Revealing hidden information using multispectral imaging
Our Conservation Science and Research team often receives requests for revealing hidden information. The manifestation of hidden information can be intentional (such as erasures and substitutions obscuring underlying text, e.g. Figure 1), or it can be a result of materials succumbing to the passing of time (archival degradation). Archival degradation is present in all materials due to natural aging and can be accelerated by usage, poor storage conditions, unsuitable humidity, mould, insect infestations, and physical damage such as fires or floods.
These conditions lead to typical deterioration artefacts including metal gall inks corrosion, ink diffusion and fading, seeping of ink from overleaf (bleed-through effect), blurred writings, fragmentation of ink or areas of ink loss. Attempting to capture what was originally underneath these artefacts can be very challenging. Each case is unique and it is often a combination of techniques which reveals information. One such technique used at the British Library is multispectral imaging.
Multispectral imaging is a non-invasive, non-destructive form of computational photography which can enhance difficult-to-read text using an extended light spectrum. The visible spectrum is the range of light wavelengths that can be detected by the human eye, and is what we often associate with the colours of the rainbow. Radiation either side of the visible region cannot be observed with the human eye, but can be detected in other ways. Image capture in multispectral imaging ranges from 420-1000 nm going beyond the visible spectrum into the ultra-violet (UV) where faded iron gall ink is enhanced, and into the infra-red (IR) where carbon underwriting and substitutions are revealed.
Multispectral imaging was recently used to help catalogue an analogue
tape from The Alan Cooban collection (C1398) for the Sound Archive. Mr
Cooban recorded BBC transmissions off-air onto open reel tape from
approximately 1957 to 1980. His metadata and labelling is excellent, for
the most part. However, the very early tapes were housed in tin containers and
in many cases the writing has worn away. Multispectral imaging was used
on one such reel whose number was barely visible on the metal container it was
housed in (Figure 2). A false colour image (which consists of an image
combining green, blue and 800 nm images) was processed to reveal a possible
number 34 or 37. Through catalogue cross-referencing and comparison with other
tape labels it was confirmed as 34. The Alan Cooban collection
comprises approximately 1900 tapes, and the Sound Archive has been
transferring them for the past year and a half. They expect to finish
Figure 2: A faded label on an analogue reel from the Alan Cooban collection (C1398) is enhanced with false colour multispectral imaging
A palimpsest is a reused writing surface from which the original text has been scraped and washed away to allow the surface to be used again. Many palimpsests are found on parchment which was sometimes recycled due to its high cost and labour intensive production. In the case of medieval codices the original text is often found to be running perpendicular to the overlaid ink due to the manner in which the parchment was folded and cut for reuse.
Or. 6581 shown in Figure 3 consists of three single leaf palimpsest fragments from the Genizeh at Cairo, housed under glass. Hebrew commentaries of the 11th or 12th century are written over the original text. The largest of the fragments (shown in the centre and right of Figure 3) has old Palestinian Syriac under the Hebrew, rather than Georgian, as is found underlying text in the central fragment (reference: George Margoliouth's Catalogue of the Hebrew and Samaritan Manuscripts in the British Museum, printed in 3 vols. 1899 [reprinted in 1965]; v. 3, p. 579, and Oliver Wardrop’s A Catalogue of Georgian Manuscripts in the British Museum, appendix to Frederick Cornwallis Conybreare’s A Catalogue of the Armenian Manuscripts in the British Museum, 1913, p406.).
The fragments were recently examined using multispectral imaging to generate
new images which may aid in the reunification of neighbouring fragments. The
style of the writing in the largest fragment resembles that of the fragment
T-S. 12. 183 of the Cambridge University Library, with both potentially
belonging to the same MS of the Bible.
Figure 3: The three palimpsest fragments of Or. 6581 housed under glass. Multispectral imaging shows the largest fragment observed in the infrared (centre) and in the ultraviolet (right) part of the spectrum
Palimpsests are easier to extract and interpret using multispectral imaging. Images of all three fragments in the infrared, visible and ultraviolet regions of the spectrum were assigned to the colours red, green and blue to create a composite image shown in Figure 4, where palimpsest detail is enhanced.
Multispectral imaging has many applications across the British Library and has helped in imaging watermarks, differentiating pigments and uncovering signatures. It has proven to be an excellent tool for information recovery and, combined with other techniques, enhances the scholarly understanding of many of our collection items.