A new scanner for in-situ digital radiography of paintings

X-ray radiography is one of the most widely used imaging techniques in the field of Cultural Heritage, both for conservation and investigation purposes. Performing radiographies in museums, thus avoiding movements of works of art, has been recently made easy by digital acquisition of images, but when the whole scan of a large painting is required, technical solutions for a portable device are still not at hand. The inherent weight of the X-ray tube, including protection shields, and of the high voltage generator makes the design of a “portable” device very difficult. In this project the solution of the puzzle was separating devices devoted to different tasks, in order to maintain each item under 50 kg weight, thus being transportable with minimum effort. The proper RX scanner is composed of two independent frames (Fig.1) in aluminium, carrying and moving the X-ray tube and the digital detector. For both units, a couple of DC servomotors (G5© by Omron Electronics Ltd, UK) are driving the movements on X and Y axis. A third frame is added, to support the painting. The distance between the two units is usually set at 1 m and the work of art is located as close as possible to the detector, to minimize magnification and penumbra effects. Both units may be moved or installed on a scaffold, to scan wider paintings. Levelling of the frames and alignment of X-ray beam with detector is performed by setting the levelling feet, collimating the beam with a small pinhole. Once aligned the units, the scan is automatically performed by remote control of motion, on horizontal and vertical axis, by dedicated software (NS-Runtime© rel.1.2 by Omron Co.). X-ray tube and detector move simultaneously and stop at the foreseen position for image acquisition. X-ray shot and digital acquisition follow, then the following position is reached. The scan is tailored to each painting, and the digital images are automatically stitched at the end of the scan. For instance, a painting of size 1 x 1 m2 requires 144 shots and about 3-hour scan. The stitching is a matter of few minutes. The X-ray tube is a Varian M-143T with tungsten anode, maximum anode voltage 49 kV, air cooled. The detector is a Teledyne DALSA (Canada) RadEye200, a two dimensional CMOS photodiode array combined with a Gd2O2S scintillator screen. It is composed of 1024 x 1000 pixel, of side length 96 µm. Depth of digitization is 12 bit/pixel. The scanning system has been designed, realized and tested. It has proved to recover each position in the scan within one pixel width and to be stable during all acquisitions.