On the surface and beyond. An new approach with Multispectral Photometric Stereo to assess Illuminated Manuscripts and their condition

摘要

MAGING WITH PHOTOMETRIC STEREO FOR MANUSCRIPT & CONSERVATION RESEARCH To document surface characteristics of illuminations, supports and pictorial layers, a Microdome was developed in at Leuven University (Belgium). A camera of 28 million pixels is digitizing the objects with omnimulti-directional lighting and export the result to 2D+. The technique is based on polynomial texture mapping, or Reflectance Transformation Imaging (RTI), a method of imaging and interactively displaying objects under varying lighting conditions to reveal surface phenomena. The underlying processing is based on the extraction of surface characteristics using methodologies such as photometric stereo and BRDF analysis (Bidirectional Reflectance Distribution Function). The module is a hemi-spherical structure with a single downward looking video camera. The object to be captured (maximum 180 to 120 mm) lies in the center and is illuminated from computer-controllable lighting directions, through the subsequent activation of multiple white LEDs (260). The different angles that illuminate the surface of the artifacts are revealing extreme details. The Microdome has a diameter of 30 cm. The Microdome is used for examination and identification of the production of scripts, miniatures, gold tooling, embossing, stamps on paper and parchment, For changes in their structures (ea. cockling, tractions, folds, lacunas, retouching, scratches, abrasion) the imaging tool proved to be an extremely accurate. Complementary, RICH is useful to monitor and measure the conservation and preservation status of an object before and after treatment, transport or exhibition. The implementation of a scaling and measuring tool with grid (1 mm) in the software enables the researcher – conservator to export graphically the dimensions and changes of topographic characteristics until 10 microns. Dome for digitizing with omnimulti-directional lighting and export the result to 2D+ For book conservation reasons, a slice of the Microdome is removed, allowing the tool to monitor in 150 degrees. The micro-dome is mounted on a special designed book conservation cradle designed by the university of Graz in Austria, so imaging can be done inside fragile manuscripts with delicate historic bindings, without risk to stress or damage. Light suction on a bar is flattening gently the paper or parchment folio's during the four minutes of imaging. The shape of the micro-dome allows to visualize into the gathering fold.