Contrast in Terahertz Images of Archival Documents—Part II: Influence of Topographic Features

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 topographi...

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Bibliographic Details
Published in:Journal of infrared, millimeter and terahertz waves millimeter and terahertz waves, 2017-04, Vol.38 (4), p.467-482
Main Authors: Bardon, Tiphaine, May, Robert K., Taday, Philip F., Strlič, Matija
Format: Article
Language:English
Subjects:
Classical Electrodynamics
Correlation analysis
Electrical Engineering
Electronics and Microelectronics
Engineering
Feature extraction
Fire damage
Image contrast
Instrumentation
Parameters
Parchment
Signal processing
Spatial resolution
Time domain analysis
Time lag
Topography
Water damage
Waveforms
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Summary: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.
ISSN:1866-6892
1866-6906
DOI:10.1007/s10762-016-0352-z