Congresso del Dipartimento di Fisica Highlights in Physics 2005

1. 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. Bertani*and L. Consolandi† * INOA-CNR 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. 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. 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 fixed pattern noise is also detectable plotting the profiles of a line at different levels of uniform illumination 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. 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. 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.