With the continual advances in printing technology, it is becoming increasingly difficult to discern counterfeit bank notes from genuine US Federal Reserve notes through visual examination. Now, writing in the online journal Optics Express, Thomas Chia and Michael Levene of Yale University report that fluorescence lifetime measurements can reliably spot the difference (Opt. Express 17, 22054–22061; 2009).

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There are three common types of counterfeit paper money: digitally copied counterfeits that use a consumer-grade colour inkjet or laser printer; traditional counterfeits printed using more sophisticated methods such as a printing press; and bleached paper money made by removing the ink from a lower denomination bill and then reprinting a larger denomination over the ink-less paper.

The paper used in authentic US bank notes is predominately a blend of cotton and linen fibres, and Chia and Levene report that when exposed to a laser beam, genuine notes emit fluorescence with a unique time signature.

With assistance from the US government, the Yale University researchers used two-photon microscopy to study a selection of official and fake bank notes. The excitation source was a pulsed (80 MHz repetition rate, 100 fs pulse duration) Ti:Sapphire laser with emission centred at 735 nm. The optical power at the sample was around 15 mW. A fluorescence lifetime decay curve was produced by raster scanning the laser beam over a 4 mm2 area and capturing the emitted photons for 60 seconds (~0.8 seconds per frame). The lifetime decay curves were fit to the two-exponential component lifetime model. For genuine US paper money, the average short lifetime component lasted 162 ± 4.6 ps and the long component lasted 2010 ± 64 ps. These values did not change significantly for old notes suffering from wear and tear. In contrast, the long component lifetime of counterfeit notes had a significantly shorter tail, allowing for repeatable and reliable discrimination. It is believed that the technique may be useful to forensic science laboratories for examining very high-quality counterfeits.