Examination of Firearms Review: 2007 to 2010 - Interpol

function of two independent parameters, specifying the direction of a point light
source. PTMs are typically produced with a digital camera by photographing an
object multiple times with lighting direction varying between images. Even a lowend
digital camera provides enough resolution to produce good PTMs, and almost
any light source can be used, such as a light bulb, LED or a flash. Hamiel and
Yoshida (16) applied this imaging technique for shoeprints and other impression
evidence, including the use of a portable unit for field studies. The PTM images
were compared to conventional sidelight and casting techniques. The application
of this technology could be more cost-effective than conventional methods and
provide higher-quality data. Results of the evaluation reveal that PTM technology
can successfully be used in the forensic field and has the potential to produce
better resolved images for the comparison of known shoe soles or tire treads to
crime scene impressions. Specific results indicated that PTM images and
enhancements improved the visibility of detail in some of the impressions under
analysis when compared to traditional photography techniques, including
improvement of the visualization of texture within a shoe or tire impression. PTM
technology thus gives the examiner the best opportunity for visualizing unique
characteristics in impression evidence. PTM technology also has the advantage of
being cheaper to operate than traditional sidelight and casting techniques.
In a recent report by Prokoski (17), 2D infrared (IR) imaging was demonstrated to
produce images of footwear impressions under dim light and under total darkness
conditions. It was also shown to produce detailed images of athletic shoe sole
patterns without controlled lighting. 3D IR imaging was also demonstrated to
produce dimensionally accurate 3D digital models of footwear and footwear
impressions simply and fast. The primary advantage of IR imaging over visible
light imaging for footwear evidence is that it produces more consistent feature
details under conditions of uncontrolled lighting. Feature contrast in IR images can
generally be enhanced by air flow over the features, which differentially heats or
cools portions of the features.
Since digital photography is becoming more and more common for capturing
shoeprint images, readily-available image enhancement enables the shoeprint
expert observing details on the shoe-mark that were initially barely visible. Wiesner
and her colleagues (18) presented two Collaborative Testing Services (CTS)
proficiency tests (04-533 and 07-533), where challenging shoeprints were
examined. The prints were made on surfaces with noisy backgrounds that
interfered with the prints. The image processing method presented in this paper,
namely color channel separation and image enhancement, reduced the
background significantly, and improved the visibility of the shoeprints.
2.1.2 Lifting and Casting
Following photography or scanning, shoeprints found on various surfaces are
usually lifted or cast, according to their nature. Recently published SWGTREAD
guides cover this area (6). Footwear impression examiners usually use
transparency lifts to compare size, design and accidental characteristics between
known shoes and questioned impressions. In certain cases, the examiner may
have to separate the lifting film from the cover sheet. In a case reported by Adair
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