Gerd Gülker

Deformation mapping and surface inspection of historical monuments

Abstract Electronic speckle pattern interferometry (EPSI) in various configurations (reference beam or shear) as well as double exposure speckle photography are applied for deformation monitoring in historical monuments. On-the-site measurements are made possible by a rigid construction of the optical head, common path configurations and portable image processing equipment. The delicate objects particularly call for non-intrusive measurement. Basic features of the techniques, their implementation and results from historical sites are presented. Inhomogeneities in plaster, paint or natural stones were detected and related to crack formation. The reversibility of deformations under mechanical load was checked. In frescos, debonds from the carrier wall could be discovered and the detrimental effects of crystalline efflorescences were studied. Correlation analysis in ESPI images showed the influence of moisture on a rough stone surface and exposed microbiological activity. Thus, valuable deformation data could be obtained that assist in the diagnostics of deterioration processes and provide information in the design of remedies.

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.

Lasers in art consevation

The excavation of several thousand life-size terracotta warriors in China has fascinated the world ever since the army was first uncovered by archaeologists in 1974. Buried in moist ground, these clay figures have remained mostly intact for over 2000 years. As soon as they are exposed to air, however, the paint on the warriors curls up and breaks into small flakes. Conservators are deeply worried.

Deformation monitoring on ancient terracotta warriors by microscopic TV-holography

Abstract In order to investigate deterioration processes on the famous 2000-yr-old Chinese Terracotta warriors, deformation measurements were performed with TV-holography. The aim of the investigations is to estimate the suitability of several conservation agents and procedures, which are developed to preserve the extremely endangered paint layers of the figures. Due to the occurrence of small-scale deformation areas the field of view had to be reduced. A microscopic system was constructed and typical object deformations due to shrinkage and swelling are presented. To investigate the individual layers of the multilayered objects a short-coherent superluminescence diode instead of a laser diode is used in a further modification. By changing the path length of one of the interfering beams it is possible to select a region limited in depth where deformations are measured even if it is located below the surface. Results on an artificial test object and on original terracotta fragments demonstrate the capabilities of this new short-coherence TV-holography system.

Detection of surface microstructure changes by electronic speckle pattern interferometry

Electronic speckle pattern interferometry is elaborated in such a way that spatially resolved image decorrelation can also be measured. While retaining the typical ESPI set-up for deformation measurements, a speckle correlation formalism is implemented based on the phase-shift method. In many practical situations decorrelation is directly related to surface microstructure changes of a test specimen. Feasibility and restrictions of the method are illustrated by measurements of water-induced changes at the surfaces of natural stones and by monitoring microbiological activity on stones.

Low-coherence ESPI in the investigation of ancient terracotta warriors

In order to investigate deterioration processes in the paint layers on the famous 2000-yr-old Chinese Terracotta warriors, a low coherence ESPI system was designed. In this modified set-up a short-coherent superluminescence diode instead of a laser is used. By changing the path length of one of the interfering beams it is possible to select a region limited in depth where deformations are measured even if it is below the surface. Results on an artificial test object and on original terracotta fragments demonstrate the limits and the capabilities of the new method.

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.

In Situ Application Of Electronic Speckle Pattern Interferometry (ESPI) In The Investigation Of Stone Decay

A miniaturized electronic speckle pattern interferometry system (ESPI) has been developed for in situ measurements of microdeformations on buildings and monuments. Direct coupling between object and the compact optical head of the system enabled detection of microdisplacements even in non laboratory environments. Several examples of applications in deterioration investigations are presented.

Microscopic ESPI: better fringe qualities by the Fourier transform method

Electronic speckle pattern interferometry (ESPI) is discussed for the detection of out-of-plane deformations in small objects. For increasing the resolution in object space a laser source of small wavelength is combined with a microscope with a high numerical aperture. Fringe quality is increased by using spatial phase-shifting and the Fourier transform method to allow deformation detection also under non-optimum conditions. The efficacy will be shown in some measurements on different specimens where deformations are successfully recorded in areas down to a few micrometers in size.