Multispectral Imaging for the Study of Historic Maps: The Example of Henricus Martellus's World Map at Yale
The purpose of this article is to describe some of the benefits that can be brought to the study of early maps by employing multispectral imaging. This recently developed technique is proving invaluable for retrieving text and other features from manuscripts and printed books that have been damaged or obscured by fading, water, fire, over painting, palimpsesting and wrinkling. The terms 'multispectral imaging' and 'hyperspectral imaging' are intended to distinguish between the use of fewer and wider (multispectral) and more numerous and narrow (hyperspectral) spectral bands in collecting data. Unfortunately these terms are not used consistently, and the general term 'spectral imaging' has much to recommend it. The process depends on the simple fact that different pigments react differently to light; this is also true for light beyond the visible spectrum, both ultraviolet and infrared. Images taken at different frequencies of light may reveal parts or aspects of damaged texts, and once the best way to combine the information revealed by several such images (hence 'multispectral' has been determined, the damaged text can often be recovered and made legible.