Alejandro Giacometti & Alberto Campagnolo [et al.]: Visualising macroscopic deterioration of parchment and writing via multispectral images.

Digital surrogates of documents have become an indispensable tool for scholars and archives, providing an easily accessible, non-degrading resource for research and study. Multispectral imaging has been utilised to examine the characteristics of documents by providing additional information about their physical properties and condition. However, current digitisation efforts have been concentrated on recording documents in their current state. In this project we aim to record the process of macroscopic document degradation using multi-spectral imaging.

A series of treatments have been applied to deteriorate 23 samples from two leaves of a deaccessioned manuscript and deemed of no historical value. We selected a series of deteriorating agents, which include both physical and chemical agents, to mimic the kinds of damage documents can be expected to incur during their lives, from technological mistakes during production, to improper use, unsuitable storage condition, disasters, and natural ageing. These deteriorating agents were selected so as to affect the properties of the parchment, but also the legibility of text in various ways, for example, shrinking or otherwise deforming the parchment, obscuring or effacing the writing via physical, chemical or biological reactions. Three samples were kept intact as controls.

Each sample has been photographed before and after the treatment using a multispectral imaging system to record the effect of the treatments on both the writing and the parchment. A series of 21 bandpass filters centred at regular intervals of the visible and near-infrared spectrum were used in combination with reflective and transmissive lighting. Each bandpass filter blocks all light except for a short range around a specific wavelength. Each pixel of the image captured by the camera is then a measurement of light as interacting with the materials present in the parchment at one coordinate. The combination of those images, photographed under each filter then provides a map of spectral responses for each pixel. The samples were photographed using a high resolution colour and a wide spectral range monochrome camera system.

Multispectral images provide information about physical characteristics of the samples, and may be used to identify materials present in the ink, parchment or even extraneous substances. In combination, images of a sample before and after the deteriorating agent has been applied can also offer clues into how the agent has transformed those identified materials. This information may further provide insights into identifying damage in other documents.

The data from our experiments will be published on a publicly accessible database under a Creative Common licence. This will comprise the whole set of multispectral images showing both the initial and degraded state of a manuscript (approximately 3000 images of samples and 300 calibration images), white-corrected, registered and analysis images, along with data manipulation methods, and image processing procedures.

Concurrent work with our dataset includes the quantitative evaluation and analysis of the effectiveness of common image processing algorithms used to digitally restore images of degraded writing. Multispectral images of the samples before the damage provide a ground truth which can be compared with digital estimates prepared using the images of the deteriorated samples. These work has the potential to redefine the practice of development of image processing methodology for use in recovery of lost information in cultural heritage materials to a more precise and objective discipline, by providing it with a metric to evaluate its work and improving its results.

Our dataset also provides direct advantages to the Conservation community as it offers a measurable classification of deterioration agents on parchment and a possible way of identifying the rate and type of damage by visual analysis. Furthermore, while our project formalises a methodology to document damage on parchment, this is also applicable to other cultural heritage materials.

Authors:

Alejandro Giacometti, Lindsay MacDonald, Simon Mahony, Stuart Robson, Tim Weyrich, Melissa Terras, Adam Gibson, University College London, Alberto Campagnolo, Ligatus Research Centre, London.