Bridging multi-light & multi-spectral images to study, preserve and disseminate archival documents

Producing relevant photographic records in collections with unique and often fragile heritage objects is a serious challenge. Combining both visual and analytic information into such images is a major asset. Over the last decade the development of the Leuven University Portable Light Dome (PLD) has produced numerous such complex datasets. Its outcome enables different visualizations and analyses on one and the same multi-light and/or multi-spectral dataset. These interactive images support various types of research questions and contain many facets of information (reflectance characteristics, surface orientations, multi-spectral). Compared to normal photography they contain much more layered information on the archived objects. Compared to other multi-light reflectance imaging solutions such as RTI, the imaging protocol of the PLD system is able to disseminate its outcome in a multi-modal manner, beyond the visual aspects of the imaged surface. Developing a suitable acquisition and dissemination method: The WL and MS Portable Light Dome Systems As the imaging-needs and demands of curators and researchers can differ, and in particular cases even conflict, several types of visual representations of heritage objects are produced. Therefore, the more information one imaging-routine can record the more diverse questions can be met; ideally providing data sets allowing interactive engagement with the object and thus opening a spectrum of potential researchor conservation-driven actions. The photographic records and data sets created with the Portable Light Dome (PLD) system, developed at the KU Leuven, provide a suitable strategy to satisfy these needs of researchers, curators and conservation scientists. Since 15 years the PLD system was used for the digitization and imaging of large, often isolated and very diverse heritage collections. The White Light (WL) PLD acquisition system was initially developed for the accurate reconstruction of the surface orientation of 3rd to 1st millennium BCE cuneiform archives and seal impressions. The viewer, with its custom-made and fine-tuned shaders, provided results supporting research; allowing genuine digital preservation of these extreme fragile artifacts and facilitated the publication of the content through the integrated automated visualization options [1][2][3]. By 2012, within the RICH project, the WL PLD hardware was adapted and miniaturized for the safe monitoring of documentary heritage objects [4][5][6][7]. These new developments enabled high-end, high-resolution analyses of the topography and materiality of book, paper and parchment heritage objects, all based on the reflective characteristics and estimated surface orientations, inherent to principles of photometric stereo applied on the acquired data sets with the multi-light reflectance system of the PLD (for results see also infra). Figure 1. The MS PLD system recording an illuminated manuscript, with one of the Green 523nm LEDs in action (© Imaging Lab KU Leuven). Joining the efforts in 2014, the RICH and EES projects opened up new paths for the research with this technology. Within this framework multi-light reflectance and multi-spectral imaging techniques were merged by integrating 5 different spectral bands in the acquisition dome. This has increased individually and conjointly the possible interpretation models vastly [8][9]. Parallel work has demonstrated the potential of this approach [10][11] and since then a number of case studies powered with by RTI tools on archaeological artifacts [12], painted surfaces [13] and papyri [14] have been published. This MS PLD system (Figure 1) enables the study of the imaged surfaces from multiple perspectives. The MS reflective responses of each registered point (pixel) can be compared 1:1 with surface orientations calculated through distinct data sets derived from IR (850nm), Red (623nm), Green (523nm), Blue (460) and UV (365nm) spectral bands. This approach enables the measurement in three dimensions of the topography of objects in a 2D+ environment (Figure 2). In addition to that, the same (MS) datasets allow the photometric stereo algorithm with the reflectance response estimations for every recorded pixel analyzing the materiality of the imaged surface [9]. Throughout the development phases of the different PLD acquisition modules and software solutions the strategy remained that the entire registration and processing routine needed to be as easy as possible, where a good and balanced time / quality ratio was pursued. An important incentive to achieve this goal is the automated all-in-one approach; once the surface has been 64 SOCIETY FOR IMAGING SCIENCE AND TECHNOLOGY This work is licensed under the Creative Commons Attribution 4.0 International License.