CHI President Mark Mudge in a training session at NYU

“Coins present a special documentation challenge. Even with direct visual examination, some details are hard to discern. CHI methods provide complete surface information.”Mark Mudge, President, Cultural Heritage Imaging

Swiss Coins: CHI's 2005 Study

In Fall 2005 the CHI team spent 14 days documenting 114 objects from 3 collections in Switzerland. Among other subjects, the team captured image data from gleaming Greek, Celtic, and Roman coins and from a Bronze Age torque (neck piece). CHI's imaging techniques revealed a delicate geometric pattern incised on the torque, and this inspired CHI to capture the first multiview RTI. Read more about this early Swiss expedition...

Roman Republic coins from the Congregation of the Grand St. Bernard

Roman Republic coins from the collection of the Congregation of the Grand St. Bernard

Related Publication

“Reflectance Transformation Imaging and Virtual Representations of Coins from the Hospice of the Grand St. Bernard”

This paper, presented at the VAST 2005 conference in Italy, examines the challenges of documenting numismatic material along with the limitations of traditional documentary techniques.

Numismatics: The Study of Coins

Detail of the obverse of an ancient Greek tetradrachma coin

Detail: Obverse of a silver Athenian tetradrachma coin, 454 BCE. RTI image with mathematical enhancement.

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.

Silver Athenian Tetradrachma (reverse) 454 BCE from Cultural Heritage Imaging on Vimeo.

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.)

Preparing coins for study

Coin being prepared for imaging at the Musée Monétaire
in Lausanne, Switzerland

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.

Unenhanced Julian Star coin, diffuse lighting mode Julian Star coin with specular enhancement

Left: An unenhanced RTI image of the Julian Star coin as seen in primarily diffuse mode. Right: The same coin with specular enhancement.

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

Merovingian Triens coin, contrasting panels

Left: Photograph of the obverse of a Merovingian Triens coin; arrows indicate obscured
surfaces due to specular blowouts. Right: The final RTI image showing complete
surface information for the coin.

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.