Thermal Quasi-Reflectography is a new optical technique, based on Mid-IR, which is demonstrated to have a great potential in the diagnostics of frescoes. Here we address the key-points: 1) basic principles and instrumentation to enable operative measurements; 2) main diagnostic results

C. Daffara

D. Ambrosini

E. Zendri

G. Marchioro

P. I. Mariotti

Name not available

1International Conference
Built Heritage 2013
Monitoring Conservation Management
C E N T R O  P E R  L A 
C O N S E R VA Z I O N E  E 
VA L O R I Z Z A Z I O N E  D E I 
B E N I  C U LT U R A L I
Online Proceedings of the Conference
BUILT HERITAGE 2013
Monitoring Conservation 
and Management
Milan, Italy
18-20 November 2013
Editor in chief M. Boriani
Edited by R. Gabaglio, D. Gulotta
2Online Proceedings of Conference 
Built Heritage 2013 - Monitoring Conservation and Management
Milan - Italy, 18-20 November 2013
Editor in chief M. Boriani, edited by R. Gabaglio, D. Gulotta
Politecnico di Milano, Centro per la Conservazione e Valorizzazione dei Beni Culturali, 
Milano, novembre 2013
ISBN 978-88-908961-0-1
1399
Built Heritage 2013 Monitoring Conservation Management
The challenge of infrared imaging of frescos: Thermal Quasi-Re-
fl ectography unveils hidden features of artworks
Claudia Daffara1; Giacomo Marchioro1; Elisabetta Zendri2; Dario Ambrosini3; 
Paola I. Mariotti4 
1 Univ. of Verona, Dept. of Computer Science, Verona, Italy; 2 Univ. Ca’ Foscari 
of Venice, Dept. of Environmental Sciences, Informatics and Statistics, Veni-
ce, Italy; 3 Univ. of L’Aquila, Dept. of Industrial and Information Engineering 
and Economics, L’Aquila, Italy; 4 Opiﬁ cio delle Pietre Dure, Florence, Italy 
1. Infrared diagnostics of multi-layered paintings
Infrared imaging is widely used in heritage diagnostics allowing the noninvasi-
ve analysis of extended painted surfaces. Wide-ﬁ eld techniques and selective 
use of proper infrared bands enable the mapping of many features both at 
surface and subsurface levels, according to the speciﬁ c radiation interaction 
properties of the artwork materials. In particular, near-infrared reﬂ ectography, 
by exploiting the capability of the near-infrared spectrum (0.8-2.5 m) to pe-
netrate the different pictorial layers, is able to reveal hidden features such as 
preparatory drawings, pentimenti (changes in the artwork made by the artist 
himself), or subsequent repaintings [Aa.Vv., 2005]. It is well known that for 
the study of canvas and panel paintings, such technique is one of the most 
powerful tools in the hands of art historians and restorers [Van Asperen de 
Boer, 1968]. The more recent spectral techniques have further boosted near-
infrared imagery both of ancient and contemporary paintings [Delaney et al., 
2010; Daffara et al., 2011]. Acquisition of images in multiple bands improves 
the observation of the different painting features, which can be detected in 
optimal spectral windows according to the optical transparency of painting 
matter in the infrared range.
From this point of view, the analysis of mural paintings is a difﬁ cult challenge. 
Traditional reﬂ ectography is less effective when a fresco layer is present, due 
to the different optical properties of the pictorial layers (different scattering 
medium).  
As recently shown [Ambrosini et al., 2010], the integration of reﬂ ectography 
and thermography represents an interesting approach in artwork diagnostics. 
These two techniques, which work in the different spectral ranges of near and 
far-infrared (7-14 ȝm), provided, jointly used, signiﬁ cant information about the 
conservation status of panel paintings. On one hand, near-infrared inspec-
tions allows going through the making of the artwork and its restoration steps; 
on the other hand, thermography allows different properties of materials, in 
addition to superﬁ cial and structural defects, to be outlined. Logical develop-
ments of this integration were the adding of the mid-infrared band (3-5 ȝm) 
and the extension to different artworks, e.g. frescos.
We have recently demonstrated a novel tool for infrared imaging of artworks, 
named Thermal Quasi-Reﬂ ectography (TQR) [Daffara et al., 2012], able to 
unlock in mural paintings details that are not revealed by the conventional 
techniques. The underpinning idea is to extract information from the mid-infra-
red energy reﬂ ected by the object, which results to be strongly related to the 
1400
Built Heritage 2013 Monitoring Conservation Management
properties of surface materials. 
Thermal bands are conventionally being used in non-destructive testing in 
relation to the emissive behavior of the target object, and widely applied to 
mural paintings for the inspection of structural defects (inner support or paint 
delamination) [Ibarra-Castanedo et al., 2008]. Here the concept of classic 
thermography, where the radiation emitted by the surface is recorded and 
then correlated to the temperature distribution (the thermogram) is taken on 
reverse. Effectively, it is a quasi-reﬂ ectography in the thermal wavelengths, in 
which the emitted radiation is minimized in order to properly record the reﬂ ec-
ted quote. The result is the mid-infrared reﬂ ectogram, which allows the discri-
mination of many features in the pictorial layers, related both to the artwork 
materials and technique and to the fresco surface decay. 
In this brief paper we trace the key-points of the new TQR technique: 1) basic 
principles and instrumentation to enable operative measurements; 2) main 
diagnostic results. Examples of the image results obtained on fresco models 
are shown, while applications on original artworks can be found in [Daffara et 
al., 2012]. 
2. Thermal Quasi-Refl ectography (TQR)
2.1. Basic principles 
In infrared imaging we can distinguish between reﬂ ective and thermal bands. 
Near-infrared imaging up to 2.5 ȝm in wavelength forms images using re-
ﬂ ected light, e.g. the infrared radiation backscattered by the painted surface. 
Conversely, the mid-infrared and the far-infrared regions are thermal bands. 
Imaging in these wavelengths records the infrared radiation emitted from the 
target and then displays this radiation as a temperature map on the target 
surface. 
The key point of the TQR method is to use a thermal camera to form images 
from the radiation reﬂ ected by the object; obviously, the obtained images are 
not related to temperature distribution on the object surface. It is shown that 
at room temperature, the emissive contribute in the mid-infrared range is ne-
gligible (about 1%) with respect to the corresponding reﬂ ective quote, there-
fore, a reﬂ ectography can be performed in this spectral range by means of a 
radiometric camera and a proper artiﬁ cial source matched in the 3-5 ȝm band, 
under the condition of limiting the heating of the surface.
The set up for measurements (sketched in ﬁ gure 1) is simple: image acqui-
sition is performed in reﬂ ection geometry, in broadband and wide-ﬁ eld mode. 
Anyway, thermal cameras working in the mid-infrared range (mainly based 
on cooled MCT or InSb array sensors), are still very expensive compared to 
near and far-infrared (bolometric) devices. A suitable source provides uniform 
irradiation over a “large” area, (e.g. ﬁ eld of view of 1 sqmt) for ﬁ eld measu-
rements. Infrared lamps controlled in power with the spectrum matched to 
the mid-infrared quasi-reﬂ ection range can be obtained starting from quartz 
elements (carbon, tungsten or alloy ﬁ lament) and a suitable ﬁ ltering infrared 
window (e.g. sapphire); halogens are peaked in the near-infrared, but also 
provide a quote of mid-infrared.
1401
Built Heritage 2013 Monitoring Conservation Management
Fig.1 - Measurement setup for TQR: mid-infrared camera, infrared source controlled in power, 
real-time image acquisition and processing. The detail outlines the layered structure of fresco: 
pictorial layers, plaster, and support
2.2. Diagnostic results on frescoes
A painting on wall, or fresco, can be described as a layered structure (see 
ﬁ gure 1). The wall support is primed with plaster layers, which serve as a 
base for the painting; in a traditional fresco, the pigment is dissolved in water 
and then applied on the wet plaster leading then to the unique carbonated 
layer. Traditional reﬂ ectography, based on near-infrared, is not very effective 
on mural paintings, although some information could be retrieved as, in many 
cases, frescos have integration and retouches in “secco” technique (separate 
paint layers with binder). The TQR technique, based on mid-infrared, has a 
big potential in the diagnostics of such complex artworks, as discussed below.
The ﬁ gure 2 compares mid-infrared imaging in reﬂ ection mode, i.e. TQR, and 
in emission mode (active thermography), showing that TQR imaging is stron-
gly related to superﬁ cial features, while thermography detects subsurface de-
fects (blue arrow).
The TQR technique has been experimented on both target samples and real 
artworks thanks to the collaboration with the Opiﬁ cio delle Pietre Dure (OPD), 
Florence.
The ﬁ gure 3 shows the application of TQR to a fresco model (a copy from old 
masters constructed in 1930, based on traditional material and technique) and 
Fig.2 - Mid-infrared diagnostics of lime plaster sample: reﬂ ection mode TQR (left) and emission 
mode thermography (right). The blu arrow indicates a subsurface defect detected in the emission 
image and not in the reﬂ ectogram. A void volume on the opposite side of the sample creates the 
subsurface discontinuity while the main surface is layered with Vinavil
1402
Built Heritage 2013 Monitoring Conservation Management
the comparison with the traditional techniques reﬂ ectography and thermogra-
phy. Mid-infrared maps the red pigment cinnabar (mouth), which is highly re-
ﬂ ective, and some features of the surface layers (skin, hat, and background) 
that are not detected in the near-infrared. As expected, far-infrared thermo-
graphy provides a clear detection of the deep texture of the support together 
with an inner void defect at plaster level (near the shoulder). The detail in 
ﬁ gure 3 compares the mid-infrared image and the reﬂ ectogram acquired with 
multispectral scanner at long near-infrared wavelenghts (2.2 ȝm) and shows 
the complementary information related to different penetration and reﬂ ectan-
ce response in the two bands. TQR in the mid-infrared detects the material 
response in the more superﬁ cial fresco layer (background area) and drawing 
traces (face contour), near-infrared detects the response in deeper painting 
layer and maps underneath drawing traces.
TQR information is related to the materials in the surface layer. Differences 
in the reﬂ ectance response allow the discrimination of some materials such 
as high reﬂ ective pigments. Mid-infrared spectrum carries back information 
related to the absorption bands of pictorial matter. The low reﬂ ectance is pro-
bably due to the absorption features of the organics materials, however an 
interpretation based exclusively on the absorption bands could be misleading. 
The mid-infrared reﬂ ectance is determined by factors related to the material 
composition, to the density and microstructure, and to the surface roughness. 
Figure 3. Analysis of a fresco model (OPD laboratories). From the left: VIS image, near-infrared 
reﬂ ectography (1m Si CCD), TQR image (3-5m), far-infrared thermography (thermogram after 
heating pulse). The detail: mid-infrared TQR (left), near-infrared scanner reﬂ ectography (2.2 m)
1403
Built Heritage 2013 Monitoring Conservation Management
As such, TQR has the potential to detect the presence of different materials 
and binders, and to discriminate the response of different surface layers. 
Pictorial materials created, layered, and ﬁ nished with different techniques 
exhibit different reﬂ ectance responses. A systematic study of the mid-infrared 
reﬂ ectance is being carried out on a set of controlled samples of different ma-
terials (pigments and binders) and execution techniques, constructed at the 
Wall Painting Laboratory of the OPD (ﬁ gure 4). 
The TQR technique is effective in the study of surface decay. As example of 
this application, a laboratory study on controlled samples is shown (ﬁ gure 5), 
where the technique is applied to map the sulphated areas in the fresco surfa-
ce. A brush was used to apply a 2% solution of Sulphuric acid in a sample of 
lime plaster. Neither in the visible spectrum nor in the near-infrared the area 
where the solution has been applied is observable, but in the 3-5 ȝm region 
the area attacked with the acid solution exhibits higher reﬂ ectance. This be-
havior is due to the different physical and chemical aspects of the material 
surface involved. The gypsum created after the sulphatation probably has a 
lower absorbance compared with lime plaster, but some reﬂ ectance incre-
ment could be related to the different morphology of the sulphated surface.
On real artworks, the TQR technique was effective for the selective mapping 
of painting materials, e.g. high reﬂ ective pigments as cinnabar or green earth 
glauconite, ﬁ nishing touches, subtle traces in bianco di San Giovanni, and a 
clear detection of different-aged organic painting integrations. Different exe-
cution techniques, e.g. fresco or tempera, also exhibit different behaviors. Ap-
plication of the TQR to the masterpiece “Resurrezione” by Piero della France-
sca, Museo Civico of Sansepolcro, can be found in the [Daffara et al., 2012].
Fig.4 -TQR applied to fresco samples (OPD laboratories). Different kind of lapis lazuli pigments 
(columns) layered with different fresco techniques (rows). Reﬂ ectance in the mid-infrared varies 
with the combinations of pigment-binder, the technique, and the surface roughness
Fig.5 - TQR diagnostics applied to a sample of lime plaster attacked by sulphate decay. From the 
left: VIS image, near-infrared reﬂ ectography (PtSi 1.2-2.5 m), TQR (3-5 m). Red arrows indica-
te the sulphated areas, which are detected only by TQR technique
1404
Built Heritage 2013 Monitoring Conservation Management
3. Conclusion
Thermal Quasi-Reﬂ ectography is a new diagnostic technique, which is de-
monstrated to have a great potential in the diagnostics of frescoes. The results 
obtained strongly encourage further developments and applications on ﬁ eld. 
Based on the recording of mid-infrared radiation reﬂ ected from the surface, 
TQR is well suited to the differentiation of surface materials. It can provide 
pigments differentiation and identiﬁ cation of repainting and retouching. TQR 
can be used as a ﬁ rst screening for a more effective sampling with point tech-
niques, e.g. spectroscopy in the mid-infrared range. Following the approach of 
imagery integration, TQR can be integrated with the other imaging techniques 
for a multi-layer representation of the artwork and a more comprehensive dia-
gnostic documentation.
An interdisciplinary effort, involving conservators and scientists, is needed. 
Thanks to the collaboration with the Opiﬁ cio delle Pietre Dure, the TQR me-
thod is being tested both on controlled samples and fresco models as well as 
on important artworks. While conservators should test TQR on real artworks in 
situ, scientists should optimize the performance in laboratory: from the optical 
acquisition chain (crucial is the mid-infrared source), to the reﬂ ectance data 
interpretation (crucial is the correlation of TQR with spectroscopy in the 3-5 
ȝm) in order to stretch the imaging technique towards materials mapping.
Acknowledgements
A special thanks is due to Cecilia Frosinini, Director of the Wall Paintings Department 
of the Opiﬁ cio delle Pietre Dure (Florence) for the proﬁ table collaboration and her pre-
cious suggestions. 
References 
Aa.Vv., 2005, Scientifi c examination of art, Proc. National Academy of Science, Wa-
shington D.C.
Ambrosini D., Daffara C., Di Biase R., Paoletti D., Pezzati L., Bellucci R., Bettini F., 
2010, Integrated refl ectography and thermography for wooden paintings diagnostics, 
«Journal of Cultural Heritage» 11, 196-204.
Daffara C., Fontana R., 2011, Multispectral infrared refl ectography to differentiate fea-
tures in paintings, «Microscopy and Microanalysis» 14, 691-695.
Daffara C., Ambrosini D., Pezzati L., Paoletti D., 2012, Thermal quasi-refl ectography: a 
new imaging tool in art conservation, «Optics Express» 20, 14746-14753.
Delaney J.K., Zeibel J.G., Thoury M., Littleton R., Palmer M., Morales K.M., René de 
la Rie, Hoenigswald, 2010, Visible and Infrared Imaging Spectroscopy of Picasso’s 
Harlequin Musician: Mapping and Identifi cation of Artist Materials in Situ, «Applied 
Spectroscopy» 64, 584-594.
Ibarra-Castanedo C., Sfarra S., Ambrosini D., Paoletti D., Bendada A., Maldague X., 
2008, Subsurface defect characterization in artworks by quantitative PPT and hologra-
phic interferometry, «Quantitative Infrared Thermography Journal» 5, 131-149.
Van Asperen de Boer J. R. J., 1968, Infrared Refl ectography: a Method for the Exami-
nation of Paintings, Applied Optics 7, 1711-1714.


The challenge of infrared imaging of frescos: Thermal Quasi-Reflectography unveils hidden features of artworks

Archivio Ricerca Ca'Foscari

1International Conference
Built Heritage 2013
Monitoring Conservation Management
C E N T R O  P E R  L A 
C O N S E R VA Z I O N E  E 
VA L O R I Z Z A Z I O N E  D E I 
B E N I  C U LT U R A L I
Online Proceedings of the Conference
BUILT HERITAGE 2013
Monitoring Conservation 
and Management
Milan, Italy
18-20 November 2013
Editor in chief M. Boriani
Edited by R. Gabaglio, D. Gulotta
2Online Proceedings of Conference 
Built Heritage 2013 - Monitoring Conservation and Management
Milan - Italy, 18-20 November 2013
Editor in chief M. Boriani, edited by R. Gabaglio, D. Gulotta
Politecnico di Milano, Centro per la Conservazione e Valorizzazione dei Beni Culturali, 
Milano, novembre 2013
ISBN 978-88-908961-0-1
1399
Built Heritage 2013 Monitoring Conservation Management
The challenge of infrared imaging of frescos: Thermal Quasi-Re-
fl ectography unveils hidden features of artworks
Claudia Daffara1; Giacomo Marchioro1; Elisabetta Zendri2; Dario Ambrosini3; 
Paola I. Mariotti4 
1 Univ. of Verona, Dept. of Computer Science, Verona, Italy; 2 Univ. Ca’ Foscari 
of Venice, Dept. of Environmental Sciences, Informatics and Statistics, Veni-
ce, Italy; 3 Univ. of L’Aquila, Dept. of Industrial and Information Engineering 
and Economics, L’Aquila, Italy; 4 Opifi cio delle Pietre Dure, Florence, Italy 
1. Infrared diagnostics of multi-layered paintings
Infrared imaging is widely used in heritage diagnostics allowing the noninvasi-
ve analysis of extended painted surfaces. Wide-fi eld techniques and selective 
use of proper infrared bands enable the mapping of many features both at 
surface and subsurface levels, according to the specifi c radiation interaction 
properties of the artwork materials. In particular, near-infrared refl ectography, 
by exploiting the capability of the near-infrared spectrum (0.8-2.5 m) to pe-
netrate the different pictorial layers, is able to reveal hidden features such as 
preparatory drawings, pentimenti (changes in the artwork made by the artist 
himself), or subsequent repaintings [Aa.Vv., 2005]. It is well known that for 
the study of canvas and panel paintings, such technique is one of the most 
powerful tools in the hands of art historians and restorers [Van Asperen de 
Boer, 1968]. The more recent spectral techniques have further boosted near-
infrared imagery both of ancient and contemporary paintings [Delaney et al., 
2010; Daffara et al., 2011]. Acquisition of images in multiple bands improves 
the observation of the different painting features, which can be detected in 
optimal spectral windows according to the optical transparency of painting 
matter in the infrared range.
From this point of view, the analysis of mural paintings is a diffi cult challenge. 
Traditional refl ectography is less effective when a fresco layer is present, due 
to the different optical properties of the pictorial layers (different scattering 
medium).  
As recently shown [Ambrosini et al., 2010], the integration of refl ectography 
and thermography represents an interesting approach in artwork diagnostics. 
These two techniques, which work in the different spectral ranges of near and 
far-infrared (7-14 ȝm), provided, jointly used, signifi cant information about the 
conservation status of panel paintings. On one hand, near-infrared inspec-
tions allows going through the making of the artwork and its restoration steps; 
on the other hand, thermography allows different properties of materials, in 
addition to superfi cial and structural defects, to be outlined. Logical develop-
ments of this integration were the adding of the mid-infrared band (3-5 ȝm) 
and the extension to different artworks, e.g. frescos.
We have recently demonstrated a novel tool for infrared imaging of artworks, 
named Thermal Quasi-Refl ectography (TQR) [Daffara et al., 2012], able to 
unlock in mural paintings details that are not revealed by the conventional 
techniques. The underpinning idea is to extract information from the mid-infra-
red energy refl ected by the object, which results to be strongly related to the 
1400
Built Heritage 2013 Monitoring Conservation Management
properties of surface materials. 
Thermal bands are conventionally being used in non-destructive testing in 
relation to the emissive behavior of the target object, and widely applied to 
mural paintings for the inspection of structural defects (inner support or paint 
delamination) [Ibarra-Castanedo et al., 2008]. Here the concept of classic 
thermography, where the radiation emitted by the surface is recorded and 
then correlated to the temperature distribution (the thermogram) is taken on 
reverse. Effectively, it is a quasi-refl ectography in the thermal wavelengths, in 
which the emitted radiation is minimized in order to properly record the refl ec-
ted quote. The result is the mid-infrared refl ectogram, which allows the discri-
mination of many features in the pictorial layers, related both to the artwork 
materials and technique and to the fresco surface decay. 
In this brief paper we trace the key-points of the new TQR technique: 1) basic 
principles and instrumentation to enable operative measurements; 2) main 
diagnostic results. Examples of the image results obtained on fresco models 
are shown, while applications on original artworks can be found in [Daffara et 
al., 2012]. 
2. Thermal Quasi-Refl ectography (TQR)
2.1. Basic principles 
In infrared imaging we can distinguish between refl ective and thermal bands. 
Near-infrared imaging up to 2.5 ȝm in wavelength forms images using re-
fl ected light, e.g. the infrared radiation backscattered by the painted surface. 
Conversely, the mid-infrared and the far-infrared regions are thermal bands. 
Imaging in these wavelengths records the infrared radiation emitted from the 
target and then displays this radiation as a temperature map on the target 
surface. 
The key point of the TQR method is to use a thermal camera to form images 
from the radiation refl ected by the object; obviously, the obtained images are 
not related to temperature distribution on the object surface. It is shown that 
at room temperature, the emissive contribute in the mid-infrared range is ne-
gligible (about 1%) with respect to the corresponding refl ective quote, there-
fore, a refl ectography can be performed in this spectral range by means of a 
radiometric camera and a proper artifi cial source matched in the 3-5 ȝm band, 
under the condition of limiting the heating of the surface.
The set up for measurements (sketched in fi gure 1) is simple: image acqui-
sition is performed in refl ection geometry, in broadband and wide-fi eld mode. 
Anyway, thermal cameras working in the mid-infrared range (mainly based 
on cooled MCT or InSb array sensors), are still very expensive compared to 
near and far-infrared (bolometric) devices. A suitable source provides uniform 
irradiation over a “large” area, (e.g. fi eld of view of 1 sqmt) for fi eld measu-
rements. Infrared lamps controlled in power with the spectrum matched to 
the mid-infrared quasi-refl ection range can be obtained starting from quartz 
elements (carbon, tungsten or alloy fi lament) and a suitable fi ltering infrared 
window (e.g. sapphire); halogens are peaked in the near-infrared, but also 
provide a quote of mid-infrared.
1401
Built Heritage 2013 Monitoring Conservation Management
Fig.1 - Measurement setup for TQR: mid-infrared camera, infrared source controlled in power, 
real-time image acquisition and processing. The detail outlines the layered structure of fresco: 
pictorial layers, plaster, and support
2.2. Diagnostic results on frescoes
A painting on wall, or fresco, can be described as a layered structure (see 
fi gure 1). The wall support is primed with plaster layers, which serve as a 
base for the painting; in a traditional fresco, the pigment is dissolved in water 
and then applied on the wet plaster leading then to the unique carbonated 
layer. Traditional refl ectography, based on near-infrared, is not very effective 
on mural paintings, although some information could be retrieved as, in many 
cases, frescos have integration and retouches in “secco” technique (separate 
paint layers with binder). The TQR technique, based on mid-infrared, has a 
big potential in the diagnostics of such complex artworks, as discussed below.
The fi gure 2 compares mid-infrared imaging in refl ection mode, i.e. TQR, and 
in emission mode (active thermography), showing that TQR imaging is stron-
gly related to superfi cial features, while thermography detects subsurface de-
fects (blue arrow).
The TQR technique has been experimented on both target samples and real 
artworks thanks to the collaboration with the Opifi cio delle Pietre Dure (OPD), 
Florence.
The fi gure 3 shows the application of TQR to a fresco model (a copy from old 
masters constructed in 1930, based on traditional material and technique) and 
Fig.2 - Mid-infrared diagnostics of lime plaster sample: refl ection mode TQR (left) and emission 
mode thermography (right). The blu arrow indicates a subsurface defect detected in the emission 
image and not in the refl ectogram. A void volume on the opposite side of the sample creates the 
subsurface discontinuity while the main surface is layered with Vinavil
1402
Built Heritage 2013 Monitoring Conservation Management
the comparison with the traditional techniques refl ectography and thermogra-
phy. Mid-infrared maps the red pigment cinnabar (mouth), which is highly re-
fl ective, and some features of the surface layers (skin, hat, and background) 
that are not detected in the near-infrared. As expected, far-infrared thermo-
graphy provides a clear detection of the deep texture of the support together 
with an inner void defect at plaster level (near the shoulder). The detail in 
fi gure 3 compares the mid-infrared image and the refl ectogram acquired with 
multispectral scanner at long near-infrared wavelenghts (2.2 ȝm) and shows 
the complementary information related to different penetration and refl ectan-
ce response in the two bands. TQR in the mid-infrared detects the material 
response in the more superfi cial fresco layer (background area) and drawing 
traces (face contour), near-infrared detects the response in deeper painting 
layer and maps underneath drawing traces.
TQR information is related to the materials in the surface layer. Differences 
in the refl ectance response allow the discrimination of some materials such 
as high refl ective pigments. Mid-infrared spectrum carries back information 
related to the absorption bands of pictorial matter. The low refl ectance is pro-
bably due to the absorption features of the organics materials, however an 
interpretation based exclusively on the absorption bands could be misleading. 
The mid-infrared refl ectance is determined by factors related to the material 
composition, to the density and microstructure, and to the surface roughness. 
Figure 3. Analysis of a fresco model (OPD laboratories). From the left: VIS image, near-infrared 
refl ectography (1m Si CCD), TQR image (3-5m), far-infrared thermography (thermogram after 
heating pulse). The detail: mid-infrared TQR (left), near-infrared scanner refl ectography (2.2 m)
1403
Built Heritage 2013 Monitoring Conservation Management
As such, TQR has the potential to detect the presence of different materials 
and binders, and to discriminate the response of different surface layers. 
Pictorial materials created, layered, and fi nished with different techniques 
exhibit different refl ectance responses. A systematic study of the mid-infrared 
refl ectance is being carried out on a set of controlled samples of different ma-
terials (pigments and binders) and execution techniques, constructed at the 
Wall Painting Laboratory of the OPD (fi gure 4). 
The TQR technique is effective in the study of surface decay. As example of 
this application, a laboratory study on controlled samples is shown (fi gure 5), 
where the technique is applied to map the sulphated areas in the fresco surfa-
ce. A brush was used to apply a 2% solution of Sulphuric acid in a sample of 
lime plaster. Neither in the visible spectrum nor in the near-infrared the area 
where the solution has been applied is observable, but in the 3-5 ȝm region 
the area attacked with the acid solution exhibits higher refl ectance. This be-
havior is due to the different physical and chemical aspects of the material 
surface involved. The gypsum created after the sulphatation probably has a 
lower absorbance compared with lime plaster, but some refl ectance incre-
ment could be related to the different morphology of the sulphated surface.
On real artworks, the TQR technique was effective for the selective mapping 
of painting materials, e.g. high refl ective pigments as cinnabar or green earth 
glauconite, fi nishing touches, subtle traces in bianco di San Giovanni, and a 
clear detection of different-aged organic painting integrations. Different exe-
cution techniques, e.g. fresco or tempera, also exhibit different behaviors. Ap-
plication of the TQR to the masterpiece “Resurrezione” by Piero della France-
sca, Museo Civico of Sansepolcro, can be found in the [Daffara et al., 2012].
Fig.4 -TQR applied to fresco samples (OPD laboratories). Different kind of lapis lazuli pigments 
(columns) layered with different fresco techniques (rows). Refl ectance in the mid-infrared varies 
with the combinations of pigment-binder, the technique, and the surface roughness
Fig.5 - TQR diagnostics applied to a sample of lime plaster attacked by sulphate decay. From the 
left: VIS image, near-infrared refl ectography (PtSi 1.2-2.5 m), TQR (3-5 m). Red arrows indica-
te the sulphated areas, which are detected only by TQR technique
1404
Built Heritage 2013 Monitoring Conservation Management
3. Conclusion
Thermal Quasi-Refl ectography is a new diagnostic technique, which is de-
monstrated to have a great potential in the diagnostics of frescoes. The results 
obtained strongly encourage further developments and applications on fi eld. 
Based on the recording of mid-infrared radiation refl ected from the surface, 
TQR is well suited to the differentiation of surface materials. It can provide 
pigments differentiation and identifi cation of repainting and retouching. TQR 
can be used as a fi rst screening for a more effective sampling with point tech-
niques, e.g. spectroscopy in the mid-infrared range. Following the approach of 
imagery integration, TQR can be integrated with the other imaging techniques 
for a multi-layer representation of the artwork and a more comprehensive dia-
gnostic documentation.
An interdisciplinary effort, involving conservators and scientists, is needed. 
Thanks to the collaboration with the Opifi cio delle Pietre Dure, the TQR me-
thod is being tested both on controlled samples and fresco models as well as 
on important artworks. While conservators should test TQR on real artworks in 
situ, scientists should optimize the performance in laboratory: from the optical 
acquisition chain (crucial is the mid-infrared source), to the refl ectance data 
interpretation (crucial is the correlation of TQR with spectroscopy in the 3-5 
ȝm) in order to stretch the imaging technique towards materials mapping.
Acknowledgements
A special thanks is due to Cecilia Frosinini, Director of the Wall Paintings Department 
of the Opifi cio delle Pietre Dure (Florence) for the profi table collaboration and her pre-
cious suggestions. 
References 
Aa.Vv., 2005, Scientifi c examination of art, Proc. National Academy of Science, Wa-
shington D.C.
Ambrosini D., Daffara C., Di Biase R., Paoletti D., Pezzati L., Bellucci R., Bettini F., 
2010, Integrated refl ectography and thermography for wooden paintings diagnostics, 
«Journal of Cultural Heritage» 11, 196-204.
Daffara C., Fontana R., 2011, Multispectral infrared refl ectography to differentiate fea-
tures in paintings, «Microscopy and Microanalysis» 14, 691-695.
Daffara C., Ambrosini D., Pezzati L., Paoletti D., 2012, Thermal quasi-refl ectography: a 
new imaging tool in art conservation, «Optics Express» 20, 14746-14753.
Delaney J.K., Zeibel J.G., Thoury M., Littleton R., Palmer M., Morales K.M., René de 
la Rie, Hoenigswald, 2010, Visible and Infrared Imaging Spectroscopy of Picasso’s 
Harlequin Musician: Mapping and Identifi cation of Artist Materials in Situ, «Applied 
Spectroscopy» 64, 584-594.
Ibarra-Castanedo C., Sfarra S., Ambrosini D., Paoletti D., Bendada A., Maldague X., 
2008, Subsurface defect characterization in artworks by quantitative PPT and hologra-
phic interferometry, «Quantitative Infrared Thermography Journal» 5, 131-149.
Van Asperen de Boer J. R. J., 1968, Infrared Refl ectography: a Method for the Exami-
nation of Paintings, Applied Optics 7, 1711-1714.


ZENDRI, Elisabetta

Archivio istituzionale della ricerca - Università degli Studi di Venezia Ca' Foscari

1International ConferenceBuilt Heritage 2013Monitoring Conservation ManagementC E N T R O  P E R  L A C O N S E R VA Z I O N E  E VA L O R I Z Z A Z I O N E  D E I B E N I  C U LT U R A L IOnline Proceedings of the ConferenceBUILT HERITAGE 2013Monitoring Conservation and ManagementMilan, Italy18-20 November 2013Editor in chief M. BorianiEdited by R. Gabaglio, D. Gulotta2Online Proceedings of Conference Built Heritage 2013 - Monitoring Conservation and ManagementMilan - Italy, 18-20 November 2013Editor in chief M. Boriani, edited by R. Gabaglio, D. GulottaPolitecnico di Milano, Centro per la Conservazione e Valorizzazione dei Beni Culturali, Milano, novembre 2013ISBN 978-88-908961-0-11399Built Heritage 2013 Monitoring Conservation ManagementThe challenge of infrared imaging of frescos: Thermal Quasi-Re-fl ectography unveils hidden features of artworksClaudia Daffara1; Giacomo Marchioro1; Elisabetta Zendri2; Dario Ambrosini3; Paola I. Mariotti4 1 Univ. of Verona, Dept. of Computer Science, Verona, Italy; 2 Univ. Ca’ Foscari of Venice, Dept. of Environmental Sciences, Informatics and Statistics, Veni-ce, Italy; 3 Univ. of L’Aquila, Dept. of Industrial and Information Engineering and Economics, L’Aquila, Italy; 4 Opiﬁ cio delle Pietre Dure, Florence, Italy 1. Infrared diagnostics of multi-layered paintingsInfrared imaging is widely used in heritage diagnostics allowing the noninvasi-ve analysis of extended painted surfaces. Wide-ﬁ eld techniques and selective use of proper infrared bands enable the mapping of many features both at surface and subsurface levels, according to the speciﬁ c radiation interaction properties of the artwork materials. In particular, near-infrared reﬂ ectography, by exploiting the capability of the near-infrared spectrum (0.8-2.5 m) to pe-netrate the different pictorial layers, is able to reveal hidden features such as preparatory drawings, pentimenti (changes in the artwork made by the artist himself), or subsequent repaintings [Aa.Vv., 2005]. It is well known that for the study of canvas and panel paintings, such technique is one of the most powerful tools in the hands of art historians and restorers [Van Asperen de Boer, 1968]. The more recent spectral techniques have further boosted near-infrared imagery both of ancient and contemporary paintings [Delaney et al., 2010; Daffara et al., 2011]. Acquisition of images in multiple bands improves the observation of the different painting features, which can be detected in optimal spectral windows according to the optical transparency of painting matter in the infrared range.From this point of view, the analysis of mural paintings is a difﬁ cult challenge. Traditional reﬂ ectography is less effective when a fresco layer is present, due to the different optical properties of the pictorial layers (different scattering medium).  As recently shown [Ambrosini et al., 2010], the integration of reﬂ ectography and thermography represents an interesting approach in artwork diagnostics. These two techniques, which work in the different spectral ranges of near and far-infrared (7-14 ȝm), provided, jointly used, signiﬁ cant information about the conservation status of panel paintings. On one hand, near-infrared inspec-tions allows going through the making of the artwork and its restoration steps; on the other hand, thermography allows different properties of materials, in addition to superﬁ cial and structural defects, to be outlined. Logical develop-ments of this integration were the adding of the mid-infrared band (3-5 ȝm) and the extension to different artworks, e.g. frescos.We have recently demonstrated a novel tool for infrared imaging of artworks, named Thermal Quasi-Reﬂ ectography (TQR) [Daffara et al., 2012], able to unlock in mural paintings details that are not revealed by the conventional techniques. The underpinning idea is to extract information from the mid-infra-red energy reﬂ ected by the object, which results to be strongly related to the 1400Built Heritage 2013 Monitoring Conservation Managementproperties of surface materials. Thermal bands are conventionally being used in non-destructive testing in relation to the emissive behavior of the target object, and widely applied to mural paintings for the inspection of structural defects (inner support or paint delamination) [Ibarra-Castanedo et al., 2008]. Here the concept of classic thermography, where the radiation emitted by the surface is recorded and then correlated to the temperature distribution (the thermogram) is taken on reverse. Effectively, it is a quasi-reﬂ ectography in the thermal wavelengths, in which the emitted radiation is minimized in order to properly record the reﬂ ec-ted quote. The result is the mid-infrared reﬂ ectogram, which allows the discri-mination of many features in the pictorial layers, related both to the artwork materials and technique and to the fresco surface decay. In this brief paper we trace the key-points of the new TQR technique: 1) basic principles and instrumentation to enable operative measurements; 2) main diagnostic results. Examples of the image results obtained on fresco models are shown, while applications on original artworks can be found in [Daffara et al., 2012]. 2. Thermal Quasi-Refl ectography (TQR)2.1. Basic principles In infrared imaging we can distinguish between reﬂ ective and thermal bands. Near-infrared imaging up to 2.5 ȝm in wavelength forms images using re-ﬂ ected light, e.g. the infrared radiation backscattered by the painted surface. Conversely, the mid-infrared and the far-infrared regions are thermal bands. Imaging in these wavelengths records the infrared radiation emitted from the target and then displays this radiation as a temperature map on the target surface. The key point of the TQR method is to use a thermal camera to form images from the radiation reﬂ ected by the object; obviously, the obtained images are not related to temperature distribution on the object surface. It is shown that at room temperature, the emissive contribute in the mid-infrared range is ne-gligible (about 1%) with respect to the corresponding reﬂ ective quote, there-fore, a reﬂ ectography can be performed in this spectral range by means of a radiometric camera and a proper artiﬁ cial source matched in the 3-5 ȝm band, under the condition of limiting the heating of the surface.The set up for measurements (sketched in ﬁ gure 1) is simple: image acqui-sition is performed in reﬂ ection geometry, in broadband and wide-ﬁ eld mode. Anyway, thermal cameras working in the mid-infrared range (mainly based on cooled MCT or InSb array sensors), are still very expensive compared to near and far-infrared (bolometric) devices. A suitable source provides uniform irradiation over a “large” area, (e.g. ﬁ eld of view of 1 sqmt) for ﬁ eld measu-rements. Infrared lamps controlled in power with the spectrum matched to the mid-infrared quasi-reﬂ ection range can be obtained starting from quartz elements (carbon, tungsten or alloy ﬁ lament) and a suitable ﬁ ltering infrared window (e.g. sapphire); halogens are peaked in the near-infrared, but also provide a quote of mid-infrared.1401Built Heritage 2013 Monitoring Conservation ManagementFig.1 - Measurement setup for TQR: mid-infrared camera, infrared source controlled in power, real-time image acquisition and processing. The detail outlines the layered structure of fresco: pictorial layers, plaster, and support2.2. Diagnostic results on frescoesA painting on wall, or fresco, can be described as a layered structure (see ﬁ gure 1). The wall support is primed with plaster layers, which serve as a base for the painting; in a traditional fresco, the pigment is dissolved in water and then applied on the wet plaster leading then to the unique carbonated layer. Traditional reﬂ ectography, based on near-infrared, is not very effective on mural paintings, although some information could be retrieved as, in many cases, frescos have integration and retouches in “secco” technique (separate paint layers with binder). The TQR technique, based on mid-infrared, has a big potential in the diagnostics of such complex artworks, as discussed below.The ﬁ gure 2 compares mid-infrared imaging in reﬂ ection mode, i.e. TQR, and in emission mode (active thermography), showing that TQR imaging is stron-gly related to superﬁ cial features, while thermography detects subsurface de-fects (blue arrow).The TQR technique has been experimented on both target samples and real artworks thanks to the collaboration with the Opiﬁ cio delle Pietre Dure (OPD), Florence.The ﬁ gure 3 shows the application of TQR to a fresco model (a copy from old masters constructed in 1930, based on traditional material and technique) and Fig.2 - Mid-infrared diagnostics of lime plaster sample: reﬂ ection mode TQR (left) and emission mode thermography (right). The blu arrow indicates a subsurface defect detected in the emission image and not in the reﬂ ectogram. A void volume on the opposite side of the sample creates the subsurface discontinuity while the main surface is layered with Vinavil1402Built Heritage 2013 Monitoring Conservation Managementthe comparison with the traditional techniques reﬂ ectography and thermogra-phy. Mid-infrared maps the red pigment cinnabar (mouth), which is highly re-ﬂ ective, and some features of the surface layers (skin, hat, and background) that are not detected in the near-infrared. As expected, far-infrared thermo-graphy provides a clear detection of the deep texture of the support together with an inner void defect at plaster level (near the shoulder). The detail in ﬁ gure 3 compares the mid-infrared image and the reﬂ ectogram acquired with multispectral scanner at long near-infrared wavelenghts (2.2 ȝm) and shows the complementary information related to different penetration and reﬂ ectan-ce response in the two bands. TQR in the mid-infrared detects the material response in the more superﬁ cial fresco layer (background area) and drawing traces (face contour), near-infrared detects the response in deeper painting layer and maps underneath drawing traces.TQR information is related to the materials in the surface layer. Differences in the reﬂ ectance response allow the discrimination of some materials such as high reﬂ ective pigments. Mid-infrared spectrum carries back information related to the absorption bands of pictorial matter. The low reﬂ ectance is pro-bably due to the absorption features of the organics materials, however an interpretation based exclusively on the absorption bands could be misleading. The mid-infrared reﬂ ectance is determined by factors related to the material composition, to the density and microstructure, and to the surface roughness. Figure 3. Analysis of a fresco model (OPD laboratories). From the left: VIS image, near-infrared reﬂ ectography (1m Si CCD), TQR image (3-5m), far-infrared thermography (thermogram after heating pulse). The detail: mid-infrared TQR (left), near-infrared scanner reﬂ ectography (2.2 m)1403Built Heritage 2013 Monitoring Conservation ManagementAs such, TQR has the potential to detect the presence of different materials and binders, and to discriminate the response of different surface layers. Pictorial materials created, layered, and ﬁ nished with different techniques exhibit different reﬂ ectance responses. A systematic study of the mid-infrared reﬂ ectance is being carried out on a set of controlled samples of different ma-terials (pigments and binders) and execution techniques, constructed at the Wall Painting Laboratory of the OPD (ﬁ gure 4). The TQR technique is effective in the study of surface decay. As example of this application, a laboratory study on controlled samples is shown (ﬁ gure 5), where the technique is applied to map the sulphated areas in the fresco surfa-ce. A brush was used to apply a 2% solution of Sulphuric acid in a sample of lime plaster. Neither in the visible spectrum nor in the near-infrared the area where the solution has been applied is observable, but in the 3-5 ȝm region the area attacked with the acid solution exhibits higher reﬂ ectance. This be-havior is due to the different physical and chemical aspects of the material surface involved. The gypsum created after the sulphatation probably has a lower absorbance compared with lime plaster, but some reﬂ ectance incre-ment could be related to the different morphology of the sulphated surface.On real artworks, the TQR technique was effective for the selective mapping of painting materials, e.g. high reﬂ ective pigments as cinnabar or green earth glauconite, ﬁ nishing touches, subtle traces in bianco di San Giovanni, and a clear detection of different-aged organic painting integrations. Different exe-cution techniques, e.g. fresco or tempera, also exhibit different behaviors. Ap-plication of the TQR to the masterpiece “Resurrezione” by Piero della France-sca, Museo Civico of Sansepolcro, can be found in the [Daffara et al., 2012].Fig.4 -TQR applied to fresco samples (OPD laboratories). Different kind of lapis lazuli pigments (columns) layered with different fresco techniques (rows). Reﬂ ectance in the mid-infrared varies with the combinations of pigment-binder, the technique, and the surface roughnessFig.5 - TQR diagnostics applied to a sample of lime plaster attacked by sulphate decay. From the left: VIS image, near-infrared reﬂ ectography (PtSi 1.2-2.5 m), TQR (3-5 m). Red arrows indica-te the sulphated areas, which are detected only by TQR technique1404Built Heritage 2013 Monitoring Conservation Management3. ConclusionThermal Quasi-Reﬂ ectography is a new diagnostic technique, which is de-monstrated to have a great potential in the diagnostics of frescoes. The results obtained strongly encourage further developments and applications on ﬁ eld. Based on the recording of mid-infrared radiation reﬂ ected from the surface, TQR is well suited to the differentiation of surface materials. It can provide pigments differentiation and identiﬁ cation of repainting and retouching. TQR can be used as a ﬁ rst screening for a more effective sampling with point tech-niques, e.g. spectroscopy in the mid-infrared range. Following the approach of imagery integration, TQR can be integrated with the other imaging techniques for a multi-layer representation of the artwork and a more comprehensive dia-gnostic documentation.An interdisciplinary effort, involving conservators and scientists, is needed. Thanks to the collaboration with the Opiﬁ cio delle Pietre Dure, the TQR me-thod is being tested both on controlled samples and fresco models as well as on important artworks. While conservators should test TQR on real artworks in situ, scientists should optimize the performance in laboratory: from the optical acquisition chain (crucial is the mid-infrared source), to the reﬂ ectance data interpretation (crucial is the correlation of TQR with spectroscopy in the 3-5 ȝm) in order to stretch the imaging technique towards materials mapping.AcknowledgementsA special thanks is due to Cecilia Frosinini, Director of the Wall Paintings Department of the Opiﬁ cio delle Pietre Dure (Florence) for the proﬁ table collaboration and her pre-cious suggestions. References Aa.Vv., 2005, Scientifi c examination of art, Proc. National Academy of Science, Wa-shington D.C.Ambrosini D., Daffara C., Di Biase R., Paoletti D., Pezzati L., Bellucci R., Bettini F., 2010, Integrated refl ectography and thermography for wooden paintings diagnostics, «Journal of Cultural Heritage» 11, 196-204.Daffara C., Fontana R., 2011, Multispectral infrared refl ectography to differentiate fea-tures in paintings, «Microscopy and Microanalysis» 14, 691-695.Daffara C., Ambrosini D., Pezzati L., Paoletti D., 2012, Thermal quasi-refl ectography: a new imaging tool in art conservation, «Optics Express» 20, 14746-14753.Delaney J.K., Zeibel J.G., Thoury M., Littleton R., Palmer M., Morales K.M., René de la Rie, Hoenigswald, 2010, Visible and Infrared Imaging Spectroscopy of Picasso’s Harlequin Musician: Mapping and Identifi cation of Artist Materials in Situ, «Applied Spectroscopy» 64, 584-594.Ibarra-Castanedo C., Sfarra S., Ambrosini D., Paoletti D., Bendada A., Maldague X., 2008, Subsurface defect characterization in artworks by quantitative PPT and hologra-phic interferometry, «Quantitative Infrared Thermography Journal» 5, 131-149.Van Asperen de Boer J. R. J., 1968, Infrared Refl ectography: a Method for the Exami-nation of Paintings, Applied Optics 7, 1711-1714.

Daffara, Claudia

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The challenge of infrared imaging of frescos: Thermal Quasi-Reflectography unveils hidden features of artworks

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