Contrast in Terahertz Images of Archival Documents\u2014Part II: Influence of Topographic Features

Tiphaine Bardon,Robert K May,Matija Strlič,Philip F Taday

doi:10.1007/s10762-016-0352-z

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.

Highlights

The ability to produce legible terahertz images of inscriptions within historical documents is dependent on the ability to discern between regions of the document with and without inscriptions
Well-contrasted terahertz images of a text written using ink with optical properties similar to that of the support might be obtained by using information related to indentations in the support left by the writing tool that is imparted on the terahertz time-domain waveforms reflected from the surface
This paper explores the information that is imparted by topographic features commonly found in archival documents on the terahertz time-domain waveforms reflected from a document, and identifies parameters and signal processing techniques that can be used to generate well-contrasted terahertz images of such topographic features

Summary

Introduction

The ability to produce legible terahertz images of inscriptions within historical documents is dependent on the ability to discern between regions of the document with and without inscriptions. If a text within a document has been written using an ink that has optical properties similar to those of the surrounding support, a terahertz image would show low contrast between regions with and without inscriptions. Well-contrasted terahertz images of a text written using ink with optical properties similar to that of the support might be obtained by using information related to indentations in the support left by the writing tool that is imparted on the terahertz time-domain waveforms reflected from the surface. Terahertz timedomain imaging could potentially reveal the presence of sieve lines and watermarks intrinsic to the paper sheets within a document Such information is of interest to curators and historians as it can help identify the date and location of manufacture of the paper sheets [6]. This paper explores the information that is imparted by topographic features commonly found in archival documents on the terahertz time-domain waveforms reflected from a document, and identifies parameters and signal processing techniques that can be used to generate well-contrasted terahertz images of such topographic features

Methods

Results

Conclusion