Image analysis developments in forensic science

The fingerprint recognition method - based on the fact that no two individuals have the same finger, palm or footprints - may be over a century old but it is not likely to be replaced soon because prints are still the most common form of forensic evidence used to make a conviction. But a recent development is now updating the processes used for capturing, storing and making arrests from fingerprint evidence.

Digital-imaging techniques are being pioneered by Esther Neate - senior fingerprinting officer of Wiltshire Constabulary in southern England - who began developing the process after extreme frustration during a case in 1995.

"We had a banknote with a fingerprint on it which could have led to a conviction but there was no way at the time of clearly separating the fingerprint from the background pattern of the note," says Esther Neate.

She investigated the new technologies she knew were becoming available, principally in the United States, and developed a prototype digital-imaging station capable of removing such patterns to achieve a clean image - and much more besides.

"If there's a regular pattern running through the background of a print, we can now remove it and retrieve a fingerprint image we can use in court," she added. The same technology can be used to amplify certain ridges in the fingerprint to increase the intensity of the ridge orientation and which can pull up to 50 per cent more information from the images.

The system works by recording a fingerprint image on an extremely high-resolution scanning camera that captures at 3600 by 4200 pixels using 12 bit technology instead of the standard 8 bit, making 4096 variations of grey scale available to the user. The image is then scanned into the software where numerous techniques can be used to make certain areas lighter, darker or larger or to remove background patterns.

Understandably, the development of these techniques presented particular concerns about verification - if the background can be altered, what's to stop an accidental shift in the ridge pattern, creating an entirely different fingerprint?

To this end, image authentication software called Veridata, which creates what Neate calls a 'digital negative', was incorporated. The system is automatic and tamperproof. As soon as the photograph is scanned into the machine it is tagged to preserve the original integrity. A duplicate working copy is then used for subsequent imaging work.

The authentication element was just one addition to Neate's initial prototype made during the development process that occurred in conjunction with company Mason Vactron of Worcestershire that produces a saleable system called DCS-121.

An additional feature called Auto-Montage allows reproduction of a full fingerprint image using a series of prints taken from a three-dimensional (3D) object. Fingerprints are often left on 3D objects rather than flat surfaces, making them difficult to photograph, because a direct photo would only allow one area of the print to appear in focus.

The way around this has traditionally been to take five photographs with different planes of focus and analyse each one individually. With Auto-Montage software, provided by Cambridge-based Syncroscopy, it is possible to create a perfectly focused digital image by overlaying the five separate shots. A depth map is also generated to show what parts of each image were used to create the final one.

"There is no chance of misalignment," assures Neate, "because the continuous ridges make it a bit like a jigsaw. If you were to fit the images together inaccurately, the ridges wouldn't line up and you'd see a series of straight lines. With Auto-Montage software, however, you see a perfectly overlaid image where the ridges are continuous."

Again, such an image will be accepted in court providing forensics can prove it resulted from verified prints. "In court we take each of the five original images which would have been individually authenticated and then with the software we reproduce the final image in front of the whole court room," explains Neate. "We have already made a lot of convictions using the image-enhancement software."

About 50 of the DCS-121 systems have now been installed in police forensic laboratories throughout the UK and South Africa, with further units on their way to the Middle East and Eastern Europe.