Examination of Firearms Review: 2007 to 2010 - Interpol

cyanoacrylate fuming, sudan black, magnetic powder, ninhydrin, and physical
developer). Different casting materials were also tested, as well as two storage
conditions (i.e., room temperature and freezer). The best method consisted in
processing the item at room temperature, according to the following sequence: first
photographing visible prints using oblique light, then casting with Mikrosil before
applying sudan black (this method will colour the underlying label, making the
fingermark appear as white ridges on a darker background).
2.3 Forensic light sources, photography and digital/chemical imaging
2.3.1 Alternative light sources (ALS)
Nd-YAG laser (340) and, for the first time, LED arrays (341) were
proposed as alternatives to the commonly used alternative light sources
based on xenon arc or quartz halogen lamps. The use of an ultraviolet
emitting light source combined with a specific digital recording was also
described (324, 342). Finally, the forensic application of infrared imaging
was discussed (343).
Lin et al. discussed the forensic application of infrared imaging (343). A digital
camera able to record infrared light combined with a filter blocking all wavelengths
below 930 nm has been used to capture bloodstains (dilution series), inks on fabric,
tire marks, gunshot residue, and charred document on dark background. Even if this
technique has not been applied to detect fingermarks, it could be interesting to note
that bloodstains were shown to absorb the near-IR light, and could be detected up to
1/4 dilution when using white light, and up to 1/8 dilution when using an IR light
source.
Following their studies on the use of a Nd-YAG laser in the ultraviolet region to detect
sebum-rich fingermarks (284), Akiba et al. reported that a 280 nm excitation source is
optimal to visualize fluorescent marks on white paper, with emission peaks at 330
and 440 nm (340). Under continuous excitation, the fluorescence at 330 nm
decreases while the one at 440 nm increases. It should be noted that this technique
seems to be successful on sebum-rich marks, but gives poor results on eccrine-rich
ones.
Besides the conventional ALS based on xenon arc lamps or quartz-halogen lamps
with filters, Takatsu et al. proposed an alternative based on LED arrays (341). LED
arrays present two major advantages: (1) the lower cost compared to traditional ALS,
and (2) the uniformity of the emitted light throughout the illuminated area. One of the
biggest disadvantages is certainly the output power of an individual LED that is
insufficient to detect weakly luminescent marks. To overcome this problem, arrays of
several LED units were considered (i.e., 50 x 60 mm arrays composed of 90 LED).
Nevertheless, the illumination intensities of LED arrays are still weaker by a
proportion of 1:7 to 1:9 compared to a xenon arc lamp such as the Polilight. The
authors tested two kinds of LED arrays (i.e., blue and green) on dye-stained
cyanoacrylate-fumed fingermarks left on plastic, and on metal-complexed ninhydrinprocessed
fingermarks left on paper. Well-contrasted images were obtained, making
of LED-based excitation systems an attractive alternative for those for which
conventional ALS are too expensive.
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