Complimentarity of digital holographic speckle pattern interferometry and simulated infrared thermography for Cultural Heritage structural diagnostic research
Abstract
In the field of art conservation there is an increased demand for non destructive and non invasive techniques able to perform remote defect detection for structural evaluation of historical structures and works of art. The techniques must have some basic important characteristics as non destructivity, accuracy, repeatability and desired features as non contact, portability, resolution, broad range of applicability. The simulated infrared thermography (SIRT) and digital holographic speckle pattern interferometry (DHSPI) are systems discussed in detail elsewhere; in this paper have been used in combined testing on art related targets according to the above criteria. Results confirm the effectiveness of both techniques in the conservation field and examples are here in presented. Each system is thus described as individual method highlighting the advantages and limitations of each. The aim is to present the suitability and appropriateness for accurate defect mapping in structural documentation reports. This work is considered preliminary and is in progress for the future hybrid synthesis of systems and data.
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Introduction
The structural diagnosis is an essential part of conservation research during the documentation of both mobile art works and immobile structures. Conservation strategy requires beforehand a detailed analysis of the condition of the art object. The condition report refers to the object as a whole and its constituent materials in terms of physicochemical deterioration and structural integrity.
Independent of the type of the object there are standard deterioration features which threaten cohesion of constituted materials and structural stability. These are two big categories of defects known in art conservation as cracks and detachments, which both are generated gradually or abruptly from natural or unusual stress related phenomena. These deterioration features depending on the age and maintenance of objects may at first be invisible under the surface but gradually elongate or expand and tend to affect the surrounding area. Through this process they become centers of structural instability and the affected areas become weaker compared to the unaffected areas. In this way parts of the structure change their mechanical properties compared to healthier parts. The magnitude of the deterioration manifests through the mechanical resistance of the object to any load. Tools that can visualise this anisotropy and detect characterise the defects and their impact on the structure become crucial if conservation and preservation plan is underway.
Conclusion
We have test and confirm that the combination of the active infrared thermography and the digital holographic speckle pattern interferometry contain highly complimentarity in the provided information in regards to the structural understanding of an artwork from surface responses. The techniques offer insight in the bulk of the materials and objects by plain and safe surface illumination. The techniques offer the important characteristics of non destructivity, accuracy, repeatability and the desired features of being remotely operated, non contact, portable, having high resolution for defect detection and structural investigation as well as broad range of applicability in different materials and constructions. The combination of the thermal distribution with the expressed reaction in terms of deformation of the surface holds high potential not only for the best description of defects and the assessment of condition reports required in art conservation and preservation of Cultural Heritage but also for the understanding of structural reactions that describe mechanisms of deterioration in general. Natural sources of deterioration and aging due to high humidity and moisture in microclimates is planned to be investigated further. The examples shown here are just some of series of original experimental data taken in the lab and on-field. The current state of research is focused on the development of workstation for providing to the data common field of view and stability conditions with next step also focused on correlation of data points and developments of exclusive algorithms for data handling.