Sep 282011

The multispectral-imaging scanner

TECHNOLOGICAL spin-outs from universities usually start in the engineering department, the school of medicine or the faculty of physics. At Oxford, however, they like to do things differently. The latest invention to emerge from the dreaming spires of England’s oldest university is the brainchild not of any of these academic Johnny-come-latelies but rather of a group who trace their origins to Oxford’s mediaeval foundation: its classicists.

The multispectral-imaging scanner developed in the Faculty of Classics, by Dirk Obbink, a lecturer in papyrology and Greek literature, and his colleagues, is able to detect traces of faded or hidden inks and paints in historical manuscripts, expose forged documents and art works, and highlight forensic evidence such as fingerprints and stains from bodily fluids. It will soon be available commercially from a firm called Oxford Multi Spectral.

Multispectral imaging works by scanning objects at a series of specific frequencies both within and beyond the visible spectrum. It is able to highlight details human eyes cannot normally see, either because they are swamped by the signal from other visible frequencies, or because they are not detectable by the rod and cone cells of the retina. Classicists at Oxford first deployed the technique in 1999, to examine papyri discovered in a villa buried by the eruption of Mount Vesuvius in 79AD. They then applied it to previously illegible manuscripts called the Oxyrhynchus papyri, which were discovered in an ancient rubbish dump in Egypt. Documents de-cyphered using it include an epic poem from the 7th century BC, by Archilochos, and parts of a lost tragedy by Sophocles, dating back to the 5th century BC.

Over the past decade Dr Obbink and his team have both improved the hardware of multispectral analysis (which was originally developed by America’s space agency, NASA), and written more sophisticated algorithms for analysing what is seen. To start with, they had to rely on a high-resolution camera mounted on a frame, and a series of filters attached to a rotating wheel, to create a set of single-frequency images from the same perspective, in order that they could be merged as desired. Now they have something that works like a flatbed document scanner, with a scanning head containing either six or 12 light-emitting diodes, each emitting light of a specific wavelength between 350 nanometres (ultraviolet) and 800 nanometres (infra-red). These diodes are switched on and off in sequence as the head moves across the instrument, and the results are recorded and fed into a computer.

Sometimes, images taken at a specific frequency provide the best contrast. For example, iron-gall ink, commonly used on ancient documents, is transparent to infra-red light and most visible in the ultra-violet region of the spectrum. In other cases the clearest picture emerges by combining images from several frequencies. The iron-and-carbon-based ink used in one of the oldest Hebrew commentaries on the Old Testament is an example of this. Much of the book, which dates from the 10th century AD, was rendered illegible in the late 19th century by misguided academics who used shellac to varnish it. Combining data from different frequencies has highlighted the old ink and allowed the document to be read.

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