Attention to detail

Visitors passing through security at the London Olympics in 2012 could have every mole and scar mapped in 3D with the introduction of a new facial recognition technology that can reconstruct the face down to every last detail.

By creating a ‘facial skin signature’ it is believed that the technique will prove a quick and accurate way of scanning and recognising faces in security applications.

The technology is being developed at Imperial College London in co-ordination with General Dynamics and leading biometric consultancy, Identity Solutions.

The system uses a technique known as photometric stereo, a technology that can map and identify faces with an extremely high level of accuracy, according to its developers.

Prof Maria Petrou, the project’s lead researcher from the university’s electrical and electronics engineering department, said the system will be able to recognise faces as quickly and easily as existing automatic number plate recognition systems.



Series of lights

Photometric stereo uses a series of lights and a fixed camera to generate an image. ‘When you have light coming from an angle on to a rough surface it creates shadows,’ said Petrou. ‘The position of the light determines where the shadow falls — and more lights means more images. You can then combine the shadow information with software to infer the 3D shape of the object.’

While photometric stereo only uses one camera it is envisaged that the completed system would use six, differently placed lights that would flash in sequence.

Each flash would be synchronised with an image taken by the digital camera. However, a user would not perceive the slight delay between each light and it would seem as if the consecutive flashes had occurred simultaneously, according to Petrou. The images from the camera are then combined to produce the 3D image.

According to Petrou, photometric stereo has a number of advantages over existing image recognition systems.

Conventional stereo imaging uses images from a number of different cameras which must then be matched. Software is needed to accurately match each pixel from each individual image. In Petrou’s opinion this process can lead to ambiguity and inaccuracy, particularly when pixels can match in different areas of the image.

The photometric approach neatly sidesteps this problem by only using a single camera, so the images do not need to match. As they are the same image taken from the same angle but with different lighting sources, it means that only the shadows differ between the shots. These occur so fast in sequence that even if the person is moving quickly through the gate it does not affect the quality of the reproduction, said Petrou.

Not only does photometric stereo provide an accurate reconstruction of facial contours, the system’s number of lights means that it can also give information on the skin tone and colour, using the reflectivity — or albedo — of the skin. Other imaging technologies do not provide this useful additional information, said Petrou.

‘This is the exact skin colour we are getting, independent of exterior lighting conditions,’ she said. ‘You can see the scratches, the moles, the roughness and colour of the face — even the skin’s pores. It is much more powerful than ordinary photography.’



Handheld scanner

While the three-year EPSRC-funded project will develop the technology to be used for crime prevention, particularly with the Olympics in mind, Petrou’s team has also received a DTI grant to develop a handheld scanner that uses photometric stereo for medical applications.

The research team is working in collaboration with the burns unit of Imperial College at Hammersmith Hospital, west London.

A device using photometric stereo technology could be used to assess skin for early signs of cancer and aid in the treatment of severe burns, said Petrou.

‘A burn is treated differently according to its depth, but this can only be determined around three weeks after the burn. We hope to create this extra-accurate surface detail and use software to infer the depth of the burn, meaning treatment can start much earlier,’ she said.

For its work in security screening the team plans to develop a full-scale working prototype, which will be set up at General Dynamics’ office. The system will be tested to see how well it can recognise a face in a crowd and how that face can be authenticated against a database image.

The possibility of using non- visible light to avoid problems for people with epilepsy who could be affected by flashing lights will also be investigated.

‘We are completely changing how the face is imaged. This is the first time photometric stereo has been used in this way and is very promising for the future,’ said Petrou.