Laser excited fluorescence imaging of latent fingerprints

[Truncated abstract] The detection of latent fingerprints is a challenging analytical problem. It involves the detection of very small quantities of specific chemical compounds in situ on potentially complex substrates. Despite the success of the current procedures for photoluminescence detection of fingerprints, many types of articles remain as difficult substrates because of intense background fluorescence, which is often spectrally broad and difficult to suppress. Very few methods are able to detect untreated latent fingerprints. This study investigated whether (i) confocal laser scanning microscopy (CLSM) equipped with Fluorescence Lifetime Imaging (FLIM) could suppress background fluorescence sufficiently to reveal good quality images of such fingerprints and (ii) whether it could further enhance fingerprints detected with conventional optical techniques. Imaging was performed with a Leica TCS multi-photon confocal system equipped with FLIM. Methodologies were developed for untreated fingerprints and fingerprints treated with ninhydrin, cyanoacrylate (CA) and CA plus fluorescent powder dusting, on various substrates chosen for either their common place in society or intractability. However, unexpected FLIM results led to the project focusing on identifying the reasons behind these results. Consequently, selected single photon lasers were employed to further investigate these findings. Exhaustive and prolonged testing revealed subtle design errors that were not in accordance with the expectations of the manufacturer...