Reflectance Transformation Imaging for Detecting Knots in Heritage Timbers

Abstract. Reflectance Transformation Imaging (RTI) is a common technique used in different cultural heritage applications. Images of an object or a part of an object are captured using a fixed camera position but changing lighting directions. With this technique small details can be captured, revealed by their shadows, even if the camera resolution would not resolve them. In this contribution we used RTI to support the detection of wooden knots in heritage timbers. We developed a handheld low-cost RTI dome that fits to create RTI models of wooden beams. To locate the knots in the context of their surroundings, 3D models were created via Structure from Motion (SfM) based on the RTI images. We used three different subsamples of the RTI images to optimize our workflow and analyse the possibilities of using different illumination setups. The models then were compared with a model that was created using common camera equipment. The models were compared based on geometry and colour. Our two case studies are addressed to a historic roof framework in a church in Bamberg (Germany) and, in addition, to more accessible newer wooden beams.