Vision System Puts Bayeux Tapestry in Focus

Preventive conservation plays an important role in preserving art and cultural assets. To preserve their condition for as long as possible, it’s essential to slow down and evaluate aging processes and other factors that alter materials. To do this, it’s necessary to gain a holistic understanding of the artifact.

Cultural heritage science and engineering methods for understanding material behavior require an interdisciplinary approach. Textiles are among the most sensitive art and cultural heritage objects. In the course of their production, handling, presentation, and storage, they are often exposed to influences that can have a damaging effect.

The French startup company S-MA-C-H has studied the Bayeux Tapestry as part of a research project and, with the help of IDS cameras, has developed a system that provides valuable insights for the conservation-optimized storage and presentation of this delicate and extraordinary artifact. The expertise of conservators, combined with state-of-the-art technology, helps to identify potentially damaging factors at an early stage to minimize them or, at best, to exclude them altogether.

The system was designed for a comprehensive study looking at the mechanical behavior of the Bayeux Tapestry, which was commissioned and funded by the Normandy Regional Office for Cultural Affairs.

“We contributed our expertise in mechanical engineering by fully monitoring the stress and strain on the tapestry,” explains Cécilia Gauvin, co-founder and managing director of S-MA-C-H. “In addition to monitoring, we helped to devise conservation solutions. This involved classifying and assessing the degree of risk and sensitivity of the tapestry in its surrounding indoor climate.”

A multidisciplinary team of textile and preventive conservators as well as heritage conservation scientists characterized the artifact’s response to appropriate environmental fluctuations. “The project addressed two main questions: ‘What are the optimal parameters for a presentation on sloping supports?’ and ‘How does the tapestry react to its microclimate?’” says Gauvin.

To be able to answer these questions, the team monitored the tapestry with temperature and humidity sensors as well as force sensors that record gravity pressure. The heart and brain of the system was a noncontact, 2D full-field digital image correlation (DIC) consisting of an IDS camera and specially developed software for digital image correlation.

Figure 1: A calibration plate is used to calculate the 2D position of the camera in relation to the object and to correct any kind of distortion caused by the camera and lens.

“Digital image correlation is an image-processing technique that can be used to analyze movements and deformations of objects,” says Patrick Schick, IDS product marketing manager. “For this purpose, high-resolution images of the object’s surface are captured and then evaluated with the help of software algorithms to detect changes in the shape, position, or size of the object.”

In the search for the right camera model for the system, parameters such as interface, sensor, and price-performance ratio were the main deciding factors. “For the analysis of the tapestry, extremely high-resolution images with low image noise are required to capture the smallest details,” says Schick. The USB 3 uEye CP meets these requirements, thanks to the particularly light-sensitive IMX183 rolling shutter CMOS sensor from Sony’s STARVIS series. With BSI technology (back-side illumination), this sensor ensures outstanding image quality even under difficult lighting conditions.

“The camera is thus ideally suited for demanding image evaluations such as this monitoring, which takes place in low light for conservation reasons,” says Schick. “The USB3 Vision camera records high-resolution images at a very low frequency over a longer period of time.” For the research project, for example, the monitoring ran around the clock for a month, with only one image capture per hour.

Figure 2: IDS camera point of view for 2D strain analysis

“The Python interface of the IDS peak API allowed us to quickly and easily develop software to parameterize the camera and capture images,” says Gauvin. The images are then processed with DEFTAC3D, a software developed by the French Prime Institute specifically for digital image correlation. This results in full-surface measurement data of the object’s surface based on thousands of 2D or 3D coordinates with very high resolution.

The displacement of the image points within the examination period is interpreted as a deformation that is due to a mechanical load or stress on the object. Based on the results, full-field strain maps as well as deformation maps are generated. Full-field strain maps illustrate the strains in a material over the entire surface. They therefore show whether and how the tapestry has deformed as a whole. Deformation maps, on the other hand, provide information about the local deformations in a material. They provide S-MA-CH staff with information on whether and how the tapestry has deformed at specific points.

Click on either image to see more detail.

The full-field strain maps are processed together with the environmental variation findings to understand the hygromechanical behavior of the Bayeux Tapestry. Hygromechanical behavior refers to the interplay between moisture and the mechanical properties of the textile. This can be influenced by a number of factors, including the type of yarn used to make the fabric and the type of weaving or embroidery technique used to make the tapestry, which adds to the study’s complexity.

To incorporate scientific data into the decision-making process, tests were carried out on both facsimiles and the tapestry itself. On the facsimiles, the full-field strain maps were combined with the stress results from the force sensor measurement and the climate measurements (temperature and humidity). The results allow conclusions to be drawn about climate regulation for optimal storage of the artifact.

The data collected by the sensors and camera system were documented in real time to analyze long-term trends. With the help of the information provided, it can be ensured that the climatic conditions in the Bayeux Museum are within the optimal limits to prevent damage to sensitive art objects or historical exhibits such as the Bayeux Tapestry.

Outlook

The market for the protection and preservation of cultural heritage is slowly opening up to structural diagnostic procedures. “Our goal is to develop appropriate systems for conservators in a plug-and-play system,” says Gauvin. In the future, the French startup wants to use vision systems to monitor structural damage to paintings in museum galleries and historical monuments. This approach across disciplines makes it possible to assess the condition of exhibits so comprehensively that the appropriate conservation measures can be taken.

“Our knowledge of mechanical engineering, combined with industrial image processing, opens up new approaches, expands the range of actions, and produces innovative solutions,” says Gauvin.

An elaboration of the conservation conditions for handling, storage, or presentation of artifacts based on these interdisciplinary research results offers protection against further damage. Historical as well as modern products of textile art can thus be preserved in the best possible way for the future. Image processing components make a valuable contribution to this.

The Bayeux Tapestry is an embroidered work created in the second half of the 11th century. Image rights: S-MA-C-H

Data and facts about the exhibit

Depicted in images and text over 68 meters in 58 individual scenes, the Conquest of England is considered one of the most remarkable pictorial monuments of the High Middle Ages because of its abundance of detailed individual depictions, well thought-out iconography, and quality of craftsmanship. Despite its name, the Bayeux Tapestry isn’t actually a tapestry but a narrative embroidery. It consists of images and text inscriptions, and comprises nine interconnected linen panels:
• 68 meters long
• 52 cm high
• Nine panels of woven linen fabric
• Four seams
• 10 colors of woolen yarn
• 58 individual scenes
• 623 people
• 202 horses
• 55 dogs
• 505 other animals
• 27 buildings
• 41 ships and boats
• 49 trees

S-MA-C-H promotes the introduction of mechanical engineering into heritage conservation research. The experimental approach acts as an interface between art and technology. Determining the mechanical behavior of complex and aged materials (including stress and damage) focuses on controlling the deformation and damage of artworks. Materials are tested at different levels using innovative and noninvasive optical and mechanical techniques. The process offers simple and accessible solutions to concrete conservation questions.

The study was commissioned and financially supported by the Normandy Regional Office of Cultural Affairs.

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