УДК 620.179.13
INTEGRATED APPROACH BETWEEN PULSED THERMOGRAPHY, NEAR-INFRARED REFLECTOGRAPHY AND SANDWICH HOLOGRAPHY FOR WOODEN PANEL PAINTINGS ADVANCED
MONITORING
S. Sfarra1, C. Ibarra-Castanedo2, D. Ambrosini1, D. Paoletti1, A. Bendada2, X. Maldague2 1 DIMEG, Las.E.R. Laboratory, University of L'Aquila, Piazzale E. Pontieri 1, Roio Poggio, L'Aquila AQ, I-67100, Italy 2 Department of Electrical and Computer Engineering, Computer Vision and Systems Laboratory, Laval University, Quebec City, Quebec, G1K7P4, Canada E-mail: stefano.sfarra@univaq.it, IbarraC@gel.ulaval.ca
Abstract: The durability of an exterior finish is affected by the characteristics of the wood. Satisfactory finish life is usually more difficult to achieve on woods of higher density. All wood shrinks as it loses moisture and swells as it absorbs moisture, but some species are more stable than others. Species that shrink and swell the most cause more stress on paint films than woods that are more stable [1]. To this end, let us recall that a painting on wood can be considered as a layered structure: The wood support is coated with a number of superposed priming layers made from mixtures of gesso and glue. A frequent fault resulting from such a system is the formation of detached regions inside the layered structure caused by the shrinkage process of the wood support [2]. Obviously, wood deteriorates more rapidly in warm, humid regions with respect to cool or dry places [3]. The influence of wood conditions on surface coatings is a critical point that should be monitored and that depends on environmental parameters such as microclimate. To prevent and control the effects, keeping costs down, a non-destructive monitoring of wood support behavior under thermal stress is needed. In this work, an integrated approach based on traditional and innovative (HI, PT and NIR) techniques was conducted on a primed support of poplar wood with a complex-shape surface containing areas of artificial defects at several depths due to the influence of the support on the various layers. The obtained results could be arranged, if integrated into a multidis-ciplinary approach, in order to define and design the conservation of the wooden artifacts.
Key words: holographic interferometry, pulsed thermography, near-infrared reflectography, defect, wood.
СОВМЕСТНОЕ ИСПОЛЬЗОВАНИЕ ИМПУЛЬСНОЙ ТЕРМОГРАФИИ, ИНФРАКРАСНОЙ РЕФЛЕКТОГРАФИИ И ПОСЛОЙНОЙ ГОЛОГРАФИИ ДЛЯ ЭФФЕКТИВНОГО МОНИТОРИНГА ДЕРЕВЯННЫХ ОСНОВАНИЙ КАРТИН
С. Сфарра1, К. Ибарра-Кастанедо2, Д. Амброзини1, Д. Паолетти1, А. Бендада2, X. Малдагу2 1 Университет г. Лаквила, Италия 2 Кафедра электротехники и вычислительной техники, лаборатория систем технического зрения, Квебек, Канада
Stefano Sfarra, PhD student, Environmental Sciences. iel. +39 0862 701901. E-mail: stefano.sfarra@univaq.it
Clemente Ibarra-Castanedo, Post-Doctor, Mechanical Engineering. iel. +1(418) 656.2131, ext. 4786. E-mail: IbarraC@gel.ulaval.ca
Dario Ambrosini, Associate Professor, Electronic Engineering. iel. +39 0862 701901. E-mail: dario.ambrosini@univaq.it
Domenica Paoletti, Professor, Physics. ien. +39 0862 701901. E-mail: domenica.paoletti@univaq.it
Abdelhakim Bendada, Associate Professor, Energy Science and Mechanical Engineering. iel. (418) 656-2131, # 3552. E-mail: bendada@gel.ulaval.ca
Xavier Maldague, Professor, Electrical Engineering. iel. (418) 656-2962. E-mail: maldagx@gel.ulaval.ca
Долговечность картины на деревянном основании зависит от характеристик древесины. Удовлетворительную долговечность труднее получить на древесине с более высокой плотностью. Все виды древесины сжимаются при потере влаги и расширяются при ее поглощении, но некоторые разновидности более устойчивы, чем другие. Разновидности древесины, которые в большей мере подвержены сжатию и расширению, вызывают большие напряжения в слоях краски, чем более стабильные виды древесины [1]. Следует, наконец, напомнить, что живопись на деревянном основании может рассматриваться как слоистая структура: деревянное основание покрывается несколькими грунтовыми слоями, сделанными из смеси гипса и клея. Часто встречающимся дефектом такой структуры является формирование отдельных областей внутри слоистой структуры под действием процесса расширения деревянной основы [2]. Очевидно, что ухудшение древесины происходит быстрее в теплых и влажных местах, чем в холодных и сухих [3]. Состояние деревянной поверхности — это тот критический пункт, который подлежит мониторингу и зависит от таких экологических параметров, как микроклимат. Чтобы предотвратить разрушение и минимизировать затраты, необходим неразрушающий контроль поведения деревянной основы под действием термических напряжений. В данной работе комплексный подход, основанный на традиционной и инновационных методиках (голографичес-кой интерферометрии Н1, импульсной термографии РТ и околоинфракрасной рефлекто-графии МЯ), был использован по отношению к грунтовому слою на древесине из тополя с поверхностью сложной формы, содержащей участки с искусственными дефектами различной глубины, которые имитировали влияние основы на различные слои. При мульти-дисциплинарном подходе полученные результаты полезны для контроля и разработки способов консервации экспонатов на древесной основе.
Ключевые слова: голографическая интерферометрия, импульсная термография, околоинфракрасная рефлектография, дефект, древесина.
INTRODUCTION
Paintings on wooden surfaces are extremely widespread, both historically and geographically. A major issue in the conservation of panels is the behavior and structural movement of the wooden support, especially due to fluctuations in environmental conditions [4]. In this work, holographic interferometry, infrared
Layer separation
Ground layer separation Picture layer separation Color layer separation Varnish layer separation
Varnish layer
Paint layer
_ Ground
Picture layer
-' Varnish layer
Color layer
Color layer
Color layer
Ground
Fig. 1. A schematic cross-section of a typical panel painting.
thermography and near-infrared reflectography have been employed in order to detect fabricated and real defects at several depths (Fig. 1). The choice of a complex surface, compared to a plane surface, usually used in the traditional panel painting, has confirmed the excellence of the techniques also on a composite structure out of the norm.
6 Дефектоскопия, < 4, 2011
The increasing deterioration of our artistic patrimony has accelerated the efforts to search for new methods for diagnosis of its state of conservation, because apart from the immense costs of restoration, the damages, in particular cases, could lead to the irretrievable loss of the artwork. The study of internal stress (due to variations of environmental parameters or external load) in frescoes, panel paintings or statues, the analysis of the behavior of small deformations or material discontinuities, in wooden or mural supports, and a knowledge of incipient and invisible flaws are a priority task for the preservation of these objects [5].
The concept of using laser interferometry for artwork structural analysis is based on the inherent property of holographic interferometry to allow a light wave, diffusely scattered by an object, to be holographically recorded and reconstructed with precision such that it can be interferometrically compared with light scattered by the same object at another instant in time. Several HI techniques exist [6, 7]. The more relevant to the present study is Sandwich Holography (SH) [8], that was found to be useful for a better localization of defects.
Nevertheless, holographic techniques are difficult to apply in situ, principally because of the strict stability requirements and high costs [9]. Other interfero-metric portable techniques, such as ESPI, might be used, although image resolution is greater for HI and can be considered as a benchmark. Another possibility is to use infrared thermography, which is a non-contact, non-invasive and nondestructive testing and evaluation (NDT&E) method [10]. However, problems such as non-uniform heating, emissivity variations, environmental reflections and surface geometry have a great impact on raw thermal data [11]. Advanced signal processing techniques must be used most of the time to improve defect contrast. In the case of panel painting inspection, three IR spectral bands are of interest: (1) the near infrared band (NIR) between 0.75 and 2.5 ^m, (2) the mid wave infrared (MWIR) between 3 and 5 ^m, and (3) the long wave infrared (LWIR) between 7 and 14 ^m. There exist some fundamental differences between the NIR and MWIR-LWIR bands being somehow complementary for the NDT&E of artworks. In one hand, NIR reflectography is employed for the assessment of ancient paintings providing information underneath the painting layers. IR thermography, on the other hand, exploits the principle of heat diffusion gradients on dissimilar materials for the detection and, in some cases, the characterization of subsurface anomalies [12]. The use of NIR reflectography, as link among holographic and thermographic techniques has been of great help in the identification and validation of simulated defects.
For the PT technique, the specimen surface is stimulated with a short heat pulse, and the cooling down process is recorded with an infrared camera for several seconds. The acquired data is then processed to improve defect contrast and signal-to-noise ratio (SNR). Several processing techniques exist, from a basic cold image subtraction to more advanced techniques [13]. The more relevant to the present study are: principal component thermography (PCT) [14], which reorganizes data into new components or projections that take into account the main spatiotemporal variances of the sequence, and pulsed phase thermography (PPT) [15, 16], which transforms data from the time domain to the frequency domain in order to obtain phase delay images or phasegrams that have an improved defect contrast. In this work, we
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