Wide-band IR imaging in the NIR-MIR-FIR regions for in situ analysis of frescoes

C. Daffara*a, L. Pezzatia, D. Ambrosinib, D. Paolettib,R. Di Biaseb, P.I. Mariottic, C. Frosininic

aINO-CNR Istituto Nazionale di Ottica, Largo E. Fermi 6, I-50125, Firenze, Italy; bDIMEG, Università dell’Aquila, Piazzale E. Pontieri 1, I-67100 Monteluco di Roio, L'Aquila, Italy;

cOpificio delle Pietre Dure, Viale Strozzi 1, I-50100, Firenze, Italy

ABSTRACT

Imaging methods offer several advantages in the field of conservation allowing to perform non-invasive inspection of works of art. In particular, non-invasive techniques based on imaging in different infrared (IR) regions are widely used for the investigation of paintings. Using radiation beyond the visible range, different characteristics of the inspected artwork may be revealed according to the bandwidth acquired. In this paper we present the recent results of a joint project among the two research institutes DIMEG and CNR-INO, and the restoration facility Opificio delle Pietre Dure, concerning the wide-band integration of IR imaging techniques, in the spectral ranges NIR 0.8–2.5 µm, MIR 3–5 μm, and FIR 8–12 µm, for in situ analysis of artworks. A joint, multi-mode use of reflection and thermal bands is proposed for the diagnostics of mural paintings, and it is demonstrated to be an effective tool in inspecting the layered structure. High resolution IR reflectography and, to a greater extent, IR imaging in the 3-5 μm band, are effectively used to characterize the superficial layer of the fresco and to analyze the stratigraphy of different pictorial layers. IR thermography in the 8-12 μm band is used to characterize the support deep structure. The integration of all the data provides a multi- layered and multi-spectral representation of the fresco that yields a comprehensive analysis.

Keywords: Infrared imaging, thermal imaging, Mid Infrared, frescoes diagnostics, data integration

1. INTRODUCTION IR imaging is one of the most suitable optical techniques for the investigation of painted surfaces. Traditionally, IR reflectography, i.e. IR imaging in the NIR (Near InfraRed) band 0.8–2.5 µm, is particularly useful to detect the features of the paintings underneath the visible surface, thanks to the partial transparency of pigments in this band1-3. Recently, the investigation in the NIR was coupled with thermography, which works in the FIR (Far InfraRed) band 8–12 µm, in a successful integration, giving much more information4-5. FIR thermography was found capable to detect structural defects in the deep structure of panel paintings and fresco models, as well as on genuine panel paintings5-6.

Wall paintings are extremely complex artworks since they are an integral part of buildings and their conservation is strictly linked to the monument and to the complex interaction between outdoor and indoor conditions; therefore, fresco diagnostics needs to be performed in situ. Moreover, wall paintings are very heterogeneous and cover large surfaces. All the above remarks make the investigation of real frescoes very different from working on models.

In this paper we present the recent results of a joint project among the two research institutes DIMEG and CNR-INO, and the restoration facility Opificio delle Pietre Dure (OPD), concerning the wide-band integration of IR imaging techniques, in the reflection and thermal bands, for in situ analysis of artworks. First applied to the analysis of panel paintings, the integrated IR approach is now extended to the crucial investigation of frescoes. If the NIR reflection band was tipically regarded as less effective on wall paintings because of their specific technique, recently, it has become increasingly clear that, even in the early Italian Renaissance, painters quite frequently employed a secco techniques so as to allow the use of a broader range of pigments. The role of these investigations also on wall paintings is warmly welcomed, allowing the detection of more features underneath the surface and the localization of the areas of different materials.

*claudia.daffara@ino.it; phone +39 055 2308280; fax +39 055 2337755; arte.ino.it

Invited Paper

O3A: Optics for Arts, Architecture, and Archaeology III, edited by Luca Pezzati, Renzo Salimbeni, Proc. of SPIE Vol. 8084, 808406 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.889891

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Previous knowledge about the joint use of NIR and FIR bands was extended to include MIR (Middle InfraRed) band 3–5 μm, so that the present work involves:

- A complete wide-band IR imaging in the reflection and thermal bands, in a multi-mode acquisition, including dual band MIR-FIR thermography and multispectral NIR imaging;

- Use of ad hoc software tool for easy integration of collected data, a graphical user interface with options such as image adjustment, overlaying and transparency variation.

- Full field in situ diagnostics. Here, after some validation on important fresco models, we report results of the investigation on the mural painting “The Resurrection”, the masterpiece by Piero della Francesca in Sansepolcro (Arezzo Italy).

High resolution IR reflectography and, to a greater extent, IR imaging in the 3–5 μm band, are effectively used to characterize the superficial layer of the fresco and to analyze the stratigraphy of different pictorial layers. IR thermography in the 8–12 μm band is used to characterize the support deep structure. Data integration provides a multi- layered and multi-spectral representation of the fresco that yields a comprehensive analysis. On the whole, the use of wide-band IR imaging is a very promising tool for the nondestructive inspection of frescoes in situ and, while maintaining the traditional visual analysis, could be usefully integrated with different diagnostic techniques.

2. THE FRESCO NATURE AND DECAY As was pointed out7, the complexity of wall paintings conservation is due to a number of special factors: the physical and aesthetic connection of frescoes with the architecture; their non-homogeneous, multilayer constitution; the fact that “artistic content” of wall paintings constitutes an extremely thin layer, which is also interface between environment and support.

Figure 1. Typical structure of a fresco and main structural defects: 1 – layer separation between pictorial layer and the

intonachino; 2 – separation between the intonachino and the intonaco; 3 – separation between the intonaco and the mural support.

Paintings on wall can be considered as a layered structure with a support. Figure 1 shows the typical structure of frescoes7 and the most important structural defects. The mural support (d) is primed with different layers of plaster, (c – arriccio, intonaco and b – intonachino) which serve as a basis for the painting (a). Several techniques were available to realize the pictorial layer:

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- Fresco, in which the pigment dissolved in water is applied on the wet plaster;

- Mezzo Fresco, in which the plaster is nearly dry;

- Secco, in which the pigment fixed with an organic/inorganic binder is applied on the dry plaster.

These layers are less thick and more fragile than the support. Expansion and contraction of the support due to daily fluctuations of ambient parameters can produce large strains and eventually cracks in the layers, as they become less flexible with age. Layer separation, which is a frequent and dangerous damage in mural paintings, may occur at all levels in the structure (see figure 1) and may be scarcely detectable; as a rule, the restorer will try to detect layer separation by careful tapping (percussion) of the picture layer.

Furthermore, abrupt changes of temperature and humidity, traffic induced vibrations, and heat exposure may also cause unpredictable stress distributions in the support with consequent damage of the painted surface. Structural stability of the walls must also be carefully considered.

3. WIDE-BAND IR IMAGING 3.1 IR interaction with fresco

IR imaging techniques are mainly applied to easel paintings and they are mostly limited to the use of the NIR region (reflectography) and, to a less extent, of the FIR region (thermography)5. Mural paintings, which are inherently different (support, materials, and techniques), are less investigated in the NIR band, and IR diagnostics is mostly limited to the use of passive or active thermography in the FIR band for the detection of support anomalies. As far as the imaging in the MIR region is concerned, in practice, the application to artwork diagnostics is still an unexplored field8.

The stratigraphy of the artwork target can be generally modeled as a set of separate layers, which then have their proper transport properties and are subject to different collision events, i.e. combined absorption-scattering, in the IR region. Wide-band IR imaging provides a result that is the integrated information from the contributes of the local matter interactions along the entire radiation path. Thus, imagery response is strongly related to both the layering structure of the irradiated object and the radiation transport inside it, including the depth penetration capability.

In the specific case of the mural painting target, we demonstrate how a joint, multi-mode use of the NIR-MIR-FIR bands is able to provide selective information about the layers.

The layered model of an easel painting includes the varnish, the painting matter, the preparation background, and the portable support, e.g. canvas or wood panel. The optical transparency of the varnish is high in the NIR, while at longer MIR wavelengths decreases, becoming translucent due to diffusion. The painting layer is a colloidal suspension of pigment and binding medium, while the preparation is generally based on chalk and animal glue. The high transparency of the painted layer in the NIR range, and the presence of a high reflective preparation are, therefore, the basis of the well known success of IR reflectography in the investigation of the underlying features, above all the preparatory underdrawings1. Multispectral imaging improves the analysis of underneath features allowing their differentiated detection9-10.

As shown, the mural painting has a different and complex stratigraphy, in which the structure and properties of the layering vary with the different techniques. When the pigment is applied in fresco technique, the pictorial layer is the thin film of calcium carbonate embedding the color particles, resulting from the carbonation process. When the pigment is applied in secco technique, the pictorial film forms a well separate layer from the plaster background, with the optical transparency properties of the tempera. The plaster preparation is not reflective in the IR spectral range. Thus, in the case of the mural painting, the different interaction properties of the layers, namely less transparent color films and less reflecting background, explain on one hand why NIR reflectography has not been traditionally used on this artworks.

On the other hand, a different use of wide-band IR imaging using both the reflection bands NIR and MIR can be proposed, mainly addressed to the inspection of the superficial color layers more than to the underneath features as it is in the traditional case of the easel painting.

Even without the contribution of a reflective background, the variegate NIR reflectance behavior of painting materials in the tempera layer, allows the NIR wavelengths to be effectively used to reveal the presence of some different regions,

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e.g. paint changes. The MIR reflectance response of some pigments and organic materials, allows the MIR wavelengths to be effectively used to investigate the thin fresco films and secco layers, finishing touches or integration. Concerning the wide-band imaging tool in the longer wavelengths FIR, we show how it can be effectively used in both emission and reflection mode to inspect both the deep support and the color layers.

3.2 The diagnostic tool for layers inspection: Reflection and Thermal bands

In the proposed integrated approach, the NIR region is used only as a reflection band; the MIR region is used, in approximation, as a reflection band; the FIR region is used both as thermal and reflection band.

In the reflection band, the propagation behavior is based on the local scattering-absorption events; the transport properties are related to the radiation wavelength and to the constituent material. Different levels of penetration provide different imaging information. The MIR reflectance imaging (dominated by the strong fundamental absorption) is of different nature from NIR reflectance imaging (where we have less absorption, thus major penetration).

In the thermal band, the propagation behavior follows the thermal Fourier conduction and the physics of emissivity; the transport properties are related to the radiation wavelength, to the temperature, and to the constituent material. Different level of penetration provides different imaging information, although quantitative methods must be used to relate thermal maps with depth maps. Both MIR and FIR give thermal contribute. Anyway, in the case of artwork investigation the temperature of the investigated object must be kept at a safe level, e.g. around room temperature of 300 K. In the approximation of grey body behavior, we deduce that the emissive contribute in the MIR range (around 1%) is negligible with respect to the reflective quote, while in the FIR the emissive and reflective quote are of the same order. Therefore, if the FIR acts both as reflection and thermal band, the MIR acts as a reflection band only.

Figure 2. IR interaction in relation with fresco layered target. NIR and MIR band are used as reflection bands for the

investigation of the surface at different level according to different penetration. MIR emission is neglected. FIR band is used both in reflection and emission mode, to investigate surface defects and inner support.

The reflective/emissive behavior of mural painting materials is variegate over the IR spectral range. The key aspect of using reflection/thermal IR bands as a tool for an integrated investigation of fresco layers is the surface temperature. Finally, the wide-band IR imaging in the NIR-MIR-FIR bands is based on the following facts, which summarize the above analysis:

- MIR information is related to the superficial thin color layer (through the diffuse reflectivity), and it can be put in relation with: 1) fresco materials; 2) fresco techniques;

- FIR information is related to the layered structure and defects (through the thermal anomalies), and provides analysis of: 1) wall support, inner defects; 2) detaches, sub-surface defects; 3) some color layering;

- NIR information is related to the thick color layer (through the diffuse reflectivity) including some subfeatures.

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3.3 NIR imaging

Wide-band imaging in the NIR band is based on acquiring the backscattered radiation in the NIR range (0.8–2.5 μm), according to the spectral sensitivity of the employed device. The artwork surface was irradiated with a set of halogen lamps with emission spectrum in the NIR.

Infrared reflectography is one of the most useful techniques for art historians to reveal features underlying the pictorial layer, such as the preparatory sketches1-3. Thanks to its capability to go through the different layers of the painting, short infrared radiation can be used to reveal some features and alterations underlying the paint layer which are undetectable to the naked eyes. Moreover this kind of analysis allows the non-destructive identification of pigments having different spectral response in the NIR region1.

Underdrawing visibility is a function of the transparency of the paint layers to NIR radiation and of the underdrawing contrast: IR absorption is high when carbon is present in the drawing (charcoal, graphite, carbonaceous pencils and inks), otherwise in presence of iron-gallium inks the underdrawing can be invisible, even if the paint layer is transparent. Reflectivity is high when the preparation is chalk-and-gypsum based. Probably, one of the best devices in NIR imaging aimed at revealing the underdrawing is the IR Scanner2 developed at the National Institute of Optics (INO – Italy); the recently introduced multispectral capability11 is particularly useful to obtain a stratigraphy of the pictorial layer in terms of both underdrawing sketch and the “pentimenti” or subsequent repaintings.

More portability and flexibility can be obtained, at the expense of image quality and “inspection depth”, by using a Canon 40D spectrum enhanced camera, having a CMOS APS-C sensor (22.2 mm×14.8 mm) with 3888×2592 pixels. Hutech Corp. enhanced this camera by replacing the rear UV/IR blocking filter with a clear filter having a transmittance better than 90% in the range 380–1150 nm.

3.4 MIR imaging

Wide-band imaging the MIR band is based on acquiring the radiation response from the artwork surface in the 3–5 μm band. The artwork surface was irradiated using a set of IR lamps. The source was controlled in power to obtain the measurement in reflection mode, i.e. by avoiding the heating of the surface. The camera device was a Nikon Thermal Vision Laird S-270 (PtSi Schottky-Barrier IRCCD). The sensor consists of 475×442 pixels, with a Stirling Cycle cooling and NETD = 0.2 K. MIR imaging was found able to detect structural defects as well as some underdrawings12, this last task with a clear loss of resolution with respect to NIR imaging. The most interesting feature of MIR imaging is its ability to differentiate materials on the surface.

3.5 FIR imaging

FIR investigation was performed using the FLIR ThermaCAM S65, based on a 320×240 pixels Focal Plane Array (FPA) uncooled microbolometer which acquires thermal images in the spectral range 7.5–13 µm, from a minimum of -40 °C to a maximum of 1500 °C, with an uncertainty of ± 2 °C. The thermal sensitivity is 0.05 °C at the full imaging rate of 50 Hz. The images can be acquired in portable format processed at 14 bit or as temporal sequence. Typically, thermographic investigations can provide information about structural and subsurface defects and/or heterogeneous materials.

4. DATA PROCESSING AND INTEGRATION 4.1 Multiview

In this paper, “multispectral” is intended in a rather extensive sense: multispectral investigation of works of art involves the integration of imaging results from different/complementary diagnostics, such as reflectography and thermography, and/or the integration of different image dataset from the same technique. According to the spectral band used and to its interaction properties with the painting materials, different characteristics of the painted layers and of the painting structure may be revealed.

4.2 Ad hoc software

A Graphical User Interface (GUI), compiled in the MATLAB® environment, was recently proposed4-5. The software allows a high quality image processing and, thanks to the easy-to-use layout interface, based on push buttons and other visual interactive commands, turns out to be a sophisticated instrument, but easy to employ. Several windows can be opened from the main one allowing a multi-view data handling to easily adjust, rotate and cut the visible image, the

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reflectogram and the thermogram. Image size synchronization, image contrast adjustment, image display functions have been implemented to support the minimal set of display features required by the reflectographic image review. Simple but useful thermographic elaboration can also be performed.

The most interesting and powerful option of this software consists in the image registration of the three datasets respectively coming from the NIR, MIR, and FIR inspections, finally aligned also to the VIS image. Once overlaid, they can be shown with different degrees of transparency in a unique multilayered representation.

Matching the corresponding features belonging to the results of the different imaging techniques is faster and easier. Thus the software provides an efficient approach for data integration. Its usefulness was tested for several case studies, collecting NIR and thermal images from different archives.

4.3 False color techniques

It is well known that the human eye is particularly able to distinguish between different colors. Therefore, an advancement in the data processing of experimental results from a wideband approach in the IR could be obtained by representing images in false color. These techniques are well known in remote sensing13 and could be very useful also in the conservation field14.

A simple way to obtain false colors image is to load the images recorded in the different bands in MATLAB® as m-by-n-by-3 arrays of number, each array representing a channel of an RGB image. Then a new image is created by combining the channels13.

MATLAB® has several useful functions to work with multispectral images. In particular, the imadjust function, combined with the stretchlim function, was used to increase the contrast of the false-color image. If the channels are not perfectly aligned (i.e., if there are some differences in the images to merge) chromatic artifact may appear. This is particularly important in artworks multispectral investigation because, usually, images in different bands may have been acquired by different devices. The MATLAB® control point selection tool contains useful commands to register with satisfactory accuracy the images to merge.

5. EXPERIMENTAL RESULTS The measurement procedure for a wide-band IR imaging for frescos investigations can be summarized as follows:

1. Take a high resolution image in the visible band (INO Scanner or spectrum enhanced camera in visible mode) for reference and possible false color (see below) analysis;

2. Use NIR high resolution imaging (INO Scanner or spectrum enhanced digital camera) to detect underdrawings, pentimenti and repaintings. Multispectral approach possible (INO Scanner with multispectral capability or spectrum enhanced camera). Could be used to reduce “false alarms” in FIR imaging5;

3. Use MIR imaging (Nikon Laird S-270) to better differentiate surface materials. Structural defects detection possible (but better in the FIR – requires lower heating), underdrawings detection possible (but better in the NIR because of higher resolution and because of the more blurred response in the MIR);

4. Use FIR imaging (FLIR ThermaCAM S65) to detect structural and subsurface defects and/or heterogeneous materials. Underdrawings detection possible (same limitations of MIR imaging).

5. Data processing and integration: multiview, ad hoc software and false colors analysis.

6. Use algorithms of quantitative thermography15 to better detect structural and subsurface defects.

5.1 The OPD Fresco targets

The proposed experimental procedure was first validated on important fresco models at the OPD laboratories (Figure 3), which are copy from Ghirlandaio realized around 1930 by the restorer Benini, and based on traditional materials and techniques. These models suffer from aging and they were seriously damaged also by the 1966 flood of the Arno river in Florence.

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Figure 3. The target models (dated around 1930) at the Mural Painting Sector of the Opificio delle Pietre Dure

laboratories, Florence.

The example results reported below demonstrate how the wide-band NIR-MIR-FIR imaging, if carefully carried out in the proposed joint, multi-mode approach, can be a powerful diagnostic tool for investigating the layered structure of fresco artworks.

In the first example (Figure 4), it is shown that the MIR reflection band is able to differentiate regions / layering related to materials with strong absorption and high inhomogeneity: part of the hematite in children’s shoulder is a superficial layer, painted over a red pigment. The NIR is used a reference map. This layering is detected also in the FIR band (less emissivity). The FIR band (emission/reflection) reveals that the hat in the hand of the man is a completely detached pigment layer, painted over a red one (thermal anomalies due to underneath void).

Figure 4. a – VIS image; b – NIR band; c – MIR band; d – FIR band. Reflection bands in the NIR-MIR, allows to

obtain information about the layering of the painted area (e.g. the child shoulder) which is confirmed also by FIR emission band. FIR maps the detached painted area (e.g. the hat in the man hand).

In the second example (Figure 5), it is shown that FAR emission band (thermogram after heating pulse) provides a clear detection of the deep texture of the support, as well as of the inner void defect probably at plaster level. The MIR band is effective in detecting the high reflective response of the cinnabar pigment in the mouth, not mapped in the NIR, and the superficial inhomogeneous area in the hat. Some information regarding the no reflective backgroung which is executed in fresco technique is provided by the MIR reflectogram (Figure 6). The Figure 6 compares the MIR wide band image and the reflectogram at the long NIR wavelength 2.3 microns, acquired with the multispectral scanner, and show the complementary information related to different penetration and reflectance response in the two bands.

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Figure 5. a – VIS image; b – NIR band; c – MIR band; d – FIR band. The FIR thermal band (in emission) allows to

obtain information of the wall support. The NIR and MIR reflective bands provide differentiated information of the painting layer: MIR detects the material response of the fresco layer of the background, secco touches, and the most superficial part of the pictorial layer. NIR provides the complementary information related to the less superficial part of the color layer including some subsurface features.

Figure 6. VIS image, MIR reflectogram, NIR reflectogram at 2.3 μm, multiband IR scanner. The MIR response reveals

complementary features that are not detectable in the NIR band, even at longer wavelengths. MIR detects the material response of the more superficial fresco layer of the background, the cinnabar secco finishing touches, and the superficial drawing traces in the contour face. NIR detects the response of deep painting layer and maps underneath features, i.e. the drawing traces.

In the third example (Figure 7) is shown the capability of the FIR band (both emission and reflection quote) in mapping the detaches and delamination of the pictorial layer. Thermal contrast-based techniques, such as the DAC computation, can be applied to the thermal sequence to enhance the detection of subsurface defects and to gain information on size and depth (Figure 8). Towards a quantitative analysis is possible to differentiate the anomalies of the deep structure (at the support/plaster level), and the detaches or delaminations (at the pictorial level).

Figure 7. (From the left) Raking light image; FIR thermogram; False color images with VIS/G-NIR-FIR channels. The

FIR band is able to map detaches and delamination.

VIS MIR 2.3 μm NIR

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Figure 8. DAC images with the FIR thermograms. DAC images computed at different time of the thermal sequence

enhance differently the anomalies related to defects of the superficial painting layer or of the inner support structure.

5.2 The mural painting La Resurrezione by Piero della Francesca

Following the measurement process tested on models, wide-band imaging in the NIR-MIR-FIR bands was performed in situ on the famous masterpiece La Resurrezione (The Resurrection) by Piero della Francesca. The artwork is a mural fresco and tempera painting, dated around 1460, and it is located in the Museo Civico of Sansepolcro (Italy). The Museum, built in XIII c., was home to the municipal hall until the 1960s and was reorganized in the present configuration around 1456. The artwork is being subject to an intensive diagnostic program (2 years) before the next restoration intervention. The conservation problem includes a particular situation relating the wall support and the building architecture. In fact, within the wall supporting the painting there is a hidden chimney, whose presence was known (there is a chimney pot on the roof!), but whose position was not exactly located. Different investigation campaigns in the FIR band were aimed at detecting the chimney position8.

The results below show the joint use of NIR-MIR-FIR bands, in the proposed dual approach reflection-thermal, for the analysis of the support structure and the superficial layer in the Resurrection mural painting.

The NIR-MIR reflection bands provide maps, related to the response of pictorial layers, which allow to extract different and complementary information on painting materials. NIR imaging was able to record the underdrawing by mapping the traces of the spolvero technique (Figure 9). MIR band is very effective in material differentiation (Figure 10), showing great capability to detect the painting integration due to response of organic material (architecture on the left), as well as some high reflective pigments, such as the high reflective mineral glauconite (green helmet of the soldier), and some finishing touches. The FIR thermal map provides information on the deep structure of the wall, and was here able to map the chimney (Figure 11), to be put in relation with the response of the mural painting support for a most comprehensive analysis of the latter.

Figure 9. “The Resurrection”, Piero della Francesca, detail. VIS (left) and NIR reflection band (right) showing the

traces of the spolvero.

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Figure 10. “The Resurrection”, Piero della Francesca, botton central part with the four soldiers. VIS (up) and MIR

reflection band (botton). The variegate MIR reflectance response allows discrimination of materials such as organic painting integration (left architecture), and high reflective pigments, e.g. the mineral glauconite (soldier helmet and shoes).

Figure 11. The Resurrection, Piero della Francesca. Photography of the mural painting and FIR thermal map of the

wall. FIR bands detects thermal anomalies in relation to structural defects, both of the deep wall (chimney) than of the painting support.

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6. CONCLUSIONS Wide-band IR imaging in the NIR-MIR-FIR spectral bands was applied to the diagnostics of mural paintings. An approach based on a joint, multi-mode use of reflection and thermal bands is proposed, and it is demonstrated to be an effective tool in inspecting the layered structure of fresco artworks, providing selective information thanks to the variegate reflective/emissive behavior of the layers in the IR. High resolution IR reflectography and, to a greater extent, IR imaging in the 3-5 μm band, are effectively used to characterize the superficial layer of the fresco and to analyze the stratigraphy of different pictorial layers. IR thermography in the 8-12 μm band is used to characterize the support deep structure. The integration of all the data provides a multi- layered representation of the fresco that yields a more comprehensive analysis. The method was validated on target model (dated 1930) and was applied to the diagnostics of the masterpiece “The Resurrection” by Piero della Francesca.

ACKNOWLEDGEMENTS

Authors wish to thank Dr. Mariangela Betti, Head of the Museo Civico of Sansepolcro, for the coordination of the measurement campaign on “The Resurrection”, and the restorer Ottaviano Caruso, restorer for creative discussion and useful suggestions. The project was partially supported by the Comune di Sansepolcro.

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