Unmasking a 'Mona Lisa' coverup.

Unmasking a "Mona Lisa' coverup

The "Mona Lisa,' on exhibit at the Louvre in Paris, no longerlooks the way it did when Leonardo da Vinci finished his painting more than 450 years ago. Layers of discolored brown varnish, an extensive network of fine cracks and repeated restoration efforts have left their marks. To get an idea of how the masterpiece may have looked originally and to recover faint or lost details, physicist John F. Asmus of the University of California at San Diego has spent the last few years applying computer image processing techniques to a high-resolution photograph of the painting.

The first image processing step required converting thephotograph into a digital image made up of 6 million pixels for each of the three colors: red, green and blue. Then, Asmus and his colleagues measured the amount of light at various wavelengths transmitted by a piece of ancient varnish. A mathematical procedure allowed the researchers to subtract the effect of the varnish from the digital image to create a brighter, more natural picture. A color reproduction of this transformed image appears in the spring issue of PERSPECTIVES IN COMPUTING.

Further digital manipulations helped remove some traces ofthe cracking pattern. Techniques such as enhancing the contrast and using false colors brought out fine details and spots where underlying patterns show through the upper layers of paint. The analysis revealed a string of dark spots below the neck, possibly a necklace that the artist eventually decided to obliterate before he was satisfied with his work. It also uncovered, in the picture's background, a faint, distant mountain ridge that may have been erased by a restorer.

Asmus's study of Leonardo's engrossing masterpiece is farfrom over. Still missing is a clear look at the Mona Lisa's lips to see how much of her celebrated, enigmatic smile is due to Leonardo and how much is the result of clumsy efforts by restorers trying to touch up the painting.