By: Ian Austen

The New York Times. September 27, 2006

TTAWA, Sept. 26—The first major scientific analysis of the ''Mona Lisa'' in 50 years has uncovered some unexpected secrets, including signs that Leonardo da Vinci changed his mind about his composition, French and Canadian researchers said Tuesday.

Photographs taken with invisible infrared light and a special infrared camera suggest that at least one of the details was hiding in plain sight, the scientists and conservators said.

The sitter in the Louvre Museum's 16th-century masterpiece, believed to be Lisa Gherardini, the wife of a Florentine silk merchant, was originally painted wearing a large transparent overdress made from gauze, they said. Under normal light, part of the garment is visible on the right-hand side of the painting, but appears simply to be part of the background.

''You can see it when you know what you're looking for,'' said Bruno Mottin, a curator in the research department of the Center of Research and Restoration of the Museums of France, known as C2RMF. He spoke at a news conference with researchers from the National Research Council of Canada.

Mr. Mottin said such transparent robes were worn by expecting or nursing mothers in 16th-century Italy. The robe's reappearance in the ''Mona Lisa'' would dovetail with scholarly research indicating that the painting might have been commissioned to commemorate the birth of Lisa Gherardini's third child.

The imaging also shows, although less clearly, that some of the sitter's hair was rolled into a small bun and tucked under a tiny bonnet with an attached veil. (The images are too cloudy to be reproduced on newsprint.)

''That is not surprising,'' Mr. Mottin said. ''The bonnet was usually worn by women in the 16th century.''

More generally, the researchers said they realized that centuries of grime had obscured some elements of the painting.

''You're seeing a lot more fine detail, showing that this remarkable painting is actually more remarkable than we believed,'' said John M. Taylor, an imaging scientist and conservator with the National Research Council of Canada.

Mr. Mottin said that two pieces of clothing had faded from view, largely because of the application of now-discolored layers of lacquer over the centuries.

While the ''Mona Lisa'' has become famous for the sitter's calm, some say enigmatic, smile, it appears that the composition was not always so restful. For example, the new images show that at one point one of her hands was painted in a clenched rather than a relaxed position.

''It was as if she was going to get up from a chair,'' Mr. Mottin said of the version Leonardo ultimately changed.

David Rosand, a Renaissance art historian at Columbia University, said it was not surprising that the ''Mona Lisa'' contained hidden secrets. ''This is a painting that has never been cleaned, that is remarkably dirty,'' he said. ''This is exactly what one would expect.''

For security and conservation reasons, scholars have rarely been able to view the painting other than through heavy glass, the researchers noted.

Indeed, Mr. Mottin, whose laboratory is within the Louvre palace complex, said that the ''Mona Lisa'' last received a complete examination after being vandalized in 1956.

Among other cutting-edge technologies, the scientists used a newly developed Canadian laser camera to construct an extremely detailed three-dimensional model of the painting.

It reveals that while the ''Mona Lisa'' may be old and dirty, it is not, as had long been thought, particularly fragile.

''We have a good handle on the physical state of the painting,'' Mr. Taylor said. While the wood panel on which it is painted is quite warped at points, he said, the 3-D model shows that it is sound and that the paint remains well bonded to its surface.

The 3-D scanner is a variation on equipment used by American astronauts earlier this month to check the space shuttle for damage before it returned to Earth. The Canadian research council, which has worked with museums around the world since the 1980's and with the French for a decade, developed a model able to resolve fine details in artworks at the limit of known optical technologies.

The pictures it produced, during two scanning sessions in 2004 when the Louvre was closed in the evening, are so detailed that special monitors had to be created to view them.

Researchers hope that their newfound ability to measure and reproduce fine detail will allow conservators and art historians to settle longstanding debates about Leonardo's sfumato painting technique, which resulted in a painting with no obvious brush strokes.

Mr. Taylor said the scan showed, as expected, that the ''Mona Lisa'' had been created by using many extremely thin layers of paint.

Mr. Mottin said many scholars believed that Leonardo first executed the light portions of his painting and then gradually built up the dark areas.

A computer-generated relief map of the painting made with the scanned data shows that, in fact, the dark areas around the sitter's mouth and eyes have the thickest layers of paint. Yet other dark areas are comparatively thin.

Over time, Mr. Mottin said, he hopes that the detailed digital image will help yield even more specific information.

''What I'd still like to know is really how the painting was done,'' he said.

Many of the researchers' findings and images are reported in a book by Jean-Pierre Mohen, Mr. Mottin and Michel Menu that has just been published by Harry N. Abrams, ''Mona Lisa: Inside the Painting.''