Deep field

The US Department of Homeland Security is funding trials early this year of a 3D X-ray scanner being developed in the UK. It could provide airport security operators with video images containing ‘hitherto unseen’ details of objects.

The trials will allow researchers, led by Dr Paul Evans, head of the vision systems group at Nottingham Trent University, to assess the benefits of 3D moving images produced by the scanner to help operators spot weapons in baggage.

The technology uses two visual phenomena, kinetic depth and stereopsis, to create a sensation of depth in which contours and lines change their length and direction simultaneously. This gives the observer far more information about the objects they are viewing.

In kinetic depth the observer gains an impression of depth through the movement of an object, said Evans. ‘It’s a partial rotation of an object, a special type of rotation on a screen which induces a very strong sensation of a 3D structure. It also gives you a limited ability to look around objects and see behind them,’ he said.

However, using kinetic depth alone can result in observers suddenly seeing a reversal of the front and back of the object. ‘Though you’re seeing depth in very fine detail, you can all of a sudden, for no change in stimulus, get a reversal of front and back,’ he said.

Stereopsis, in which differences in an image shown to each eye create depth, removes this ambiguity, according to Evans. To create stereo video images the system displays an identical sequence to both eyes but with a slight time lag between the two.

The scanner, which uses a single X-ray source and a number of sensors underneath the conveyor belt, can create moving colour images, with different colours to indicate the various types of material the radiation passes through. The object is imaged from a range of angles, making it virtually impossible to fool the system.

If the trials prove successful the system could be built very quickly, said Evans. The group is also working with the Home Office’s Police Scientific Development Branch and vision systems specialist 3D X-Ray, which has developed a static 3D scanner and special glasses based on research at the university. These have been tested at Heathrow.

Evans will also begin an EPSRC-funded project in April to develop image-processing techniques that should reduce the number of X-ray sensors needed by the system. The technique, known as image synthesis, involves simplifying the intermediary views between different sensors, without affecting the kinetic depth or stereopsis, to allow the intermediary sensors to be removed.

This could significantly lower the cost of the technology to create a practical system, he said.