Computer analysis of a picture allegedly taken of Madeleine McCann in Morocco is unlikely to provide more evidence that it is her than simply looking at it, experts said today.

So-called 'facial recognition' technology - which analyses an image of a face and seeks to match it with existing images of the subject - was not capable of making a more accurate assessment of a photograph than a human.

At best, all that a computer might conclude was that "this was a photo of a person who resembles Madeleine McCann a bit," Dr Simon Prince, a senior lecturer in the Department of Computer Science at University College London, said.

The quality of the image was not even good enough to allow for a match - or what referred to as a "facial verification" - using the criteria that are typically applied in the analysis of, say, CCTV footage, Dr Prince said.

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The number of 'pixels' between the centres of the girl's eyes - a standard measurement in facial recognition - was 10, whereas usually 20 would be the "bare minimum" required in order for a computer to determine that two subjects matched.

"A typical digital camera might have 5 megapixels, meaning there are 5 million pixels in each image," Dr Prince said. "In this photo, the area covered by Madeleine's face makes up about 200 pixels, of which 10 are between her eyes, and that's not enough. In addition to the resolution being too low, the face is partially occluded by the shadow coming from the woman's arm, which further hampers identification."

Facial recognition technology was principally used to search vast databases of pictures - for instance CCTV footage or the internet - that would take humans far longer to examine, he said.

The Child Exploitation and Online Protection (CEOP) Centre uses facial recognition to search through the thousands of photos that are seized in child sex offender investigations, and some cameras now also have facial recognition applications built-in which allow customers to organise their photo collections.

"The technology works by taking various reference points from a subject's face - the distance between features, for instance - and then comparing those with the same measurements on different pictures," a CEOP spokeswom said.

One common application is in building security systems, which can - by taking a picture - confirm that a person attempting to enter a certain area is who they say they are.

"Facial recognition works well when it is comparing passport-style photographs where the subject is front-on, the lighting is consistent, and the facial expressions are similar," Dr Prince said.

"There can be problems, however, when the pose of the subject, as well as the lighting and expression, are different - that's where all current research in the area is focused."

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