Ordinary thermal paper, used for retail and ATM receipts and once the bane of crime scene investigators and fingerprint scientists, works because it is impregnated with dye; heat the paper and you change its colour and see writing. For fingerprint identification, however, most techniques involve a chemical solvent that reacts with the dye and turns it black.
But chemist John Bond from the UK’s University of Leicester developed a method for applying just enough heat to bring up the fingerprints. ‘The area of contact between fingerprint sweat and dye preferentially coloured the dye and you saw the image of the print as a black image,’ he explains. Now, he has recalibrated his device to account for new thermal paper that has begun to appear around the world, particularly in China and the US, and which requires higher temperatures.
But Bond has also gone one step further. He has invented a device that will tell investigators whether thermal paper contains prints or not. ‘It is a high-intensity UV LED with a special filter on the front,’ he explains, which requires no physical contact with paper whatsoever. It works because thermal paper is impregnated with optical brighteners, compounds that absorb UV and then emit a visible blue light. Years ago these were used in soaps to add a sterile blue shine to clothes.
‘This works because the area of the paper that has had contact with fingerprints and where sweat is present reacts ever so slightly with the dye, due to amino acids like lysine. It is not enough for you to see it at room temperature, but enough that it inhibits the absorption of UV from the light source,’ Bond explains. Point the device and the paper emits a visible blue light, but any fingerprints present appear black.
‘Receipts are something handled all the time and they can be really good for evidence,’ says finger-print expert David Goodwin at Forensic Equity. For example, a car thief may rifle through the glove box and touch receipts there. ‘Receipts are very common in submission to fingerprint labs,’ he adds.
Bond showed off the new device at the Forensics Europe Expo in April 2014 in London and says a number of parties expressed an interest in manufacturing it. He estimates it should cost between £300 and £400 and would be hand-held and portable.
Fluorescence is often used now at crime scenes, so in principle is not new to police and forensic experts. ‘Police forces will be interested in something costing £400. Nowadays, price is a really important driver for all of them [UK police forces],’ says Goodwin.