Using UV imaging technology, according to the special absorption 、reflection 、scattering and fluorescence characterization of the various residues in fingerprints (fatty acid ester, protein, and carboxylic acid salts etc) to the UV light, weaken or eliminate the background disturbance to increase the brightness contrast of fingerprints with the background, and design、setup the illumination optical system and UV imaging system, the noninvasive detection of latent fingerprints remaining on various object surface are studied. In the illumination optical system, using the 266nm UV Nd:YAG solid state laser as illumination light source, by calculating the best coupling conditions of the laser beam with UV liquid core fiber and analyzing the beam transforming characterizations, we designed and setup the optical system to realize the UV imaging uniform illumination. In the UV imaging system, the UV lens is selected as the fingerprint imaging element, and the UV intensified CCD (ICCD) which consists of a second-generation UV image intensifier and a CCD coupled by fiber plate and taper directly are used as the imaging sensing element. The best imaging conditions of the UV lens with ICCD were analyzed and the imaging system was designed and setup. In this study, by analyzing the factors which influence the detection effect, optimal design and setup the illumination system and imaging system, latent fingerprints on the surface of the paint tin box、plastic、smooth paper、notebook paper and print paper were noninvasive detected and appeared, and the result meet the fingerprint identification requirements in forensic science.
Latent fingerprints present a considerable challenge in forensics, and noninvasive procedure that captures a digital image
of the latent fingerprints is significant in the field of criminal investigation. The capability of photography technologies
using 266nm UV Nd:YAG solid state laser as excitation light source to provide detailed images of unprocessed latent
fingerprints is demonstrated. Unprocessed latent fingerprints were developed on various non-absorbent and absorbing
substrates. According to the special absorption, reflection, scattering and fluorescence characterization of the various
residues in fingerprints (fatty acid ester, protein, and carbosylic acid salts etc) to the UV light to weaken or eliminate the
background disturbance and increase the brightness contrast of fingerprints with the background, and using 266nm UV
laser as excitation light source, fresh and old latent fingerprints on the surface of four types of non-absorbent objects as
magazine cover, glass, back of cellphone, wood desktop paintwork and two types of absorbing objects as manila
envelope, notebook paper were noninvasive detected and appeared through reflection photography and fluorescence
photography technologies, and the results meet the fingerprint identification requirements in forensic science.