NRC Scientists Help Solve the Mysteries behind the Mona Lisa

For five hundred years she's been the source of endless real and fictional mysteries, from the magic of her eyes – always seeming to follow the viewer – the enigma of her smile, to author Dan Brown's blockbuster The da Vinci Code. Now, using a sophisticated 3D colour laser scanning system, NRC scientists helped reveal the secrets behind the most famous painting in the world.

In the process, they're not only helping protect and better understand this cultural treasure, they're finessing a technology that's making space exploration safer, has allowed car companies to engineer motors to the exactness of a micron (10-20 times narrower than a human hair), fuelling cutting-edge Hollywood animation, and could one day bring 3D laser colour imaging to your home.

The 11-person research team from NRC's Institute for Information Technology (NRC-IIT) is the only North American group involved in a current benchmark study of the Mona Lisa. The study, organized by the Centre de Recherche et de Restauration des Musées de France (C2RMF) and involving more than three dozen scientists, is using state-of-the-art imaging technology to document La Joconde's state of preservation. The NRC-IIT group's job is to create the first high-resolution archival quality 3D digital model of the masterpiece. The study is also shedding light on DaVinci's unparalleled sfumato technique (see sidebar).

Solving da Vinci's Sfumato Secret

Leonardo da Vinci called this painting technique sfumato – derived from the Italian word for smoke. But what's really lost in the smoke of time is how the technique was actually applied to the Mona Lisa.

With the high-resolution of the NRC-IIT 3D colour laser scanning technology, the relief pattern of brush strokes on a typical painting appear like ripples on the sea.

"With the Mona Lisa we don't see any signs of brush stroke detail," says John Taylor. "It's extremely thinly painted and extremely flat, and yet the details of the curls of hair, for example are extremely distinct. So, the technique is unlike anything we've ever seen before. Leonardo was in a league of his own."

So how did da Vinci actually paint the Mona Lisa? Taylor says that although it is thought he may have used his fingers there aren't any indications of finger prints on the Mona Lisa – as have been found on other da Vinci paintings. Art experts know that the sfumato technique involved overlaying translucent layers of color to create the perception of depth, volume, form and lighter or darker areas.

"With the scanning we've demonstrated that the darker areas, such as the eyes, are indeed thicker indicating that they are composed of a succession of thinly applied glaze layers," says NRC-IIT scientist François Blais.

However, how the Renaissance master actually applied his layers of pigment and oil medium is still a mystery. Now the NRC-IIT team is working with commissioned replicas of sfumato on wood in order to better understand the interaction between the 3D laser scanning and the material in the hope of one day cracking da Vinci's sfumato code.

During the past 25 years the NRC-IIT proprietary 3D colour laser technology has developed from an idea on paper — by research scientist Marc Rioux — to a tool capable of resolving an object, in colour, at less than the width of a human hair. The technology's development has been driven by a unique meeting of art and science. For two decades the 3D scanner has been used and improved to document major artefacts and works of art in Canadian and international museums.

"Our work with the Mona Lisa has driven the development of the technology further than any other art work that we've ever done," says NRC-IIT research scientist François Blais, who leads the development of the 3D technology.

Before leaving for Paris, the NRC-IIT team performed detailed simulations of the scanning process. Traditionally, the Mona Lisa, painted between 1503 and 1506, is only removed from her environment-controlled chamber for a check-up once a year, and only for a single night. The NRC-IIT researchers had two precious nights, between October 18 and 20 2004, to perform the back, front and side 3D scans. Each night they worked from the Louvre's closing until dusk and the paintings return to its place of prestige for the next day's visitors.

On that first evening, the NRC-IIT team waited in the environment-controlled photo studio in the Louvre's basement as the painting was wheeled in face down by a Louvre conservator, the only person to touch the painting. NRC-IIT's John Taylor, who coordinated the team's scientific examination of the Mona Lisa, had analyzed fabulous paintings before from Renoir to Tom Thomson, and sculptures from the likes of Michelangelo to Bill Reid, but Mona Lisa, he says – is in a class of her own.

"When the Mona Lisa was rolled in, I thought Wow! We're working on the world's greatest work of art, the product of an incredible mind. Then there was the sense that time was ticking and we had a job to do," recalls Taylor.

The NRC-IIT team had developed a portable version of the 3D laser colour scanner specifically for this project. The portable version was first tested at the C2RMF on several Renoir paintings in May of 2004. Mr. Blais had also developed new algorithms, mathematical models, to handle Mona Lisa's gradually changing shades of colour on the paintings extraordinarily smooth surface.

The data collected in 16-hours of 3D scanning in 4 centimetre wide bands required more than a year's worth of analysis. The 3D model accurately recorded and documented the extent of the warping on Mona Lisa's panel. The poplar wood panel on which da Vinci painted his masterwork has a convex warp on the middle right side that's 12 millimetres higher (about the length of your pinkie's nail) than its surroundings. But this historic warp doesn't appear to threaten Mona Lisa's smile.

With a depth resolution down to one-hundredth-of-a-millimetre the NRC-IIT scan was able to provide the most detailed analysis to date of the paintings craquelure, the network of surface cracks. "What our results show, and this corroborates the other studies, is that the paint layer itself, despite all its craquelure, is very well bonded to the poplar substrate," says Taylor. "We didn't see any sign of paint lifting. So for a 500-year-old painting it's very good news. And if they continue to keep it the way they have in an environment-controlled chamber, it could remain like that for a very long time."

Not only was the depth resolution sufficient to see differences in the height around cracks, it could also resolve differences in the thickness of the varnish and even beyond to reveal the master's first conception of the Mona Lisa.

"The 3D imaging was able to detect the incised drawing to provide us with DaVinci's general conception for the composition," says Dr. Christian Lahanier, Head of the Documentation Department of the C2RMF. The results of the research are contained in a new book Mona Lisa: Inside the Painting (Harry N. Abrams, publisher) and in an upcoming scientific text on the research.

Mr. Blais says that the Heritage sector has been an excellent teacher and testing ground for the decades-long development of the NRC 3D colour laser scanning technology. "If it works for works of art, then it's almost a sure thing for the industrial applications, " he says.

The NRC 3-D technology, camera and processing software have been licensed to nine Canadian companies, which employ more than 300 people and inject an estimated $50 million annually into the Canadian economy. Among these, an Ottawa-based company has used the technology to create a 3D camera currently onboard the NASA space shuttle Atlantis. This camera serves to examine the changing state of the shuttle's heat tiles during a voyage. Licensee XYZ RGB Inc. uses the technology for cutting-edge film animation, including the Lord of the Rings trilogy and the Matrix sequels.

Mirroring the development of digital scanners from industrial tools to off-the-shelf consumer goods, Blais believes that within 20 years Canadians will be able to buy their own 3D colour laser scanners. And the technological push to get there is still coming from a 500-year old painting. The NRC-IIT team continues its work to unravel the Mona Lisa's secrets, including how to digitally capture her mysterious eyes.

"The 3D image that we have is so close, it's so close, to reality, but you still know you're looking at a digital copy," says Blais. "Now we've begun thinking about how to create a visualization algorithm that will capture and recreate the sense of luminous depth in Mona Lisa's eyes. This incredible painting is still pushing our technology to a higher level."

NRC Scientists Help Solve the Mysteries behind the Mona Lisa

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