Edge detection combined with optical and infrared NDT techniques: an aid for wooden samples with complex surface and subsurface defects

To explore wood is to realize its complexity, its diversity, and its variability. The union of wood and paint is as old as the human desire to protect an object, or simply to decorate a surface. The link between paint and wood is therefore at the heart of any approach to conservation of these objects. Panel paintings are increasingly being investigated using advanced non-destructive infrared and optical measurement techniques. In the present work, a wooden sample having a complex surface and realized following the Cennino Cennini rules, containing natural and fabricated defects (Mylar® inserts), was investigated by stimulated thermography, near-infrared reflectography, double-exposure (DE) and sandwich holographic (SH) interferometry. The stimulated thermography technique consists in depositing energy, whatever be the means of deposition and the type of energy (sun, flash lamp, laser, and hot air flow), into the observed system (in the present case a wooden sample with complex surface and subsurface defects) and in monitoring the temporal and/or local evolution of the surface temperature field of the system caused by this thermal stimulation. Infrared reflectography is a non-destructive testing imaging technique based on the different optical behaviour of visible and near-infrared (NIR) radiation through a thin pictorial layer. This effect is a consequence of both lower NIR absorption and reduced NIR scattering due to the particle size smaller than the wavelength. The acquisition of NIR images using LED lamps working at different wavelengths, seems a very promising method in this field. However, NIR and DE are not dynamic techniques, while SH is a dynamic technique. In the latter, a number of holograms can be made, each one recording a single state of the object, in a temporal sequence. Since enhancing the edge of a detached area identified by SH, means improving the detection of the defect’s position, this idea was applied in the present research. Instead, the defect’s depth was retrieved working with phase analysis, i.e. using the pulsed phase thermography (PPT) technique. Finally, the results coming from optical and infrared NDT techniques were compared each other in order to explore the advantages and disadvantages of the methods used.