Thomas Fricke-Begemann

Speckle interferometry: three-dimensional deformation field measurement with a single interferogram

An electronic speckle interferometer, arranged for out-of-plane sensitivity and with an off-axis reference beam to produce spatial phase bias, is used for three-dimensional deformation field measurements. The complex amplitude of the object wave is calculated by application of a Fourier-transform method to a single interferogram. The change in phase after object deformation yields the out-of-plane component of the displacement field. The two in-plane components are obtained by cross correlation of subimages of the reconstructed object waves intensity, a method that is also referred to as digital speckle photography. The Fourier-transform algorithm is extended and modified, leading to random measurement errors that are below widely accepted theoretical limits and also to an extended measuring range. These properties and the mutually combined information improve the accuracy of both methods compared with their usual single implementation. The performance is evaluated in experiments with pure out-of-plane, pure in-plane, and combined deformations and compared with theoretical values. An example of a practical application is given.

Three-dimensional deformation field measurement with digital speckle correlation

Digital speckle correlation is based on a detailed analysis of changes in speckle images that are recorded from laser-illuminated rough surfaces. The two in-plane components are obtained by cross-correlation of corresponding subimages, a method also known as digital speckle photography. The local gradient of the hitherto inaccessible out-of-plane component is determined from the characteristic dependence of the speckle correlation on the spatial frequency. A detailed experimental study is carried out to analyze the new technique for systematic and random measuring errors. For moderate decorrelation the accuracy of the out-of-plane measurement is better than lambda/10 and thus comparable with interferometric techniques. Yet the extremely simple and robust optical setup is suited to nondestructive-testing applications in harsh environments. The quality of the deformation maps is demonstrated in a practical application.

Speckle correlation for the analysis of random processes at rough surfaces

The importance of technological processes like corrosion, ablation or deposition causes interest in the quantitative monitoring of changes at rough surfaces. Thus, there is a need for effective methods to measure the statistical parameters characterizing changes in the profile or the material composition of such objects. The speckle field scattered from the surface is used as information carrier and its change is measured by correlation. This is realized by sophisticated data acquisition and digital processing techniques. An important issue is the interpretation of the correlation output in terms of statistical parameters describing the surface change. For many random surfaces a geometrical relation between surface profile and optical phase proves satisfactory. This allows to determine the standard deviation of the profile change. For a verification, speckle decorrelation in model surfaces of known deviation is measured. The paper introduces the speckle correlation concept, outlines some history and current setups and describes methods for data evaluation. The reliability of the quantitative interpretation of the speckle decorrelation is demonstrated. The method is illustrated by studies of metal corrosion and material removal in the cleaning of historical objects by laser ablation.

Corrosion monitoring with speckle correlation

The changes in the microtopography of a metal surface during a corrosion process are measured by decorrelation of the scattered speckle fields under coherent illumination. For that purpose a quantitative relation between the decorrelation of the scattered light fields and the rate of corrosion is established in a theoretical model, based on the statistics of phase and reflectivity changes of point scatterers at the surface. The speckle fields are recorded by a CCD camera and processed numerically in a computer, yielding the standard deviation of the topography changes with nanometer sensitivity. From the analysis of a series of images taken at equal time intervals during the corrosion process, the degree of interrelation among subsequent topography changes is calculated.

Measurement of random processes at rough surfaces with digital speckle correlation.

We present a detailed investigation of digital speckle correlation to measure small changes in the microstructure of random rough surfaces. The corresponding alterations in the scattered-light field are recorded by an electronic camera with subsequent numerical correlation. Among the classical theoretical approaches to describe the scattering at random rough surfaces, the composite-roughness model is advanced to calculate the speckle correlation in terms of parameters of the changes in surface microstructure. For an experimental verification, surfaces with similar microstructure are fabricated with a photolithographic technique. They are employed for comparative measurements with high-resolution scanning force microscopy and for correlation measurements under variation of experimental parameters. A good agreement between theoretically predicted and experimental correlation data can be observed. The results allow a quantitative whole-field monitoring of surface processes by remote optical means.

Mural inspection by vibration measurements with TV-holography

A commonly encountered problem in the conservation of historical murals is the identification of sections in the plaster that have detached from the wall and thus threaten to fall off. Commonly, walls are inspected by the acoustic response to a gentle finger-tapping (percussion method). Since this is a costly and cumbersome technique, means for a more automatic inspection are searched for. A TV-holography system of increased sensitivity in combination with acoustic excitation of the object is shown to be a new and powerful tool for monitoring of loose areas. It has the advantage of non-contact and remote operation which, for example, is extremely useful in large buildings. Principles of the method, experimental results obtained at an artificial wall in the laboratory, and a thorough comparison of results from historical sites gained by the traditional percussion method and the new technique are presented. The latter shows very good agreement in the assessment of wall quality and thus is evidence of the suitability of the optical equipment for tasks in conservation.

Analysis of microstructure changes and dynamic processes on rough surfaces using speckle correlation

The decorrelation of the characteristic speckle pattern, produced by the scattering of coherent light from a random rough surface, is used to monitor changes in the microtopography of the surface. In a basic optical setup minute alterations down to a few nanometers can be measured. To enlighten the basic relations between the speckle field decorrelation and the surface changes a theoretical model is developed, numerical calculations are carried out and comparisons with high resolution AFM measurements on two frozen states of a surface process are undertaken. Whereas the reproducibility of the AFM measurement turns out to be not precise enough to measure nanometer changes on a micrometer scale rough surface, the theoretical model is consistent with results gained from experimental variation of wavelength and angle of illumination and a-priori knowledge about the surface. It is then used to obtain information about magnitude, time constants and statistical parameters of a corrosion process.

Analyse dynamischer Vorgänge an technischen Oberflächen mit Speckle-Korrelation / Analysis of Surface Processes by Means of Speckle-Correlation

Veränderungen des Mikroprofils rauher Oberflächen werden analysiert, indem die durch Streuung von Laserlicht jeweils entstehenden Specklemuster als digitale Bilder im Rechner korreliert werden. Ein theoretisches Modell und Vergleichsmessungen an Musterobjekten erlauben die quantitative Interpretation von Dekorrelationen. Profiländerungen in der Größenordnung von 10nm sind bereits meßbar. Der Korrosionsvorgang an einer Eisenprobe und die Laserablation einer Patinaschicht auf Naturstein illustrieren den Anwendungsbereich des Verfahrens.

Monitoring of plaster detachments in historical murals using TV holography

Deterioration of historical murals often leads to partial detachments of the paint carrying plaster layers. To identify such regions an acousto-optical method is described. Loose portions of the plaster are excited by sound waves and the resulting vibrations are detected by sensitized analog TV-holography.

Vibration Monitoring by TV-Holography ­ a Diagnostic Tool in the Conservation of Historical Murals

A TV-holography system is designed to monitor detached plaster areas in historical murals. Defects in the wall respond to sound irradiation by vibrations. These are detected optically. A feature of the system is the enhanced sensitivity obtained by reference wave modulation. Experimental results obtained at a historical site show very good agreement with data gained by the traditional percussion method.