THIEVES who leave faint fingerprints on plastic banknotes or photographs in newspapers could find themselves fingered by a technique that shows up previously undetectable prints.
The most sensitive methods for detecting fingerprints involve either dusting a surface with fluorescent powder, which sticks to the oily residue left by the pattern of ridges on the skin, or using a chemical that reacts with the print to produce a fluorescent image. This surface is then photographed through an optical filter that blocks all but the fluorescent light to produce an image of the print. But some materials, such as certain newspaper inks and plastics, emit light at the same frequency as the fluorescent powder. This makes the pattern difficult to distinguish.
Now a team in Australia has found it can do a better job with a device known as a chemical imaging spectrometer. It is commonly used in the pharmaceutical industry for working out the chemical make-up of drugs and polymers. It combines the techniques of spectroscopy, which identifies specific chemicals, and digital imaging, which creates an image of a surface. 鈥淐hemical imaging is giving us the sort of fingerprint evidence that we would simply not otherwise have had,鈥� says Chris Lennard, director of operations support for forensic services at the Australian Federal Police in Canberra. The AFP is working with a team led by Claude Roux at the University of Technology in Sydney.
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The print is first treated with a chemical to make it fluorescent, which binds to the oily pattern of ridges. An imaging spectrometer then produces a series of images taken at different wavelengths which can be compared to find the frequency at which the print shows up most clearly.
The team has shown that prints on newspaper are revealed in sharp relief at visible light frequencies (Journal of Forensic Sciences, vol 48, p 1047). Previously undetectable prints on plastic banknotes, such as those in circulation in Australia, show up at mid-infrared frequencies.
With the imaging equipment costing around A$400,000 (US$295,000), the technique is unlikely to be used widely in the short term. But Lennard says the AFP could use it in high-profile cases within 12 months.
