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SCIENTIFIC IMAGE: 鈥淚 like the idea of a gentle form of science, of a gentle form of imaging,鈥 Jackson says of using terahertz technology to analyze cultural and historical artifacts. (Photo: Adam Fenster)Bianca Jackson was ready to give up on the 鈥淭hree Men.鈥 Now a postdoc- toral researcher at the Institute of Optics, she had analyzed seven sections of the fresco in the Louvre with nothing to show for her efforts but a growing sense that maybe the faded paint and plaster had no secrets to yield to her scanning technology after all.
As she worked across the painting, Jackson moved a carefully controlled beam of terahertz pulses across a postcard-sized section of the fresco and repeated a time-consuming cycle of recording the signal as it was reflected to her equipment. There, a computer program she wrote calculated the maximum amplitude of each returning pulse, adjusted the noise-to-signal ratio, and presented a pixel-by-pixel string of data across her laptop screen.
鈥淚 was ready to give up because we weren鈥檛 really seeing anything, and you have to keep doing the same thing to each section,鈥 Jackson says from her office in Goergen Hall. 鈥淚f the layers are very thin and very close, what you鈥檙e looking for is very hard to identify. I think I was on the eighth one when we started to see something.鈥
That 鈥渟omething鈥 made international news this spring when Jackson reported the results at the American Chemical Society鈥檚 national meeting. She and her colleagues reported that below the surface of the left hip of one of the Romanesque men in the fresco, 鈥淭rois hommes armes de lances鈥 (鈥淭hree Men with Lances鈥), the signals indicated the outline of what appeared to be a face. At a depth of a fraction of a millimeter, the equipment picked up a noticeable difference, a change in contrast, between the fresco鈥檚 plaster and another material that formed the outline of what seemed to be a separate, older work of art.
Was it possible that someone had painted over an original Roman-era work of art?
鈥淲e could not believe our eyes as the image evolved on the screen,鈥 Jackson says. 鈥淲e were seeing what likely was part of an ancient Roman fresco, thousands of years old.鈥
Using new technology to peer鈥攁t least metaphorically鈥攊nto layers of cultural, archaeological, and artistic history has become something of a specialty for Jackson. A leading researcher on the use of terahertz imaging to examine historical artifacts and paintings, she has traveled the world during the past half decade, honing the application of the technology as a potential research tool for archaeologists, historians, art conservators, and curators.
FACE TIME: Jackson鈥檚 research indicated that an outline of a face lay underneath the surface of a fresco at the Louvre that was suspected to be a forged example of a Roman-era work. (Photo: Dominique Martos-Levif and Bianca Jackson (terahertz scan); C2RMF (fresco))In addition to the Louvre, she has undertaken research at 脟atalh枚y眉k, Turkey, a 9,000-year-old U.N. World Heritage site that鈥檚 considered the oldest example of a Neolithic-era human settlement; she鈥檚 scanned for decorative motifs that were painted over at an 800-year-old medieval cathedral in Riga, Latvia, and scanned Orthodox icons in Nizhny Novgorod, Russia; she鈥檚 climbed to the top of Chartres Cathedral in France for a project; and she鈥檚 demonstrated the technology鈥檚 possibilities by scanning Egyptian bird mummies at the Oriental Institute at the University of Chicago.
Gerard Mourou, a former faculty member at the Institute of Optics who is now a professor of physics and the director of the International Center for Zetta-Exawatt Science and Technology at the Ecole Polytechnique in Paris, says Jackson was one of the first researchers to demonstrate that terahertz technology could be used to identify 鈥渦ndercover鈥 art鈥攊mages that have been covered over by layers of paint, plaster, and other materials.
鈥淪he won鈥檛 tell you this, but she really is a pioneer,鈥 Mourou says. 鈥淏ianca introduced a new electromagnetic frequency band to the archaeologists鈥 panoply. She鈥檚 made seminal contributions to the field.鈥
While terahertz technology is probably best known for its potential use in some scanning machines in airport security lines, the field has been undergoing a scientific renaissance since the 1980s. Historically considered something of a 鈥済ap鈥 when it came to understanding the electromagnetic spectrum, the terahertz band lies between frequencies that can be readily measured electronically with antennas or optically with detectors. Falling between microwaves, like those used in the common kitchen appliances, and the infrared light used in TV remote controls, the terahertz frequency region is invisible to humans.
And like all physical phenomena that are part of a spectrum, terahertz radiation not only shares properties of nearby frequencies but also has properties that set it apart. That鈥檚 tantalizing for researchers hoping to capitalize on its potential.
On the plus side, terahertz frequencies are relatively weak as far as electromagnetic radiation goes. Unlike x-rays or ultraviolet radiation, terahertz waves don鈥檛 鈥渆nergize鈥 the electrons of the molecules that absorb them. That means those electrons don鈥檛 change their state, altering the underlying atomic structure in the way that living tissue can be affected by x-ray radiation or a work of art will change color if left exposed to the sun. Also in the plus column, terahertz waves travel deeply into dense material, transmitting especially well in materials that contain little moisture.
On the minus side, while terahertz waves propagate, their higher frequencies are easily absorbed by water and other molecules in Earth鈥檚 atmosphere, meaning that unlike microwaves or radio waves, they can鈥檛 travel far in the open air. And because terahertz radiation has relatively large wavelengths鈥攐n the order of 300 to 1,000 times larger than infrared and ultraviolet鈥攖he level of detail at which terahertz images can be resolved is more limited than that of other imaging technology.
Xi-Cheng Zhang, the director of the Institute of Optics who is considered one of the leading experts in understanding the terahertz band, recognizes the trade-offs, but he says the technology has proven itself in several applications beyond imaging, particularly for quality control, nondestructive testing, and other manufacturing processes.
鈥淚 like to talk about terahertz technology as a complementary method to x-ray and infrared spectroscopy,鈥 says Zhang, the M. Parker Givens Professor of Optics. 鈥淭here are certain materials for which terahertz wave technology does a better job than x-ray or infrared. For example, for low-contrast materials, such as low-density foam, terahertz wave technology has been demonstrated as having a better contrast ratio than x-ray images; on the other hand, for infrared imaging of some optical opaque materials, such as paper, plastics, or cloth, terahertz penetration capability is better. That鈥檚 why now many airports use sub-terahertz wave imaging, and why we continue to push the frequency to the terahertz range.鈥
Working with terahertz pioneer David Auston and others at Columbia University in the late 1980s, Zhang was among the first to demonstrate that terahertz signals could be consistently generated and measured using semiconducting materials and lasers. In 2012, he brought his internationally regarded program to Rochester when he moved to the Institute of Optics after a 20-year career at Rensselaer Polytechnic Institute, where he was director of the Center for Terahertz Imaging, acting head of physics, applied physics, and astronomy, and the Erik Josson Professor of Science.
He and his group are currently leading work to explore whether terahertz technology can be harnessed for remote sensing and long-distance communications. Zhang invited Jackson to spend a year working at Rochester after learning about her work with terahertz imaging on several cultural heritage projects. He considers her work as part of a larger effort to build strength in terahertz at Rochester.
In many ways Zhang鈥檚 initiative taps into a larger history of leadership in terahertz research at Rochester. As a scientist at the Laboratory for Laser Energetics in the 1970s, Mourou led a team that reported the first work in producing and detecting terahertz pulses. In 1988, he moved to the University of Michigan in Ann Arbor, where one of his former students, Steve Williamson 鈥82, would later found Picometrix, one of the world鈥檚 leading manufacturers of terahertz equipment. The company鈥檚 key principals also include Irl Duling 鈥85 (PhD) and Janis Valdmanis 鈥84 (PhD). In 2005, Picometrix was purchased by Advanced Photonix Inc., a public company.
At Michigan, Mourou first met Jackson, whose PhD advisor was John Whitaker 鈥88 (PhD), a scientist at Michigan鈥檚 Center for Ultrafast Optical Science. Whitaker himself had worked on applications of terahertz radiation in electrical transmission lines as a graduate student in electrical and computer engineering at Rochester.
鈥淚t鈥檚 like a circle almost,鈥 Jackson says of the connections between Rochester, Michigan, and terahertz research.
In the mid-2000s, Mourou had returned from a conference at the Hermitage in St. Petersburg, Russia, where he had heard the latest research on using infrared, ultraviolet, x-rays, and other frequencies to image art work and historical objects, and he had an idea: could pulsed terahertz waves be used for imaging cultural artifacts? He turned to Whitaker, who had been advising Jackson on terahertz-based work to examine the ceramic coatings of jet engine turbine blades.
鈥淲e were basically looking for defects in the coatings on the blades,鈥 says Whitaker.
He suggested to Jackson that she undertake the research on artwork. To do that, Jackson and Mourou鈥檚 daughter established a mini-art studio in the Michigan lab, using historically accurate painting materials and compounds to draw and paint images, such as butterflies, and then cover part of each work with different kinds of plaster, including gypsum, the mineral in modern drywall that鈥檚 also found in the plaster at the Neolithic site in Turkey.
鈥淰isibly, you would see half of a sketch, and then white plaster,鈥 Whitaker says. But with the terahertz equipment, it was possible to 鈥渟ee鈥 more. 鈥淓ven though it was under the plaster, you could still see the image of the butterfly.鈥
Says Jackson: 鈥淲e would have a 4- or 5-millimeter thick piece of plaster, and we could see the paint patterns behind it.鈥
The findings were promising for imaging, Whitaker says, but they also demonstrated that pulsed terahertz waves could be used for spectroscopy, or analyzing the molecular make-up of materials by the frequency at which they reflect electromagnetic radiation. By 鈥渢uning鈥 the frequencies at which the pulses were emitted, Jackson could examine the makeup of individual compounds under the plaster, the 鈥渘itty-gritty spectroscopy of plaster and pigment.鈥
鈥淥ne of the beauties of using terahertz pulsed imaging to look at art is that you鈥檙e not only doing imaging, but you鈥檙e also doing spectroscopy at the same time,鈥 Whitaker says.
Understanding the relationships among fre- quencies and the contrasting properties of different types of materials when they absorb radiation is a key area of research, Jackson says.
鈥淲hen you see something, you鈥檙e not really seeing the 鈥榯hing,鈥欌 she says of one of the tenets of imaging science. 鈥淵ou鈥檙e seeing the difference between the 鈥榯hing鈥 and what鈥檚 around it. If you have two materials that are very similar and neither of them is particularly absorbing, it鈥檚 going to seem like the 鈥榯hing鈥 is invisible or transparent. You may see edges, but you won鈥檛 really see the whole object.鈥
For example, with x-rays, materials like plastics and polymers are difficult to detect because the materials do not absorb much of the radiation. In other words, the atomic density of plastic is closer to air than it is to something more substantial like bones, brick, or metal, allowing the radiation to pass through it.
鈥淲ith terahertz, the structure of the material鈥攖he specific atoms that make up the material鈥攁ren鈥檛 as significant,鈥 says Jackson. 鈥淪o you can have carbon-based materials or organic materials that are very visible with terahertz because there鈥檚 still some absorption, there鈥檚 still some contrast between that material and the air around it.鈥
To analyze paintings, the molecular composition of some pigment compounds traditionally used by artists, such as red iron oxide, aka 鈥渞ed ochre,鈥 have very low contrast when it comes to imaging. On the other hand, a fundamentally similar one, like iron oxide hydroxide, aka 鈥測ellow ochre,鈥 might be identified well.
鈥淪o red ochre, no; yellow ochre, yes,鈥 says Jackson. 鈥淭hat鈥檚 the kind of information I started picking up from my early experiments.鈥
An early idea to scan the Mona Lisa, for example, eventually failed to come to fruition after Jackson found that if Leonardo DaVinci used red ochre, as was traditional for him and other artists of his era, indications of drawings underneath the surface paint would be nearly impossible for her to pinpoint.
After graduating from Michigan, Jackson began working in the field, where she discovered that, because most art exists in a world of clumps, splotches, and uneven surfaces, much of her work requires fine-tuning computer algorithms to adjust for such variables, a programming challenge that she takes on herself.
鈥淭erahertz works well for layered artifacts, like old wall paintings, because it can scan at different depths,鈥 Jackson says. 鈥淎s work is painted or plastered over, each new addition adds a layer. Sometimes the previous work was erased, but sometimes it was simply plastered over. At sites like 脟atalh枚y眉k, that means centuries of plaster.鈥
That requires developing new algorithms to account for the layering, which colleagues reported with Jackson this spring in the journal Optics Express.
TEACHER: As part of her work, Jackson teaches in a terahertz lab, where Jing Zhang, a visiting PhD student from Huazhong University
of Science and Technology (pointing), and Rochester optics PhD students Xuan (Betty) Sun (left) and Fabrizio Buccheri conduct
research. (Photo: Adam Fenster)On the minus side, while terahertz waves propagate, their higher frequencies are easily absorbed by water and other molecules in
From academic positions in France and elsewhere, Jackson has become a go-to person in the world of terahertz applications in cultural heritage. For the 鈥淭hree Men鈥 project, curators from the Louvre had turned to her and her team to evaluate the fresco鈥攑art of the Louvre鈥檚 collection of pieces that once belonged to 19th-century collector Giampietro Campana鈥攂ecause they knew it was not an authentic Roman-era work. Art historians have long known that Campana was not above 鈥渞estoring鈥 pieces in his collection because he thought it important to save historical work such as frescoes.
Working with a portable system made by Picometrix, Jackson was able to conduct her work within the Louvre, something that she sees as an important benefit to terahertz imaging. With other scanning technologies, like x-rays or computer tomography (CT) scans, artwork has to be taken to the equipment, usually requiring transportation to a hospital setting. That can make curators and conservators uneasy about letting researchers examine artifacts that are often priceless and one-of-a-kind, and sometimes too large to move.
Jack Green, chief curator at the Oriental Institute Museum, says that conservators have long been interested in being able to 鈥渄issect objects digitally,鈥 especially for artifacts like mummies, which cannot be opened without destroying them. The Oriental Institute invited Jackson to scan a 2,000-year-old Egyptian bird mummy as part of a larger research project on the institute鈥檚 collection of Egyptian artifacts. While the fragile mummies had to be transported to the University of Chicago鈥檚 hospitals to undergo CT scanning, Jackson conducted her work in the institute鈥檚 conservation lab. Jackson contributed a chapter to the catalog for the exhibition, Between Heaven and Earth: Birds in Ancient Egypt, on display at the Oriental Institute through July 28.
The results were not as detailed as the CT scanning, but Green says experimenting with new technologies is important to understanding cultural history. Jackson and the exhibition鈥檚 guest curator, Rozenn Bailleul-LeSeur, plan to conduct a more detailed comparison of the CT scanning and terahertz imaging that was done on the bird mummy. 鈥淥bviously, we favor nondestructive testing,鈥 Green says. 鈥淎nd a tool like this can be incredibly useful if you鈥檙e able to reveal information about the object layer by layer.鈥
Gillian Walker, a researcher at the School of Systems Engineering at the University of Reading in the United Kingdom and a frequent collaborator with Jackson, says the noninvasive nature of terahertz imaging is one of its most compelling features. One of her first projects was trying to develop a way to detect skin cancer using terahertz imaging. 鈥淭he mathematical problem is exactly the same as detecting layers of paint in plaster,鈥 Walker says.
And although she鈥檚 intrigued by the opportunity to help conservators and archaeologists gain new insights into cultural history, 鈥渉ow that enhances the study of cultural history and archaeology is up to the archaeologists to decide,鈥 she says. 鈥淏ut it is where the impact of my work is, so it is important to work with good experts who are open and able to interpret the data I provide.鈥
Whitaker says he expects terahertz data will become increasingly prevalent, particularly in areas such as biomedical imaging, the pharmaceutical industry, and in materials testing and quality control. Two challenges to address, he notes, include enhancing the power that terahertz systems can produce and improving the signal processing of the systems.
鈥淚mproving the signal-to-noise ratio is crucial,鈥 Whitaker says, noting 鈥渢hat鈥檚 also something that will improve the resolution of the images over time.鈥
Jackson says she鈥檚 up for the challenge, excited by the chance to demonstrate the applicability of science and research to a problem that improves other people鈥檚 lives. When people ask about her work, 鈥淚 always say 鈥業 do terahertz.鈥 I think it鈥檚 great technology.鈥
鈥淚鈥檝e always been interested in science and cultural history,鈥 says Jackson, who describes her father as a 鈥渢echie person鈥 who taught her how to program using the BASIC computer language when she was in elementary school. Her mother is an artist who made her living as an art history teacher and who often had her students try to reproduce artifacts as a way of understanding art.
鈥淪o it worked out that I was able to use both sides of my brain growing up,鈥 Jackson says.
鈥淏ut as a scientist, I鈥檓 interested in being innocuous, but also being helpful and useful. I like the idea of a gentle form of science, of a gentle form of imaging. After 9/11, I think the interest in using the technology in providing a service and in making people safer gave me more incentive.鈥
In 2014, she begins a Marie Curie Fellowship IntraEuropean, a program of the European Commission to support the work and geographic mobility of young investigators. She will be based at the University of Reading.
At Reading, Walker has finished scanning St. Thomas鈥檚 Church in Salisbury with the French team, and has scheduled work to scan a contemporary work of art and a church in France this summer. Jackson will inherit the data about the Salisbury Church.
鈥淚 tend to think of myself as being more aligned with terahertz and being a promoter of terahertz,鈥 says Jackson. 鈥淏ut I like having fun. I like art. I like traveling. But that鈥檚 mostly secondary to someone who wants to promote terahertz.鈥