Last week we all convened in sunny Philadelphia to begin imaging stains from the Chemical Heritage Foundation and Penn Libraries manuscript collections.
With the generous help of Mike Toth from R.B. Toth Associates and Sarah Reidell, Margy E. Meyerson Head of Conservation of the Kislak Center for Special Collections, Rare Books and Manuscripts, the multispectral imaging system was set-up in a small windowless–i.e. perfect for imaging!–room within the Conservation Studio.
Over the course of two intense days we imaged stains from the pages and covers of fourteen manuscripts ranging from the 13th to the 16th century, thus beginning to build our dataset of stains. The manuscripts include nine alchemy texts from the Othmer collection, Chemical Heritage Foundation (Othmer MS 1 pictured below) and five medical texts from the Schoenberg Institute for Manuscript Studies and Penn Libraries collection.
We used a Phase One IQ260 Achromatic camera, a 60 megapixel 16-bit monochrome digital back with 8964 x 6716 pixel CCD array at 6.0 micron pixel size, with an iXR body and 80mm lens producing 675 ppi resolution images. The special illumination necessary for multi-spectral imaging was provided by a third-generation LED light system designed by Dr. William (Bill) Christens-Barry of Equipoise Imaging that produces very specific and narrow bands of illumination, ranging from ultraviolet light (370nm) to the near infrared (940nm).1 Because of the nature of the project, we also utilized long-pass green and red filters to detect fluorescence energy: the filters remove the illumination wavelength, but let through longer fluorescence emission that can be recorded in the captured image, thus allowing the characteristic spectra of substrate, colourant, or contaminant substances to be more completely determined and analyzed.
The camera-light-filter system is integrated within a software that simplifies the operation and records unified metadata at each step.
The result of the imaging is a sequence of photographs, one for each different illumination and filter setting, as it can be seen below.
Different materials react differently to each wavelength, and details that are not visible in natural light begin to appear and be clearly noticeable. Notice, for example, how the stain in the cover above appears and disappears, depending on the illumination.
One detail of particular interest is a writing in the upper part of the cover that was almost invisible to the naked eye, but that becomes immediately distinguishable and readable under infrared light (see detail images below).
Capturing the photographs (and managing the metadata) is only the first step. For a deeper understanding of the data recorded and the variety of material responses to the different wavelengths, one needs to process the stack of images and analyze the data through statistical algorithms capable of simplifying it and of finding patterns in it.
This kind of analysis, thanks to colour reference cards positioned in the scene, can also reconstruct colour images, despite the fact that the camera is achromatic, i.e. agnostic to colour information (see below).
One output than can prove particularly useful in distinguishing different components — i.e. materials reacting in different ways under the different lights — is a false colour image, where different components are assigned an arbitrary colour to help discerning similar and dissimilar light responses.
It is through this kind of data analysis that we’ll try to distinguish and characterize stains in the coming months.
We thank Mike Toth, Bill Christens-Barry, James (Jim) Voelkel, William (Will) Noel, Doug Emery, and Sarah Reidell and everyone else involved with our imaging session at the University of Pennsylvania for their help and support.
We thank CLIR for their constant assistance (above and beyond financial support) and encouragement.
1. We imaged at: 370nm (UV); 448nm (deep blue); 476nm (blue); 499nm (cyan); 519nm (green); 598nm (amber); 636nm (red); 740nm (IR1); 850nm (IR2); 940nm (IR3). UV in italics, visible light in roman characters, and infrared frequencies in bold.
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Labeculae Vivae 