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Optical Methods for Damage Assessment of Artworks

Introduction

The protection of cultural heritage is a topic of great interest to museums and galleries.In order to preserve artwork, art conservators need to constantly record the condition of objects and, if necessary, to apply methods and materials which stabilize their condition without affecting the integrity of the object. Therefore, a thorough understanding of the exact preservation condition is necessary. This can be achieved mainly by structural diagnosis techniques, in particular, interferometric signatures of artwork obtained by shearography can be used to identify that damage has occurred.

Principle

Shearography is a full-field interferometrical technique sensitive to displacement gradients. To perform a measurement the object is illuminated by theexpanded beam from a laser. The light scattered by the optically rough surface of the object forms a speckle pattern which is imaged through a shearing interferometer (see Figure 1). This generates an interferogram, which can be recorded by a camera.This interferometer is common path and as such is relatively insensitive to environmental disturbances, such as rigid body motion. The object to be investigated (e.g. sculpture, canvas,..) is loaded  by using an infrared lamp and shearography is used for detecting deformations (or more precisely deformation gradients) due to loading.These deformations contain the information about the mechanical structure of the object and in regions wheredefects are present (voids and cracks under the surface, delamination of the paint layer from the substrate, etc…) the deformation will differ from the homogenous surrounding, creating discontinuities in the interferograms.

  

 

 

Figure 1

 

Results

Figure 2 shows an example of measurement of the Stuppacher Madonna before the restauration. Different kind of defects have been detected.

  

 

 

Figure 2

 

References

[1] Groves, R., Osten, W., Dulgeridis, M., et al: “Shearography as part of a multifunctional sensor for the detection of signature features in movable cultural heritage”, Proc. SPIE Vol. 6618(2007), 661810-1.
[2]  Groves, R., Derauw, D., Thizy, C., Alexeenko, I., Osten, W., Georges, M., Tornari, V.: Automated phase map referencing against historic phase map data. . Fringe 2009, Springer Heidelberg 2009, S. 192-196.
 
[3]

Groves, R.; Pradarutti, B.; Kouloumpi, E; Notni, G.; Osten W.: “2D and 3D non-destructive evaluation of a wooden panel painting using shearography and terahertz imaging“. NDT E International 42(2009)6, 543-549

[4] Groves, R., Pedrini, G.; Osten, W.: “Real-time extended dynamic range imaging”. In: Shearography. Applied Optics 47 (2008) 30 pp. 5550 – 5556.