New high resolution IR-colour reflectography scanner
for painting diagnosis
Raffaella Fontanaa, Maria Chiara Gambinoa, Marinella Grecoa, Luciano Marrasa, Marzia Materazzi∗a,
Enrico Pampalonia, Luca Pezzatia, Pasquale Poggia
a
Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6, 50125 Firenze, Italia
ABSTRACT
Infrared reflectography is a prominent optical technique for non-destructive diagnostics of paintings, which allows the
visualisation of details hidden by the paint layers, because of their transparency characteristics to IR radiation. Highresolution reflectography was introduced around the end of the 80s by the Istituto Nazionale di Ottica Applicata, where a
prototype of an innovative scanner device was built. This technique was recently improved with the introduction of a
new optical head, able to acquire simultaneously the reflectogram and the colour image, perfectly superimposing. The
technical characteristics of the IR-colour scanner guarantees: high spatial resolution (16 points/mm2), high tonal
dynamics (thousands of grey levels), uniform lighting of the scanned area, high sensitivity and punctual superimposition
of the colour and IR images. Moreover we can print distortion-free reflectograms and colour images on scale of 1:1. The
quality of the acquired reflectogram is presently greatly higher than that obtainable with any traditional detection system,
like CCD or Vidicon cameras. The point-by-point comparison between the reflectogram and the colour image of the
painting, along with digital processing of the recorded images, open new possibilities for the analysis of the
reflectogram. Some examples of application of this device to the study of ancient paintings are shown.
Keywords: infrared reflectography, scanner device, non destructive testing , imaging.
1.
INTRODUCTION
Infrared reflectography is one of the most suitable optical techniques traditionally employed in non destructive
diagnostics of ancient paintings1. Applied to panel, canvas and wall paintings, this method of investigation allows to
reveal features underlying the pictorial layer thanks to transparency characteristics to NIR radiation (0,8-2 µm) of the
materials which composed its.
Reflectography allows the revelation of pictorial layers hidden by superimposed ones, due to the artist will or to
subsequent restorations, as well as the visualization of the under-drawing, executed by the artist on the preparation before
laying the colours. If the preparation under the pictorial layer is characterized by an high reflectivity in the NIR spectral
range (a typical case is chalk-and-gypsum-based preparations), the reflectogram can be obtained by acquiring with a
suitable device the image of the back-scattered radiation. The visibility of the underlying features depends on the paint
layers thickness and on the chemical composition of the materials which form the under-drawing and the pictorial layer.
The paints transparency generally increases with the radiation wavelength, and for wavelength of 1.6-1,7 µm and typical
thickness of about 0,1mm, nearly all the pigmented compounds are at least partially transparent.
The detection of underlying pictorial layer is allowed by the different transparency characteristics of the pigments to the
radiation incident and back scattered by preparation. The visibility of the under-drawing essentially depends on the
contrast between the radiation reflected by the preparation and that one absorbed by the drawing itself. It is maximised
when the former is traced out with carbonaceous pencils or inks on a gypsum ground. On the contrary, the drawing may
not be visible when it is realized with iron-gallic inks, which are nearly transparent to IR radiation.
∗
[email protected]; phone +390552308238; fax +390552337755; www.arte.ino.it
This technique gives precious indications both on the realization phases and on the state of conservation of the artwork.
Furthermore it helps the historical placing of the painting, and in some cases it can confirm or deny the attribution to an
artist2.
Infrared reflectography dates back to approximately 30 years ago and is based upon the fundamental work of Van
Asperen de Boer, who laid the theoretical and experimental bases for this technique, and introduced the use of PbS
Vidicon cameras 1,3. The use of Vidicon detectors allowed to analyse underlying features of the paintings in a more
effective way than the up to date used IR films. Photographic films, sensitive in the 0.7-0.9 µm spectral range, produce
poor quality images because radiation in this range passes through a few kind of paints, most likely reds and whites,
whereas the greens and the blues are nearly opaque. On the contrary, Vidicon tubes work between 0.9 and 2.0 µm, where
nearly all the pigments are transparent.
The most widely used device for reflectography is still today the Vidicon camera. However this detector has scarce light
sensitivity, so that measurements request an intense illumination, which can induce a detrimental warming of the
painting surface. Moreover the images are characterized by a very low contrast due to the limited number (some tens) of
grey levels, and are affected by geometrical distortion, due to the camera lens and to the device intrinsic characteristics.
To obtain a high spatial resolution the measured area must be very small (e.g. 10×10 cm2 to have a resolution of 4×4
pixel/mm2, needed to resolve the finest under-drawing lines). The reproduction of a large panel thus requires the
collection of several images (more than 100 for 1 m2) that are successively combined in a mosaic. Since the single
images are brighter in the centre and darker on the borders for the non-uniform sensitivity of the detector’s area, the
resulting reflectogram generally appears tiled, in many cases even after equalization (Fig.1).
Figure 1: Flemish painting of XVI century, Maddalena, National Gallery, London, UK.. The reflectogram detail is composed by a
mosaic of images acquired with a PbS Vidicon camera.
The use of low-cost CCD detectors does not meaningfully improve the quality of the acquired images: they have a higher
intensity resolution, the geometrical distortion in the image is only due to the camera lens and the image has a much
better uniformity than in Vidicon case. Nevertheless they have a sensitivity limited up to 1.1 µm so that the acquired
images have the same scarce informative content as IR photography. Special highly-priced CCD devices can strongly
improve the visibility of the underlying features. However, a great number of images must still be collected to reproduce
a large painting with high resolution. The mosaic realisation requests long calibration and cleaning operation of the
acquired images, owing to the non-uniform lighting condition and the misalignment of images, which depends on the
precision of the scanning mechanical system and on the distortion at image boundaries.
High-resolution reflectography was developed by the Istituto Nazionale di Ottica Applicata (National Institute of
Applied Optics, in Florence) in about 19904. At that time, an innovative scanning device for imaging in the infrared
spectral range was built, able to acquire reflectograms with spatial resolution and tonal dynamics that are still now
unobtainable by any of the techniques traditionally used for infrared reflectography (Vidicon or CCD camera).
Recently its performances were improved and the scanner can simultaneously acquire high spatial resolution
reflectogram and colour image of a painting, which can be perfectly superimposed one on the other.
In the next sections we describe the IR scanning device and its improved IR-colour version and eventually we present
some interesting applications of this device to the diagnostics of some ancient paintings.
2.
THE SCANNER FOR HIGH RESOLUTION IR REFLECTOGRAPHY
Two orthogonal-mounted, motorized translation stages move the optical head, consisting of the lighting and the detection
systems, scanning a surface of nearly 1m2 with a spatial resolution of 4×4 pixels/mm2 (Fig.2). The acquisition rate is 500
Hz. The safety distance of the optical head from the panting has been set to about 150 mm.
The lighting system is built of two low-voltage halogen lamps which are fixed on the opposite sides of the detector,
being at 45o to the normal to the painting surface and illuminating a small area of nearly 10 cm2. Since the lighting
system moves jointly with the detection system, the surface warming is minimized and the uniform illumination is
assured both during the scanning as well as in subsequent measurements. Illumination stability in time is obtained by
powering the lamps through a current-stabilized generator.
A standard lens with 73,5 cm focal length, gathers the reflected radiation from the scanned point on the painting and
focuses it on the sensitive area of a photodetector. The 2f-2f working configuration of the lens guarantees to have unitary
magnification so that the sampled spot on the painting has an area equal to that of the photodetector. An iris placed
behind the doublet allows to change the working f-number which in most practical cases is set in the 9-16 range.
Figure 2: The scanning device for high resolution IR reflectography : two orthogonal-mounted, motorized translation stages move the
optical head (shown in detail on the bottom), consisting of the lighting and the detection systems, scanning a surface of nearly 1m2
with a spatial resolution of 4×4 pixels/mm2.
The detection system is based upon an InGaAs photodiode with 200mm-diameter sensitive area and spectral response in
the 0.9-1.7 µm spectral region. A black filter cuts the wavelength lower than 1µm. Spectral sensitivity up to so long
wavelength determinates a substantial visibility under most of materials which forms the colour layer. Moreover, thanks
to the wide sensitivity spectral range, the reflectogram is a superposition of the different painting layers response also if
without regard for the position of the layer within the paint structure.
The use of a single sensitive element, instead of spatially-extended sensors, such as matrices or arrays, solves both the
problem of non-uniform lighting of the sampled area as well as that of finding aberration-free lenses for NIR spectral
region and it avoids the geometric deformations introduced by the traditional detection systems. The high accuracy of the
translation stages jointly with the limited field depth of the device contribute further to the acquisition of metrically
correct images. The accuracy depends on painting intrinsic characteristics (principally the paint warping) but in practical
cases it’s always higher than 0,1 mm. The capability to acquire distortion free images is of basic importance for metric
comparison between the reflectogram of an artwork and that of other works by the same artist or his contemporaries 2,5.
Figure 3: Top part on the left: Flemish painting of XVI century, Maddalena, National Gallery, London, UK, a detail of the
reflectogram acquired with a PbS Vidicon camera. Top part on the right: Pontormo, Madonna con bambino e S. Giovannino, Galleria
degli Uffizi, Firenze, Italy, a detail of the reflectogram acquired with a CCD camera. On the bottom part are shown the same details
acquired with the INOA scanner. In the case of Vidicon detection the visibility of the under-drawing is compromised by the limited
tonal dynamics of the detector; on the other hand the limited performance of the CCD camera is due to the spectral range sensitivity
limited to 1,1 µm.
The output signal of the photodetector is processed by a 12 bit A/D converter, which permits a tonal dynamics of
thousands of grey levels. The digitised data are recorded and handled via a purpose-developed software running on a
standard PC. The reflectogram is stored in a standard image format and can be printed on the scale of 1:1 with the
original artwork.
A comparison between the results obtained with the scanner device and both the traditional techniques is presented in
Fig.2: on the top part are shown a detail of a Flemish painting’s reflectography (on the left), acquired by a PbS Vidicon
camera, and a detail of a Pontormo’s painting reflectography (on the right), acquired with a CCD camera; on the bottom
part are shown the same details acquired with the INOA scanner device.
3.
THE NEW OPTICAL HEAD
Recently, a new optical head was developed which allows the simultaneous acquisition of the reflectogram and the
colour image of the painting, perfectly superimposing, without increasing the acquisition time.
Figure 4: A schematic representation of the recently developed new optical head. An achromatic doublet focuses the radiation,
reflected by the scanned spot on the painting, on the terminations of four optical fibres. An iris placed behind the doublet allows to
change the working f-number. The detection system is based upon three suitably filtered miniaturized photomultipliers for acquisition
in the red, green and blue bands and an InGaAs photodiode, with spectral response between 0.9 and 1.7 µm. A black filter before the
photodiode cuts the wavelengths lower than 1µm. The fibre terminations are mounted in a purpose-built support that keeps the coreto-core distance of 250 µm. The terminations are fixed at a slightly different distance from the doublet, depending on the effective
focal length associated with the carried wavelengths.
A standard achromatic doublet, with f-number equal to 4.6, focuses the radiation, reflected by the scanned spot on the
painting, on the terminations of four optical fibres, each coupled with a photodetector (Fig.4). Also in this case an iris
placed behind the doublet allows to change the working f-number which in most practical cases is set in the 9-16 range.
The detection system is based upon three miniaturized photomultipliers, suitably filtered for acquisition in the red (R),
green (G) and blue (B) bands (Kodak wratten colours gelatin filters n. 25, n. 58, n. 47B), and an InGaAs photodiode,
with spectral response between 0.9 and 1.7 µm (mod. H5784, Hamamatsu). A black filter before the photodiode cuts the
wavelength lower than 1µm.
The doublet is characterized by an 85% transmittance in the IR range of interest. The IR axial chromatic aberration
(1mm for wavelength between 900 nm and 1600 nm) is negligible because of the image pixel dimensions (250 µm side).
The 2f-2f working configuration of the doublet guarantees to have unitary magnification so that the sampled spots on the
painting have an area equal to that of the optics fibres which have a core diameter of 200 µm .
The fibre terminations are mounted in a purpose-built support that keeps the core-to-core distance equal to the sampling
step (250 µm). The terminations are also fixed to the support at a slightly different distance from the doublet, depending
on the effective focal length associated with the carried wavelengths.
Figure 5: Raffaello, Madonna del Cardellino, Galleria degli Uffizi, Firenze, Italy. Actually under restoration at Opificio delle Pietre
Dure, Firenze. The four details show how acquired images could be digitally composed and processed to obtain differently aimed
reproductions of the artwork. The first two squares represent details of the superposition of the reflectogram and the colour image
which show the correspondence between the final aspect of the artwork and the different techniques adopted for realizing the underdrawing. The second two squares show two details of the false-colours image, a technique which is used for the non destructive
discrimination of indistinguishable-at-sight pigments.
Figure 6: Bronzino, S. Giovanni Battista, Galleria Borghese, Roma, Italy. The reflectogram has revealed a consistent pictorial trace of
a Medicean gentleman, whose contours are pointed out in the figure, under the visible painted layer. The point by point superposition
of the colour image and the reflectogram permits to perfectly compare the definitive version of the painting with the under-painted
figure and makes easier to discriminates one from the other.
The fibres which guide the radiation to the photomultipliers can be set out of axis with respect to the doublet. In fact the introduced
field negligibly degrades the performances of the doublet.
The output signal of the four photodetectors is processed by a 12 bit A/D converter, corresponding to a tonal dynamics of
thousands of grey levels for each acquired image, with an accuracy of a few grey levels . The digitised data are recorded
and handled via a dedicated software running on a standard PC. The four monochromatic images are stored in a standard
image format by using another specifically developed software. Storing the images in the 8 bit standard format it can be
chosen both to convert all the grey scale, thus compressing the intensity resolution, and to convert only a limited part of
the gray scale to have the highest intensity resolution in a certain part of the reflectogram.
Due to the optical system configuration the acquired images are perfectly superimposing with a simple translation of 250
µm.
4.
APPLICATION
The colour image of the artwork is obtained by the superposition of the images recorded in the primary colours R, G, B,
bands by using any commercial graphic software. The colour images acquired with photographic methods have two main
problems: the geometric deformation (optical and perspective) and an insufficient resolution with respect to the
reflectogram. The geometrical distortions do not allow for a perfect superposition of the colour and IR images, and the
lack in resolution can hinder the analysis of fine details in the reflectogram, that could not be seen in the colour image.
On the contrary the high spatial and tonal resolution of the reflectogram and the colour image acquired with the scanner,
jointly with their pixel by pixel comparison, permit the individuation of the finest under-drawing details and favour the
understanding of the correlation between the under-drawing and the final pictorial results, allowing restorers and art
historians to analyse with more ease the different phases of the painting realization from the original idea to the definitive
version. Moreover the colour image is a high resolution, chromatically faithful reproduction of the artwork which is
conserved in a digital format and can be printed on scale of 1:1 with the original artwork.
The acquired images could be digitally composed and processed to obtain differently aimed reproductions of the artwork.
Figure 5 shows some examples of digital processing of the measurements carried out on the Madonna del Cardellino
painted by Raffaello. The first two windows represent details of the superposition of the reflectogram and the colour
image which show the correspondence between the final results and the different techniques adopted by Raffaello for
realizing the under-drawing: the draw of the head of S.Giovannino and of the dress of the Virgin Maria has been
executed by pounce technique, whereas the draw of the landscape behind the Virgin’s shoulders appears as quickly freehand traced. The second two windows show two details of the false-colours image6, obtained by the superposition of the
infrared, red and green band images treated as they were the red, green and blue band images respectively. The falsecolours technique is frequently used for the discrimination of indistinguishable-at-sight pigments. The false colours
brilliant red appearance of the Virgin’s blue mantle (third window) reveals that it has been painted with lapis lazuli. In
the fourth window the false-colours rendering permits to exactly identify the XIV century pictorial integration of
Virgin’s mantle (on the left of the S. Giovannino’s foot): it looks in fact differently coloured with respect to the lapis
lazuli painted areas, being executed with azurite.
The analysis of the superimposed infrared and colour images makes also easier the individuation of painting layers lying
under the artwork surface, revealing changes between the original purpose and the artwork final aspect. In some cases
the artist himself changed and re-painted either some details or the whole painting, in other cases some corrections, due
to restoration or evolution of taste, were introduced by a different painter.
In Figure 5 is shown the main result obtained by scanning with the new optical head the S. Giovanni Battista painted by
Bronzino. The reflectogram has revealed a consistent pictorial trace of a Medicean gentleman under the visible painted
layer: the collar trimmed with lace, the left hand holding some sheets and the right hand which hold a pen are clearly
visible and pointed out in figure. The point by point superposition of the colour image and the reflectogram permits to
perfectly compare the definitive version of the painting with the under-painted figure and makes easier to discriminate
one from the other.
5.
CONCLUSION
The realisation of the INOA scanning device for infrared imaging greatly improved reflectography technique. In fact the
image quality obtained by this device is still now not achieved with traditional techniques based on CCD or Vidicon
camera acquisitions. The high spatial and tonal resolution of the acquired reflectogram permits to discriminate the finest
details of the under-drawing and if any under-paintings. Moreover the capability to acquire distortion free infrared
images is of basic importance for metric comparison between the reflectogram of an artwork and that of other works by
the same artist or his contemporaries.
The recent realization of a new optical head, which simultaneously acquires the reflectogram and the colour image, at the
same spatial resolution and without increasing the time of acquisition, is a further, important evolution of this technique.
The perfect superposition of the reflectogram and the colour image greatly increases the interpretation of the
reflectography results thanks to the point by point comparison with the colour image of the artwork and to other possible
digital processing of the recorded images. It helps to individuate the hidden particulars and the different realization
phases of the artwork, it makes easier to discriminates if any under-painted layers from the definitive version of the
painting and the relations between the underdrawing and the final pictorial realization. Finally, the colour image is a
high-resolution, chromatically faithful reproduction of the painting, stored in magnetic or optical support, which can be
printed on scale of 1:1 with the original artwork.
At present we are working on the development of a new optical head for multi-spectral analysis of paintings to be
integrated to the presented scanning system. The device will be able to simultaneously acquire high spatial resolution,
metrically correct, multi-spectral images of a painted surface in the 380 nm-2200 nm spectral range. It will also
simultaneously acquire the shape of the surface.
AKNOWLEDGEMENTS
The development of the new optical head has been carried out in the context of the RIS+ Toscana Project (art.10 FESRFondo Europeo di Sviluppo Regionale) in collaboration with Falcon Instruments s.r.l. (Firenze) and Opificio delle Pietre
Dure (Firenze). We are indebted to Roberto Bellucci and Cecilia Frosinini (Opificio delle Pietre Dure) for daily giving us
the possibility of a constant comparison with the real problems of artwork conservation.
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