Bardon, T., May, R. K., Taday, P. F., & Strlič, M. (2017). Contrast in Terahertz Images of Archival Documents—Part II: Influence of Topographic Features. Journal of Infrared, Millimeter, and Terahertz Waves, 38(4), 467-482.
Abstract
We investigate the potential of terahertz time-domain imaging in reflection mode to reveal archival information in documents in a non-invasive way. In particular, this study explores the parameters and signal processing tools that can be used to produce well-contrasted terahertz images of topographic features commonly found in archival documents, such as indentations left by a writing tool, as well as sieve lines. While the amplitude of the waveforms at a specific time delay can provide the most contrasted and legible images of topographic features on flat paper or parchment sheets, this parameter may not be suitable for documents that have a highly irregular surface, such as water- or fire-damaged documents. For analysis of such documents, cross-correlation of the time-domain signals can instead yield images with good contrast. Analysis of the frequency-domain representation of terahertz waveforms can also provide well-contrasted images of topographic features, with improved spatial resolution when utilising high-frequency content. Finally, we point out some of the limitations of these means of analysis for extracting information relating to topographic features of interest from documents.
For information about the group see https://iris.ucl.ac.uk/iris/browse/profile?upi=MSTRL90
For full paper see https://link.springer.com/article/10.1007/s10762-016-0352-z
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