Numismatics: The Study of Coins
Reflectance Transformation Imaging (RTI) offers a powerful method of documenting and communicating numismatic data.
Numismatic photographic documentation has been challenged by the characteristics of often highly reflective metallic materials with a great variety of delicate surface patinas and a small, fine-featured, low-relief morphology. The photographer of coins has had to balance the desire to limit data obscuring shadows and reflections with the need to portray as much of a coin’s surface shape information as possible. This has often led photographers to compromises that inevitably leave some valuable visual information undocumented.
CHI's Initial Numismatic Work Using PTM
CHI first experiment in the imaging of coins was done in collaboration with Tom Malzbender and Dan Gelb at Hewlett-Packard Labs. The team used Polynomial Texture Maps (PTM), an early version of RTI developed by Malzbender, Gelb, and other researchers at the Labs. (Learn more about PTM and the origin of RTI on the HP Labs web site.)
From the beginning, PTM images were understood to have the attribute of muting sharp specularities and softening hard shadows, but the extent of this property was unknown. The goal of the first tests at HP Labs was to determine the quality of the PTM data that could be captured based on the highly specular characteristics of two coins. In spite of the highly specular nature of the coins, it was found that the PTM image capture was sufficient to accurately determine surface normals and color values over the entire surface of both coins. This experiment laid the foundation for CHI's work in numismatics at the Hospice of the Grand St. Bernard in 2005.
Since this initial work, CHI has imaged many more coins at a variety of museums and universities. Coins are often subjects in RTI training. The ability to see fine details in the coins, bring out the details hidden by wear, and “fingerprint” the individual examples by documenting scratch and wear patterns, makes the RTI technique a great tool for the documentation and study of numismatics.
The Julian Star: Coin Example
In a publication presented at the VAST 2005 conference, Mark Mudge and Carla Schroer presented the results of their initial study of RTI and coins.
In the two figures above, you see two views of the “Julian Star”, a bronze coin issued by Octavian, the adopted son of Julius Caesar, the first Roman emperor Augustus. A comet, the Sidus Iulium, or Julian Star, appeared just after Julius's assassination in 44 BCE and remained for days during the funerary games. When the comet departed, historians state that it was thought to have been a sign of his divinity.
The left panel clearly shows the surface abrasion of the coin. The comet that appeared is circled and then magnified. The right panel above shows the same coin in specular enhancement mode, where one can detect the four rays of light surrounding the comet, along with the triangular comet's tail extending behind it at the “five o’clock” position.
The Merovingian Triens: Another Coin Example
The two panels in the figure above are captures of one side of a Merovingian Triens coin, with a bust of the Merovingian ruler wearing pearl diadem. The left panel is one of the twenty-four input images for the Polynomial Texture Mapping (PTM, an earlier form of RTI) and shows where specular reflections have caused data loss. The right panel shows that there is complete surface information for the coin, even though in some input images data was lost due to specular reflections. The complete surface can be calculated because the coin has been sampled from a variety of light positions and data for the entire surface is contained in the full set of images.