Abstract
An inlay sample with artificial defects was inspected via the pulse-compression thermography (PuCT) technique. The sample belongs to the cultural heritage field, and it was realized by a professional restorer based on his long-time experience, imitating historical art crafting styles. The tesserae composing the inlay were not treated by any protective paints, so that external thermal stimuli may cause physical/mechanical alterations of the cell walls, with consequent colour changes, cracks, and eventually damage to its surface. To avoid any alteration of the sample, the PuCT technique was used for inspecting the inlay sample as it allows the heating power to be very low, while assuring enough thermal contrast for the defects to be detected after the exploitation of the pulse-compression algorithm. Even if a maximum ΔT slightly exceeding 1 °C was detected during the PuCT test of the inlay sample, it is shown that this is enough for detecting several defects. Further, image processing based on the Hilbert transform increases defect detection and characterization. In addition, a novel normalization technique, i.e., a pixel-by-pixel data normalization with respect to the absorbance estimated by considering a characteristic value of the compression peak, is introduced here for the first time. The proposed normalization enhances the defect detection capability with respect to the standard pixel-by-pixel amplitude visualization. This has been demonstrated for two experimental setups, both exploiting the same LED chips system as heating source but different thermal camera sensors, i.e., one in the mid-infrared spectrum, the other in the far infrared one. Thus, the present work is also the first small-scale test of a future portable system that will include low-power LED chip feed in DC by metal-oxide-semiconductor field-effect transistor (MOSFET) devices, and a handy far-infrared camera.
Highlights
In recent decades, strong efforts have been put into the research of innovative techniques in the field of physics and chemistry applied to cultural heritage (CH) for diagnostic and conservations, based on both the applications of new materials and investigation methods
Pulse-compression thermography has been shown to be a valuable technique for cultural heritage
Pulse-compression thermography has been shown to be a valuable technique for cultural items and artworks inspection, as it is capable of contextually providing good defect detection, as well heritage items and artworks inspection, as it is capable of contextually providing good defect as a very low and smooth temperature increment, which is extremely important for such items
Summary
Strong efforts have been put into the research of innovative techniques in the field of physics and chemistry applied to cultural heritage (CH) for diagnostic and conservations, based on both the applications of new materials and investigation methods. Processing methods working in the infrared spectrum have been described and applied by Vrabie et al [9], Gavrilov et al [10,11], Bendada et al [12] and Sfarra et al [13]. In this scenario, a novel strategy based on modulating the heat source via a pseudo-random binary excitation was used in [14] to reduce the risk associated with the use of high-power heating source. A new contrast enhancement metric was developed to demonstrate the quantitative efficiency of the algorithm
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