Visible (420 – 720 nm) Hyperspectral Imaging Techniques to Assess Inks in Historical Documents

Understanding multispectral imaging of cultural heritage: Determining best practice in MSI analysis of historical artefacts

“…Since then images have been captured for a range of historical artefacts in order to recover lost text and features, such as the Dead Sea Scrolls [21], [29], Petra and Herculaneum scrolls [22], the Archimedes Palimpsest [30], and the letters of David Livingstone [31]. It has been widely used to recover undertexts in palimpsests ( [23], [31]- [34]) as well as to locate watermarks in paper documents [35][36] and underdrawings in paintings [25], [37][38][39]. However, research into the optimal application of MSI in cultural heritage is still relatively scarce, and where it exists it tends to be specific to reporting the results of a particular artefact (see [30][31], [34], [40][41]).…”

“…Extra light panels at a lower angle can capture raking light images of the artefacts, highlighting surface features [23], [26], [43], [100]. Transmission imaging, where the source of illumination is beneath the artefact, has been used to successfully detect watermarks [26], [35], [36]. Diffusers attached to the light panels soften the lighting and disperse shadows, ( [35], [50], [100], [112], [113], [117]), but can also soften details and edges [80].…”

“…Flat Field Correction (where a uniformly-reflecting surface is captured under identical conditions to the capture of the object to remove the non-uniform illumination or camera sensitivity) is commonly described [19], [21], [112], [113], [133], [135][136][137], [23], [25], [34][35][36], [38], [45], [82]. The material used should be flat, uniformly reflective, with minimal surface texture and no fluorescent brighteners.…”

Color and hyperspectral image segmentation for historical documents

“…Spectral imaging is a promising approach to address specific degradation aspects present in historical handwritten documents and successful results have been previously obtained [9], [10], [11], [1], [12], [13]. Processing multispectral images, the faded writing from historical manuscripts was recovered by detecting the invisible ink with a constrained energy maximization filter [11].…”

“…However, when dealing with documents and inks, the traditional spectrometric instruments (e.g: spectrophotometer), have the measurement port wider than the ink line (3-8mm diameter in comparison to 1mm), so that it outputs the spectra of the ink combined with the spectra of the surrounding paper [14]. By contrast, hyperspectral imaging has the advantage of capturing spectral information for every pixel along the spatial dimension [10], so it is possible to acquire the spectra of the thin ink without major influence from the surrounding substrate [14].…”