Assessing stains on historical documents using hyperspectral imaging

Hyperspectral imaging can be an important tool for the assessment and documentation of the state of preservation of an object. Over time, documents that have experienced heavy usage will inevitably show evidence of handling, which can include staining. In this paper, the use of hyperspectral imaging is described for enhancing the assessment of the visual properties of stains. The use of imaging software (ENVI) is also described for quantitatively assessing the extent of staining in two different documents. Single 10 nm bandpass images can be useful assessing darker stains with well defined boundaries. In one document (a treaty), the faint discolouration on one page made the extent of staining difficult to assess visually. A false colour density slice (450 nm) provided a topographical image which was useful for enhancing the contrast between stained and unstained paper. In this type of image, the degree of discolouration could be correlated to optical density and the amount of staining on a page could then be related to the number of pixels for a given absorbance range. In a second document (a prayer book), the staining was more extensive and some of the stains were dark in appearance. This document also contained a lot of text that was written using a dark irongall ink, which limited the use of a density slice at a single bandpass. In this document, pixel unmixing was successfully used to quantitatively determine the extent of staining. The measurement tool provided with the Nuance? Imaging System made it possible to quantitatively describe the size of the stain in terms of the number of pixels as well as its appearance in terms of average optical density.