Paper watermark imaging using electron and low energy x-ray radiography

Historians and librarians are interested in watermarks and mould surface patterns in historic papers, because they represent the “fingerprints” of antique papers. However, these features are usually covered or hidden by printing, writing or other media. Different techniques have been developed to extract the watermarks in the paper while avoiding interference from media on the paper. Beta radiography provides good results, but this method cannot be widely used because of radiation safety regulations and the long exposure times required due to weak isotope sources employed. In this work, two promising methods are compared which can be used to extract digital high-resolution images for paper watermarks and these are electron radiography and low energy X-ray radiography. For electron radiography a “sandwich” of a lead sheet, the paper object, and a film in a dark cassette, is formed and it is exposed at higher X-ray potentials (\u3e 300 kV). The photoelectrons escaping from the lead sheet penetrate the paper and expose the film. After development, the film captures the watermark and mould surface pattern images for the paper being investigated. These images are then digitized using an X-ray film digitizer. The film employed could potentially be replaced by a special type of imaging plate with a very thin protection layer to directly generate digital images using computed radiography (CR). For the second method, a low energy X-ray source is used with the specimen paper placed on a digital detector array (DDA). This method directly generates a low energy digital radiography (DR) image. Both methods provide high quality images without interference from the printing media, and provide the potential to generate a “fingerprint” database for historical papers. There were nevertheless found to be differences in the images obtained using the two methods. The second method, using a low energy X-ray source, has the potential to be integrated in a portable device with a small footprint incorporating user safety requirements. Differences obtained using the two methods are shown and discussed

Paper-Based Watermark Extraction with Image Processing

This thesis presents frameworks for the digitisation, localisation, extraction and graphical representation of paper-based watermark designs embedded in paper texture. There is a growing need for this among librarians and antiquarians to aid with identification, wider accessibility, and providing a further level of document imaging for preservation. The proposed approaches are designed to handle manuscripts with interference such as recto and verso writing, and defects such as non-uniform paper structure, physical damage, etc. A back-lighting scanning technique is used for capturing images of paper, followed by a selection of intelligent image processing operations, rather than alternatives such as radioactive techniques. This technique requires low cost equipment, and produces a fast and safe solution to capturing all details on paper, including watermarks, and laid and chain lines patterns. Two approaches are presented: the first takes a bottom-up approach and deploys image processing operations to enhance, filter, and extract the watermark, and convert it into a graphical representation. These operations determine a suitable configuration of parameters to allow optimal content processing, in addition to the detection and extraction of chain lines. The second approach uses a model of the back-lighting effect to locate a watermark in pages of archaic documents. It removes recto information, and highlights remaining ‘hidden’ data, and then presents a statistical approach to locate watermarks from a known lexicon. Work is further presented on reconstructing features of the paper mould by aggregating the success of the foregoing steps: this permits an analysis of ‘twin’ watermarks. Results are presented from comprehensively scanned eighteenth and nineteenth century manuscripts, including two unusual copies of the Quran, an Islamic Prayer, and various historical documents