Examination of Firearms Review: 2007 to 2010 - Interpol

The use of reflected ultraviolet (such as RUVIS) is recommended as an efficient
optical method allowing the detection of marks before any chemical treatment or in
specific occasions (e.g., bitemarks or post-blast evidence) (324). However, one of the
difficulties inherent to this technique is the photographic recording of the detected
marks since reflected ultraviolet is invisible to the eye. Sanfilippo et al. discussed this
issue and gave an overview of the advantages related with the use of reflected UV,
as well as some experimental recommendations for a successful digital recording
(342). First, they recommended to replace old camera loaded with panchromatic
films with a recent digital camera able to record in the ultraviolet region (without the
need of a light intensifier) and equipped with a “live-view” system, such as the Fujifilm
FinePix S-3 Pro UVIR or IS-Pro. The sensitivity of the current sensors is sometimes
better in the infrared region than in the near-ultraviolet light. To circumvent this
problem (that could cause an unwanted loss of contrast), the use of highly efficient
filters able to block all non-UV radiations (i.e., visible and infrared radiations up to
1,100 nm) is recommended in replacement of the classical barrier filters (which might
let pass some near-infrared light). The authors cite, for example, the Baader Venus
filters. A less efficient alternative could consist in stacking two classical barrier filters,
as a way to maximize the blocking of all non-UV radiations. Finally, the light source
used to illuminate the evidence has a major role to play, since a pure near-ultraviolet
light is preferred to an incandescent light (that emits near-infrared light as well).
2.3.2 Photography
When facing multicoloured backgrounds, full benefit should be taken from
digital imaging functions, such as high dynamic range (HDR) (344) or the
Black & White adjustment function included in the Adobe Photoshop
software (345). When dealing with fingermarks left on rear-view mirrors, it
could be necessary to remove the back reflective layer before attempting
to capture the marks (346). Finally, a discussion about the image quality is
proposed (347).
The issues related to the digital recording of cyanoacrylate-processed fingermarks
found on the interior of rear-view mirrors (i.e., inside vehicles) has been addressed
(346). Indeed, due to the double reflection effect caused by such mirrors, ridge
patterns may be split into two overlapping ones, creating fuzzy areas reducing the
overall quality of the mark (even using coaxial lighting). To overcome this problem,
the author proposed to remove the reflective layer at the back of the mirror by using a
paint remover and cleaning abrasive metal oxide.
Day proposed a tutorial explaining the advantages of using high dynamic range
(HDR) images to extend the luminance of some fingermark images (344). An HDR
image is built through the fusion of different photographs of the same scene but
taken with different exposure times. Such operations are now easily achieved in
some commercially available imaging software, such as Adobe Photoshop. This
technique could be of interest in the case of scenes with strong differences between
dark and bright regions, or presenting difficult contrast conditions (e.g., powdered
mark on a soda can or ninhydrin-processed fingermark on a pink paper). HDR
processing allows producing ridge detail of greater quality and contrast compared to
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