The Penn State research team delivered the scanning stage to Xuan Liu’s optics lab at New Jersey Institute of Technology. From left to right, Liu, Yi Yang, Xing Chen, Disha Patel and Eric Carlidge. [Image: Yi Yang, Penn State Abington]
Art conservators have enlisted a bevy of imaging techniques, including optical coherence tomography (OCT), to study valuable paintings and ensure their long-term preservation. Even though OCT can generate 3D images of scattering media with micrometer resolution and without destroying its subject, the method’s narrow field of view limits its ability to examine broad regions of an artist’s canvas.
Researchers at two U.S. universities have built a hybrid scanning platform that couples OCT with a sampling algorithm to create 3D reconstructions of impressionist oil paintings (Appl. Opt., doi: 10.1364/AO.390326). Besides helping art historians to understand the creative techniques of artists and to repair damage to their paintings, the 3D reconstructions could be reproduced as tangible models to help people with visual impairments understand these works of art.
Zooming in on art
The team leader, Yi Yang of Pennsylvania State University Abington, USA, says that he enjoys visiting art museums, but has long noticed that museum patrons were not able to closely examine paintings and the artists’ techniques, due to security and conservation concerns. That realization motivated him to investigate ways for art students and patrons to “zoom in” on the details of artworks without the dangers of getting too close.
Yang and his collaborators had previously developed a raster scanning platform to stitch together OCT data for a wider field of view (Appl. Opt., doi: 10.1364/AO.55.010313). However, the linear motors in that platform created redundant sampling errors and introduced undesirable artifacts into the image.
A hybrid scanning platform
The researchers acquired OCT imaging data of an impressionist-style oil painting (left image). OCT images within the dotted line were digitally stitched together to produce the 3D images shown in the panels on the right. [Image: Yi Yang, Penn State Abington]
This time around, Yang, Xuan Liu of the New Jersey Institute of Technology, USA, and their colleagues attached an OCT system to a platform with two galvanometers that moved the incident light beam in two dimensions for scanning the target artwork. The swept-source OCT system operated at 1.06 μm with 100 nm bandwidth and axial scanning frequency of 100 kHz. The output beam passed through a fiber optic Michelson interferometer.
The robotic scanning system gathered data in partially overlapping rows and columns of rectangular-shaped sections. To remove the artifacts that had plagued previous attempts at wide-field OCT viewing, the researchers wrote a data-processing algorithm that employed cross-correlation analysis.
To demonstrate the hybrid system’s performance, the team scanned a small canvas painted with heavily textured brushstrokes of oil paint in an impressionistic style. The researchers concentrated their efforts on a region measuring 27 mm by 18 mm, even though the field of view of the base OCT system (without robotic scanning) was 3.8 mm by 2.3 mm. The system took about 900 s to acquire the data. In the computer, the researchers merged the volumetric OCT data with a full-color photograph of the painting to produce a realistic 3D model of the region of interest.
Understanding the art perspective
Yang notes that he and Liu have backgrounds in biomedical imaging and optical communications. “The biggest challenge,” he says, “is to go beyond our assumptions and really understand the problems from the perspective of art historians and conservators.” Fortunately, they enlisted the help of art historians and conservators, including Penn State art history assistant professor Heather McCune Bruhn, on the team to share their expertise and provide advice. “I think we all learned a lot from each other,” he adds.
In addition to helping art students and visually impaired persons, the OCT system could be used to create “digital backups” of paintings and other cultural artifacts to guard against museum fires, terrorism, theft, floods and other disasters.