Software developed to recognise terrorist faces is being adapted to solve the mystery of portraits of unidentified people.
In certain cases, cutting-edge "face recognition" technology could identify faces from digital images, detecting similarities in facial constructs. The data will come from scans of known features of individuals, such as in a death mask or identified sculpture.
A feasibility study is being conducted by two art historians and an electronic engineer at the University of California. They describe FACES (Faces, Art and Computerised Evaluation Systems) as a "new tool for art historians". The project has received a $25,000 government grant.
Conrad Rudolph, professor of art history at the university, said: "Before the advent of photography, portraits were, almost by definition, depictions of people who were important in their own worlds. But, as a walk through almost any major museum will show, a large number of these unidentified portraits from before the 19th century have lost the identities of their subjects."
Frans Hals' The Laughing Cavalier, the 1624 masterpiece in the Wallace Collection, London, is among famous portraits whose sitters remain unknown. The picture's title was coined in the 19th century.
Jeremy Warren, the Wallace's director of collections, said: "With the Laughing Cavalier, everyone accepts that name, but actually he's not laughing and he's not a cavalier ... I'd love to know who he is. If this technology can help us do it, we'd be absolutely delighted."
Bendor Grosvenor, a specialist in portraits at the Philip Mould Gallery, London, would particularly like to identify a "rather beautiful portrait" by an anonymous 17th-century hand – currently on show at the National Portrait Gallery.
He said: "It was traditionally called The Duke of Monmouth on his deathbed, but it isn't him as the dates don't work. Deathbed portraits are relatively rare, so who was important enough, or loved enough, to have been painted in such a moving portrayal by a good artist? I would love to know."
But he added: "Most unknown sitters are unknown because they were only painted once, and there is no other likeness with which to compare them. So the new programme will most likely only help with portraits of people for whom we already have other portraits."
Professor Rudolph accepts that "difficulties are inherent" through variations in expressions, age, angle of pose and lighting. But initial tests – on identified 15th-century portraits of Lorenzo de' Medici, the Florentine ruler – have shown how faces can be reduced to labelled graphs and matched up.
Famous nobodies: Unknown sitters
1. Girl with a Pearl Earring: Little is known about this portrait by the Dutch painter Johannes Vermeer, from about 1667, despite the novel of the same name, later adapted as a film, which speculated she was one of his servants.
2. The Laughing Cavalier: Frans Hals' work, painted in 1624, has an inscription which reveals the one known fact about the sitter: he was 26.
3. Portrait of unknown woman: This portrait, painted around 1570, was thought to depict Mary, Queen of Scots, but the sitter bears no similarity to portraits of her. Her dress suggests a noblewoman.
4. Unknown man: This portrait of a man on his deathbed was thought to depict James Scott, Duke of Monmouth, the eldest illegitimate son of Charles II, who was beheaded in 1685. However, the hair and style suggest a date in the 1640s.