Abstract
Severely damaged historical documents are extremely fragile. In many cases, their secrets remain concealed beneath their cover. Recently, non-invasive digitization approaches based on 3-D scanning have demonstrated the ability to recover single pages or letters without the need to open the manuscripts. This can even be achieved using conventional micro-CTs without the need for synchrotron hardware. However, not all manuscripts may be suited for such techniques due to their material and X-ray properties. In order to recommend which manuscripts and which inks are best suited for such a process, we investigate six inks that were commonly used in ancient times: malachite, three types of iron gall, Tyrian purple, and buckthorn. Image contrast is explored over the complete pipeline, from the X-ray CT scan and page extraction to the virtual flattening of the page image. We demonstrate, that all inks containing metallic particles are visible in the output, a decrease of the X-ray energy enhances the readability, and that the visibility highly depends on the X-ray attenuation of the ink’s metallic ingredients and their concentration. Based on these observations, we give recommendations on how to select the appropriate imaging parameters.
Highlights
We performed a scan with a horizontal book placement setting the cover orthogonal to the rotation axis and compared it to a scan with a vertical placement
We showed that book placement plays an important role for improving the output quality, such as reducing metal or cone-beam artifacts
We make a recommendation for a set of scan parameters, based on our experiments, to enhance the ink visibility
Summary
The volumetric scan.The scans were performed on a 3-D X-ray micro-CT using cone-beam geometry.The calculations were made by using the CONRAD framework[42]. The scans were performed on a 3-D X-ray micro-CT using cone-beam geometry. From the resulting set of projections, a 3-D volume is calculated by applying an appropriate reconstruction algorithm. We performed three 360° scans with 1800 projections. For the 50 kV scan, we used an additional copper pre-filtration of 0.25 mm to narrow the polychromatic X-ray spectrum[43]. The drawback of such low X-ray energies is increased noise, so a trade off between noise and ink visibility is necessary. The ratio of source-to-object (SOD) (710 mm) to source-to-detector (SID) (1377 mm) distances and the detector pixel size of 0.2 × 0.2 mm[2] results in a voxel size of 103 × 103 × 103 μm[3]
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