When Scotland decided to change from cotton banknotes (called dollars in the U.S.) to polymer versions to increase counterfeit resilience and durability, they probably didn’t give much thought to how simple or difficult latent fingerprint detection would be.
Instead, forensic scientists found out the hard way that conventional latent fingerprinting techniques were ineffective on the new polymer notes. Compared with the old cotton paper notes, the polymer notes are non-porous, have a large amount of background pattern interference, and lack surface uniformity.
“With banks introducing polymer banknotes, it is important forensic scientists are able to continue to recover fingerprints from money seized under the Proceeds of Crime Act, which has been used by convicted criminals in Serious and Organized Crime,” said Carina Joannidis, who lead a recent study on the best way to detect fingerprints on the new banknotes. At the time, Joannidis was a student at the University of Strathclyde; she has since taken a position as a Mark Enhancement Recovery Officer at the Scottish Police Authority Forensic Services. The methods described in the study are not being used operationally by the police department.
The study, published in Forensic Science International, tested three common latent fingerprint techniques on £5 and £10 banknotes—cyanoacrylate fuming, powder suspension, and magnetic powder and IR powder. The techniques were used in conjunction with different light sources and light filters.
According to the study, 10 fingerprints “donated” on the banknotes were given a score between zero and four for visibility, with 0 indicating no visible fingerprint and 4 indicating a fully detailed fingerprint. The scores across each series of 10 were then added together to give total scores out of 40, while each series was given a second overall score out of 10—this showed how many of the 10 fingerprints gave a score of one or more.
Cyanoacrylate fuming using PolyCyano UV powder followed by black magnetic powder received the highest score—by far. In this process, the bacnknotes were subjected to cyanoacrylate vapors, then photographed using a UV light (350–380 nm). Hours later, black magnetic powder was applied in an attempt to enhance prints not previously visible. The notes were photographed again, and examined a second time using an IR light (730−800 nm).
Powder suspension under IR light was determined to be the second-bets technique, closely followed by IR powder plus powder suspension under IR light. Cyanoacrylate fuming alone was third, while IR fluorescent powder alone performed very poorly.
While all techniques showed a general decline from examination at day 7 to examination at day 28, there was a slight increase in score toward the end of the examination period in multiple cases. Interestingly, the outer edges of the banknotes were examined at the end of the time period.
“This increase is…a useful observation because individuals are more likely to handle the notes around the edges, and therefore more information is likely to be extracted from these marks,” the authors write in the study. “The transparent window of each note is located at one end of each note type, and the remaining surfaces of the notes are opaque. The higher scores observed at all the outer edges of the notes suggests that, rather than being a result of fingermarks being more effectively recovered from transparent areas of the banknotes, the higher scores result from lower levels of background pattern interference in these areas.”
“This is a good example of research being put into practice, as the methods in the study are now being used operationally,” said Penny Haddrill, a Teaching Fellow at the University of Strathclyde’s Centre for Forensic Science and a co-author of the study.
Next, the researchers intend to extend their study to cover £20 banknotes issued by Royal Bank of Scotland, Clydesdale Bank and the Bank of Scotland.
Photo: The Royal Bank of Scotland’s £5 polymer banknote. Credit: RBS.