Imaging

Almost every method described in this guide is technically a form of imaging, but here we use the term broadly to refer to an analytical method that produces a two- or three-dimensional visual representation of a complete object or its constituent parts (e.g., a manuscript folio, an ostraca fragment). The imaging methods used at the CSMC — multispectral imaging (MSI) and infrared reflectography (IRR) — are designed to reveal otherwise-imperceptible textual and material information by photographing the objects with wavelengths of light outside the narrow range of human vision (400–700 nm).

For two-dimensional imaging, there are four distinct modalities: reflectance, fluorescence, raking, and transmissive. MSI uses a combination of all four imaging modalities, while IRR uses only reflectance. Reflectance occurs when light hits an object and gets absorbed or reflected. The reflected wavelength determines what colour we perceive an object to be: grass appears green on a sunny day, for example, because it absorbs all wavelengths of sunlight except for those around 550 nm. Fluorescence occurs when high-energy light, usually ultraviolet (UV), gets absorbed and reemitted as lower-energy light in the longer, visible wavelengths. Most people are familiar with fluorescence when their white shirt ‘glows’ under a blacklight, but the same phenomenon also happens to a lesser extent when UV light encounters writing material, such as parchment, paper, gum arabic, or animal glue. In this case, the substrate ‘glows’ while the surface materials (e.g., inks or pigments) do not fluoresce and thus appear darker against the background, making it easier to see faint traces of writing. The CSMC’s MSI system takes full advantage of this phenomenon by capturing both visible fluorescence and any reflected UV radiation using photographic filters that block specific wavelengths of radiation. Raking light uses lights positioned at a 10–15° angle to the object so that the shadows make any topographical imperfections (such as scratches, wrinkles, bends, abrasions) more visible. Raking light has successfully recovered text on incised palm leaf manuscripts, drypoint text, worn seals or coins, and more. Reflectance transformation imaging (RTI) uses the same principles to produce an interactive image in which users can manipulate the light source to reveal detailed topographical features; the CSMC does not do RTI, but our raking lights can be used to perform an initial assessment of an object. Transmissive light information is gathered by placing the object between the light source and the camera so that the photograph captures the light shining through the object. This modality is useful for revealing structural characteristics, such as watermarks and the fibre structure of paper and papyrus. It can also be used for text recovery when an iron-gall ink has been completely removed but the ink has corroded the parchment and left it slightly thinner where the letters used to be.