This is a continuation in a series of posts. The previous post in this series covering photogrammetry can be found here.
Hyper Spectral Imaging
We decided to employ this method for only one of our artefacts, the abstract painting. This method essentially allows us to compare the reflective signatures of different materials and to discover the subtle differences of the material elements of the artefact.
At Maynooth University we have access to the ForensicXP-WA-4010 D system. This hyper spectral scanner is able to scan for the reflectivity or adsorption properties of materials along the spectrum from 400 nanometers to 1000 nanometers and ultraviolet light at 365nm for transmitted light as well as 313nm and 256nm.
The scanner was not capable of capturing the entire canvas at once so it was scanned and examined in four sections, each relating to one corner of the rectangular canvas.
The first scan immediately displayed underdrawings when examined in the region of 1000nm to 700nm or the invisible infrared portion of the spectrum. When turned anticlockwise at 90 degrees a clear image of eyes and perhaps some details of eyebrows and a head became visible. This material was reflected at a frequency that the layers of paint were not visible at, essentially letting us look beneath the paint. This discovery did not require much fine tuning to capture an effective record of this feature. The below image was captured at 978nm using backlights to view transmitted light.
The second section of the painting scanned highlighted a single fingerprint, distinct from those that had been discovered in the R.T.I. model previously discussed. This fingerprint was captured using a high level of zoom in the red/orange portion of the spectrum at 623nm. The scanner also allows for measurement to be taken once the machine has been configured. This process, which involves defining a known length, can be used to find both distance and area, provided the focus and zoom functions are not changed since initial configuration. The dimensions of the fingerprint were recored as having an area of 344mm2, width of 13.72mm and length of 25.28mm.
In the third segment a text was visible when scanned again throughout the invisible portion of the spectrum. This quote was actually very slightly visible to the naked eye when the canvas was scrutinised but certainly would not have been remotely legible without the scanner. While scanning through this portion a signature below the text became briefly visible between around 700nm and 635nm; a much shorter period than the main text itself, indicating that the signature was made with different material. It is possible that this signature would have been more easily captured had transmitted light been scanned, however it was written across the stretched segment of canvas which had timber behind it so could only be seen under LED spotlights. The signature appeared to read ‘Michelangelo’ and indeed the text is a quote attributed to the Italian artist. Both features are shown below.
A final hidden feature was discovered at 90 degrees to the ‘Michelangelo’ text. Another piece of underwriting, this time spelling out ‘Untold Story’ was discovered. It appeared to perhaps be surrounded by more text but if this existed it could not be made visible despite considerable experimentation. Different integration times, which are similar to shutter speeds on traditional SLR cameras, were tried in an effort to clean up the accompanying text but they were ultimately inconclusive. A capture of the feature using a higher integration time is below followed by the darker lower integration time capture. They were both photographed at 720nm.
This method required quite a bit of perseverance but as with the previous methods discussed, the captured features were certainly worth the effort. The H.S.I. process is all the more satisfying as it allows you to see the unseeable and can return truly unknowable data. The nature of the technology, which feels very much like scientific apparatus as distinct from the shiny user friendly software like photoscan, can be a challenge to get to grips with. There is an assumed level of knowledge of the physics surrounding light and materiel reflectance that one must come to terms with as a scholar of the humanities but, as we have seen, the potential of this technology within such fields as art history, fine art, literature, history, archeology and myriad others is immeasurable.