Congresso del Dipartimento di Fisica
Highlights in Physics 2005
11–14 October 2005, Dipartimento di Fisica, Università di Milano
Imaging techniques for the analysis of Cultural Heritage
D.
* INOA-CNR
*
Bertani
and L.
†
Consolandi
e Centro di Riflettografia Infrarossa e Diagnostica dei Beni Culturali-Università di Milano
† Istituto di Fisica Generale Applicata, Università di Milano
Digital imaging techniques are more and more used in the field of works of art for
study and documentation purposes. High resolution is always a must because the
recording of small details is always requested along with the reproduction of a
large colour gamut or of a large number of grey tones in the case of black and
white images. This implies that in many research fields, such as the examination
and reproduction of ancient paintings, parchments and papyri in the visible and in
different bands of the near infrared, all the process from recording down to printing
has to be considered as a whole. Here the problem of high quality black and white
and colour recording and reproduction is far from resolved also because the
request is that the final results be printed in real size.
Infrared examination of ancient paintings
To examine ancient paintings, infrared reflectography is
used to store images in the near infrared between 1 and 2
micron because the radiation of this wavelength penetrates
the pigment layers and makes visible the underdrawing that
was sketched on the gypsum preparatory layer. In a
reflectogram one wants to detect all the shades in the
drawing, its thinnest lines and the craquelure of the paint [1].
Because no high resolution devices are commercially
available, we designed a prototype which is based on an
InGaAs CCD focal plane array (FPA) that automatically
scans the image plane of a lens for large format
photography [2]. The reflectogram stored is the mosaic of a
large number of images, typically a few hundreds.
The FPA has 40x40 micron elements: this implies very strict
constraints to the mechanical design for what concerns the
perpendicularity and precision of the X-Y translation axes
and the attachment of the FPA to the moving Y stage.
Although the CCD is Peltier
cooled to reduce its thermal
noise, its performance is far
from perfect. Images taken at
uniform illumination show
vertical lines, bad pixels and a
remarkable edge effect.
The fixed pattern noise is also detectable plotting the profiles
of a line at different levels of uniform illumination of the CCD.
An other effect to be
compensated is the intensity
fall off at the image boundary
due to the lens. This is
clearly visible in the image of
a uniformly illuminated grey
panel, combined with the
checkerwise pattern due to
the non uniform response of
the CCD.
The performance of the overall correction for the system
defects is evident in the reflectogram of S. Giustina by G.
Bellini (Bagatti Valsecchi Museum - Milan) before and after
compensation.
The fact that the gain of each element of the CCD is different
is shown by the variation of the shape of the image histogram
under different illumination intensities.
An important improvement of this prototype is the possibility
to change the optical magnification at will, according to the
characteristics of the underdrawing. The device is compact,
transportable and much faster than previous systems.
Therefore the system had to be calibrated and the calibration
data are taken into account by the dedicated software that
drives the translation axes, corrects the images, stores them
and assembles the final mosaic.
Infrared examination of papyri
Papyri written with a carbon based ink are examined with
silicon CCD sensors both in the visible and in the near
infrared up to about 1 micron, in order to make legible with
optimum contrast even the faintest remains of badly damaged
texts [3]. High resolution imaging in the near infrared is
particularly effective in making visible the text of "dark papyri".
References
1. Oltre il visibile - indagini riflettografiche, Milano: Hoepli (2001)
2. D. Bertani, L. Consolandi, High resolution imaging in the near
infrared for the examination of paintings and papyrus manuscripts in
Digital Heritage, Elsevier 2005 (in print)
3. Papiri dell'Università degli Studi di Milano - VIII: Posidippo di
Pella, Epigrammi (P.Mil.Vogl. VIII 309), Milano: Il Filarete (2001)
Acknowledgements
The authors are very indebted to prof. C. Gallazzi of Università degli
Studi di Milano, dr R. Visigalli of FESTO Italia s.p.a., dr R. Sommacal
of Hewlett-Packard Italia, dr Lucia Pini of Bagatti Valsecchi Museum
- Milan for their help throughout the work.