New methodological approaches in Reflectance Transformation Imaging applications for conservation documentation of cultural heritage metal objects

Imaging techniques, along with their subsequent processing and analysis, are of utmost importance in the visual documentation of conservation processes of cultural heritage (CH) objects. Amongst them, Reflectance Transformation Imaging (RTI) is being used as a tool for the enhancement of surface topography, such as decorative details. This paper proposes a new approach based on advanced RTI data processing and analysis to document the condition of metal artefacts or monitor their conservation treatments. First, the methodology for mapping geometric and statistical information from the stack of RTI image data and their relation to the surface topography and the appearance attributes resulting in feature maps is described. Additionally, the possibility of quantifying intra- or inter-surface changes based on saliency and distance measurements by applying the Mahalanobis distance (MD) on feature maps is demonstrated. This methodology is then used for documenting the condition and monitoring the cleaning treatment of a late Roman coin.