The scientist used a multispectral camera to project intense lights on to the painting while measuring the reflections. This helps expose what happened between the paint layers.
Art historian Martin Kemp was skeptical, despite recognizing Cotte's techniques as "highly innovatory," he told CNN.
"There are considerable changes during the course of the making of the portrait -- as is the case with most of Leonardo's paintings. I prefer to see a fluid evolution from a relatively straightforward portrait of a Florentine women into a philosophical and poetic picture that has a universal dimension."
Art historian Andrew Graham-Dixon, who has made a BBC documentary about the discovery, believes this changes history. "It's jawdropping," he told CNN. "The Louvre are going to have to change their label."
