the turbulence level. By resolving the time delayed covariance of the received intensity (TDC), into coherent and incoherent terms, it is shown that for large time delays, the time delayed covariance is determined by the incoherent fluctuations and for poor vacuum speckle contrast ratio, the time delayed covariance is not very sensitive to the wind velocity. Finally it is shown that due to the atmospheric perturbation the probability density function of the received intensity changes from an M-distribution or a sum of exponential distributions in vacuum to a K-distribution or a weighted sum of K-distributions in the presence of the turbulent atmosphere.

Thesis (Ph. D.)--Oregon Graduate Institute of Science and Technology, 1992.

When an optically rough surface is illuminated by laser, a grainy pattern is produced at a distance away from the surface, which is known as laser speckle pattern. This pattern is random in intensity with its grains separated by an average amount controlled by the intermediate optical system or the diameter of the illuminated area of the surface in absence of an optical system. It has been shown that the grain spacing of such pattern may be correlated to yield a singly-peaked power spectral density of an intensity ensemble which is found by spatial ensemble averages. This correlation holds only if the sampling is done in a plane referred to as the constant plane. On an average basis such a characteristic peak persists with an in-plane rigid-body motion of the test object and within a uniformly rough area of the surface. The behavior of such peak with an induced in-plane strain is shown to be non-linear and of second order for the test range, when frequency values of such peak is plotted versus strain. Further, the slope of such curve changes sign when a strain of opposite sign is sensed. Finally, the best responses were obtained when no intermediate optical system was employed in the observation field.

Speckle noise is an integral part of any laser projection display because it is the nature of laser (coherent) illumination to form interference patterns with high visibility. The granularity of the image due to the speckle formed on the viewer's retina degrades the image quality, thus stimulating the need for speckle reduction techniques applied to laser displays. An instrument to measure image plane speckle contrast was built based on a linear CCD detector array interfaced to an LSI-11 microcomputer. Speckle reduction techniques were then evaluated by comparing the contrast obtained with each method. The effect of the spatial frequency response (MTF) on the measured contrast was studied, along with the statistical significance of the measurement which is limited by the finite sample space of 1024 detector pixels per CCD frame. The lowering of the contrast due to the array MTF can be minimized by working at extremely high F numbers (> 100). The sample space can be widened by taking more than one frame of data and treating all the frames as a single data set. Techniques to reduce speckle noise in laser displays fall into two broad categories: reducing the coherence of light forming the speckle, and incoherently adding multiple uncorrelated (or partially correlated) speckle patterns. The first technique (effective only for monochromatic displays) was implemented by coating a screen with various dyes, phosphors, or fluorescent paints. Using the 514 nm line from an Argon laser, the contrast can be reduced by almost 30% by spraying a thin layer of fluorescent paint on the screen. More speckle reduction can be achieved with an accompanying loss in image brightness. The second technique involved creating a multiplicity of partially correlated speckle patterns that appear from the same position on the screen over the integration period of the eye. The different speckle patterns are produced by changing the angle of illumination while keeping a portion of the laser spot focused on the same point on the screen. The scan angle method (applicable to multi-color displays), can be implemented by properly synchronizing an acousto-optic modulator with the scan optics, and imaging the modulator on the screen. Using a beaded screen and a reasonable laser dither of 10 millirads, the contrast can be reduced by half.

Thesis (M.S.)--Oregon Graduate Center, 1976.

Power transformers play an important role in electric networks. Copper windings wrapped in kraft paper and immersed in a tank of mineral oil are their main composition. Oil-impregnated paper is a very common insulator in power transforms, due to its durability and endurance. Kraft paper, a wood-based fiber from cellulose, can gradually degrade due to the ambient effects such as high-voltage electric field, temperature, and moisture in the normal working condition of transformers. Aged paper is more prone to experience electric breakdown which finally results in transformer failures. It is essential to monitor paper insulation condition. Conventional methods for evaluating kraft paper are mainly based on chemical analyses which require sampling of the surrounding oil and are usually time-consuming and expensive. Investigation on and classification of aged kraft paper with four different levels of thermal ageing using a potentially fast, simple, and inexpensive optical setup based on optical speckle is the main subject of this thesis. Speckle interference patterns from a laser diode source have been captured by a CCD camera. Classification based on textural features shows very good discrimination between ageing classes making this technique promising for industrial applications.

Thesis (Ph. D.)--Oregon Graduate Center, 1988.

Le speckle laser est un effet d'interférence longtemps considéré comme néfaste lors de l'utilisation de sources de lumière cohérente. Cependant, pour certaines applications, cet effet peut être bénéfique et utilisé comme source d'information. C’est le cas du domaine biomédical.Ainsi, le speckle laser est utilisé depuis des décennies pour la surveillance du flux sanguin microvasculaire. Il commence à être considéré aussi pour l'extraction de paramètres de la macrocirculation sanguine. Ce travail vise donc tout d’abord à démontrer que le speckle laser permet d'évaluer les paramètres hémodynamiques de la macrocirculation avec fiabilité et à partir d’une technique identique à celle employée dans l’étude de la microcirculation. Ceci conduira à une intégration rapide du dispositif dans les instruments existants. Par ailleurs, l'un des problèmes les plus importants du speckle laser,empêchant l’obtention d’une analyse totalement quantitative, est l'effet des diffuseurs statiques. Ce type de diffuseurs influence fortement le contraste de speckle, conduisant à une mauvaise interprétation des données. Le second objectif de ce travail est donc d'étudier l'effet des diffuseurs statiques sur la corrélation et le contraste du speckle laser. Nos résultats montrent tout d’abord que le speckle laser est un phénomène intéressant pour extraire les paramètres hémodynamiques de la macrocirculation. Par ailleurs, nos études révèlent que le calcul de la corrélation du speckle laser permet d'estimer le rapport entre diffuseurs statiques / dynamiques avec une bonne fiabilité. En outre, le contraste temporel permet de déterminer les diffuseurs dynamiques possédant des vitesses différentes. / The laser speckle is an interference effect that has been considered as a main drawback in the use of coherent light sources. However, for a specific set of applications, this effect can become a source of information. Among these applications there are the biomedical ones. The laser speckle has been used for decades to monitor microvascular blood flow but only now starts to be considered as a method that can also be used for macrocirculation parameters extraction. This work first aims at demonstrating that laser speckle can be used for macrocirculation assessment with good reliability, using the same technique as the one employed in microcirculation assessment. The use of the same methods could lead to a rapid inclusion of this new evaluation in the existing devices. Furthermore, one of the most important laser speckle issues, that prevents a fully quantitative analysis, is the effect of static scatterers. This type of scatterers strongly influences the speckle contrast, leading to a wrong interpretation of the data. The second objective of this work is to study the effect of statics catterers on the laser speckle correlation and contrast. Our results show that the laser speckle is an interesting phenomenon to extract hemodynamic parameters in the macrocirculation. This work also demonstrates that the laser speckle correlation is able to estimate the ratio between static/dynamic scatterers with good reliability. Moreover, the temporal speckle contrast achieved a very good performance in discerning dynamic scatterers with different velocities.