Mona Lisa's enigmatic smile draws millions of viewers from across the world, all eager to see the art world's most famous female face. But is it?

An Italian art detective is arguing that research backs his long-standing claim that Leonardo Da Vinci used both a female and male model to create the acclaimed portrait that hangs in Paris' Louvre museum.

While the identity of the woman is not certain, historians believe Lisa Gherardini, the wife of Florentine silk merchant Francesco del Giocondo, sat for Da Vinci for the painting.

But Silvano Vinceti, who heads Italy's National Committee for the Promotion of Historic and Cultural Heritage, says he used infrared technology to examine the painting and made key findings in its first layer.

"In that layer we can see that she was not smiling and joyful but looked melancholic and sad," he said, adding the second model was Gian Giacomo Caprotti - Da Vinci's male apprentice, known as Salai.

Using Photoshop, Vinceti compared the "Mona Lisa" face to other Da Vinci works Salai is believed to have posed for, including "St John the Baptist".

"We have used all the paintings in which Leonardo used Salai as a model and compared them to the 'Mona Lisa' and certain details correspond perfectly; so he used two models and added creative details which came from his own imagination," he said.

"I believe that this goes with a long-time fascination of Leonardo's, that is, the subject of androgyny. In other words, for Leonardo, the perfect person was a combination of a man and a woman."

Vinceti also bases his theory on claims by 16th Italian art historian and painter Giorgio Vasari that Gherardini's husband hired clowns to try to make her smile for the sitting.

Salai's name has in the past been linked to the "Mona Lisa", but other historians have dismissed the claims.