Reflectance Transformation Imaging (RTI)
Scanning Methods used by USC
Three main RTI scanning set ups are employed by USC for scanning archaeological items. These cater for small items such as coins and cups, larger objects up to statues (up to around 5 m in height) and cylindrical items such as scroll seals and vases.
Small objects are placed under a dome with evenly spaced lights which are turned on and each image captured in turn by the camera which sits at the apex of the dome (as shown left).
Cylindrical objects are placed upon a small turntable with s static two piece light source and images captured at timed intervals. The images then have to be calibrated and adjusted after capture as although the time intervals are evenly spaced, due to mechanical inertia etc., the rotational movement is not and so the resulting image sequence can end up a little skewed and has to be corrected.
For larger objects, a camera is set up on a tripod and the light source has to be moved physically. As the light source has to be roughly equidistant from the object being scanned, the string method is used - one person moves around the object holding the light source at the string length distance whilst a seconf person captures each image. This is actually very effective and has the advantage of being simple and portable, i.e. can be used out on site.
Software has been developed to capture the images from camera, store and process them and display them such that the end user can then display a single image and move around the screen to simulate different light sources on the resulting image. These methods are now in use at the Cyprus Institute.
The InscriptiFact Viewer
The InscriptiFact Project is a database designed to allow access via the Internet to over 30,000 high-resolution images of ancient inscriptions from the Near Eastern and Mediterranean Worlds. The target inscriptions are some of the earliest written records in the world from an array of international museums and libraries and field projects where inscriptions still remain in situ. Included are, for example, Dead Sea Scrolls; cuneiform tablets from Mesopotamia and Canaan; papyri from Egypt; inscriptions on stone from Jordan, Lebanon and Cyprus; Hebrew, Aramaic, Ammonite and Edomite inscriptions on a variety of hard media (e.g., clay sherds, copper, semi-precious stones, jar handles); and Egyptian scarabs. These ancient texts represent religious and historical documents that serve as a foundation and historical point of reference for Judaism, Christianity, Islam and the cultures out of which they emerged. [Source: http://www.inscriptifact.com/aboutus/index.shtml].
The InscriptiFact viewer has been developed to allow end users to view each of the images using RTI. Each image can be uploaded into the viewer, moved around and digitally enhanced and enlarged for closer inspection. Comparisons can be made from images taken many decades with modern versions. Other techniques such as use of infra red photography can differentiate text from the background substrate (where both are very dark) and fragments can be brought together and viewed as a whole.
Discoveries using RTI
RTI has been used to reveal changes made to sculpted friezes and tablets, sometimes due to mistakes being corrected and in other cases where a change in circumstance has led to alterations. Once such example is a 5m tall statue of an Assyrian prince in a museum in Chicago which was scanned by the Western Semitic Research unit at USC. When the scanned images were scrutinised, it was noticed that the figure had originally been carved with a beard and ponytail and that these had been removed at a later date, indicating that the prince had been demoted. Another example is that by inspecting the skin texture of the Dead Sea Scrolls, researchers have been able to identify the animals the skins have come from. On some scrolls, the ink writing has been absorbed in places and not in others which can indicate the stroke order used when written.
Opportunity for Prospective RTI Users
USC have obtained funding from the Mellon Foundation for four people to receive a week’s training and to lend equipment for short term RTI scanning pilot projects. See here for further details.