Princeton University Library’s (PUL) digital imaging technicians are helping researchers look beyond the surfaces of PUL Collections using a piece of technology inspired by the way scientists capture images of the moon.
In July 2023, the Library acquired a Selene Photometric Stereo System (SPSS),a non-contact and ultra high-resolution recording system and workflow designed to capture the color and three-dimensional topography of low-relief surfaces. Developed by the Factum Foundation, the Selene takes photos of items under varied lighting directions to extract high-resolution data from an item’s surface.
As advanced as it is, the Selene is also unique: PUL has the only machine of its kind in the United States.
The underside of the SPSS. Photo credit: Brandon Johnson.
“We first encountered a similar, earlier version of PUL’s Selene during a visit to Oxford in the fall of 2022,” said Jon Stroop, Deputy University Librarian. “The project immediately enticed us as an opportunity to extend the library's imaging services and, more specifically, to collaborate with new partners on technology that has the potential to unlock information that is literally embedded in the pages of our collections.”
After PUL purchased the system, Jorge Cano, the lead designer of the system and Head of Technology at Factum Foundation, along with Carlos San Juan from the R&D Department at Factum Foundation, visited the Library to set it up. The system is a custom design consisting of a series of strobes, a camera system, a sequencing board, a housing frame, and accompanying software.
“There are four strobes attached to a camera system, a Canon EOS R5, with either 50mm or 100mm RF lenses,” Roel Muñoz, Library Digital Imaging Manager, explained. A small sequencing board and custom software made by Cano allows the system to capture consecutive photographs from illumination from 90-degree orientations.
Though the Digital Imaging Studio had the physical space needed to accommodate the new system, Cano and San Juan encountered trouble regarding some mis-sized aluminum parts that anchor the lighting system.
“Fortunately, within hours, we redesigned the part in 3D in Madrid, sent it for printing in Princeton [at the PUL Makerspace] and, the next day, we were able to assemble the system as planned,” Cano said.
Most of the initial image processing happens with the help of consumer software — like Adobe Photoshop and Lightroom. The second phase of processing, which is the core of the pipeline, involves the three-dimensional reconstruction from the captured images. This is performed using custom software called SeleneProcessor, which implements various reconstruction methods based on photometric-stereo. The captured data can be used to, among other things, simulate the light being moved 360 degrees around the object.
“A single image set will allow for seeing the object in normal color, and as an albedo image that mitigates light noise and hom*ogenizes exposure,” Muñoz said. By eliminating reflections and capturing shadow data, the Selene software can analyze the topography of an object and render a depth map that allows for a 3D view of surface details.
Capabilities and challenges
In its current form, the Selene, which was named for the mythological goddess of the moon, can image items up to a size of about 45-inches by 40-inches. One example of such an item from PUL’s collections, an Aztec map on deerskin, took about 40 hours to create a composite of 320 photographs from 80 different coordinates.
“This is about the largest item we can capture completely with the Selene. As we continue to work with Factum Foundation, improved software processing efficiency has reduced recording time significantly,” Muñoz said.
Smaller items accordingly take less time to image, with something like a Byzantine coin needing just 30 minutes to output a 3D file for printing.
The Digital Studio does face challenges with using the Selene, namely in its lack of commercial availability. “There’s no help desk [for the Selene],” Muñoz said, noting he is working to get all of his staff trained on using the system. Members of the Digital Studio also meet monthly with Factum Foundationand Oxford to share notes on how they’re using the device and the types of material being imaged.
Selene in action
Technicalities aside — what does the Selene allow PUL to do? In the case of Alan Stahl, Curator of Numismatics, images from Selene transformed a 538-9 CE bronze coin into a series of medals given to participants at the “From Solidus to Stavraton” conference.
Left - The obverse of a bronze follis depicting Justinian I. Right - A mold of the same coin created from SPSS imaging. Photo credits: The Digital Imaging Studio and Brandon Johnson.
“We are producing a medal for distribution at our upcoming conference on Byzantine coinage, one side of which is modeled on a large bronze coin in our collection,” Stahl said. “We had the artist hired for producing the medal (through casting in bonded bronze) meet with Roel and discuss the best format for transmitting a 3D scan of the coin to him for pursuing the rest of the casting process.”
Despite some irregularities in the coin, the Selene’s file output allowed the artist to seamlessly translate them into a mold that would be used to cast the medals in a process he learned while working for the U.S. Mint.
Next door in Special Collections, Stahl’s colleague Graphic Arts Librarian Molly Dotson also had an idea for an item that would put the Selene through its paces. Dotson requested imaging of the original 16th-century woodblock for the frontispiece of Realdo Colombo’s “De re anatomica libri XV,” an anatomical text that was Colombo’s only published work and was originally to be designed by Michaelangelo.
“I wanted to see Selene’s potential for illustrating the materiality and dimensionality of prints and their process materials,” explained Dotson. “Having both the original woodblock and a copy of the book made this an ideal candidate.”
The Princeton University Art Museum also got in on the action, with Associate Professor of Art and Archaeology Carolyn Yerkes providing two prints by Jacques Callot for a book and exhibition she’s working on with Professor of Art and Archaeology Bridget Alsdorf.
“The prints were both impressions of Callot's The Temptation of Saint Anthony, a wild demonic scene,” Yerkes said. “Callot made the etching early in his career, around 1616–17, and as he was working on it, he destroyed the plate by burning it with acid.”
She added, “Princeton has an extremely rare state of the print that Callot made before the plate was ruined—the only one in an American collection—and also one made after it was destroyed. The high-resolution Selene scans allow us to compare these before-and-after versions of an unusual and fascinating Callot etching, and formulate new hypotheses about his working methods.”
Future and comparison to other methods
With the Selene in relative infancy—there are just three systems set up worldwide—Cano noted that Factum Foundation is focused on its continual development. “One important aspect is project documentation. We believe that thorough documentation is crucial, especially as the system reaches a stage of maturity,” Cano said.
To that end, Factum Foundation is also working on diversifying the types of surfaces that the system can scan.
“Photometric stereo imaging is ideally suited to flat objects with limited relief,” Muñoz said. Cano added, “Along these lines, we are implementing a cross-polarization-based capture system, which has been shown to significantly improve the capture of particularly shiny surfaces. In addition, we are exploring the integration of alternative depth sensors in order to extend the depth range of scannable surfaces.”
Roel Muñoz adjusts the lens of the SPSS, during a training for the Digital Scholarship team. Photo credit: Brandon Johnson
In the meantime, the Selene exists as part of a suite of tools that PUL uses to image its items. Over the last few years, the Digital Studio has experimented with other methods of image capture — the lab has used photogrammetry to capture objectively 3D objects, while in 2022 the staff hosted Mike Toth, who spectrally imaged collection items under various wavelengths of light. Techniques like photogrammetry are compatible and complementary with Selene for documenting higher relief objects. The Selene system, by capturing photographs from different positions, is perfect for automating photogrammetry captures. Additionally, the data extracted from photogrammetry and photometric stereo can be unified into a single model.
“These are obviously different techniques than our traditional photography,” Muñoz said. “Generating digital surrogates of our physical collections to democratize access to our collections remains a principal studio task.”
