Examination of Firearms Review: 2007 to 2010 - Interpol

normally exposed images subsequently processed with standard digital
modifications.
When speaking about fingermarks (or about reference fingerprints), the image quality
is of a first importance if it is defined as the apparent visibility of the existing ridges
and ridge details. This subject is discussed by Smith in an article whose aim is to
encourage the readers to think about such issues (347).
A tutorial was dedicated to the use of a function named “Black & White adjustment”
(B&W), included in the Adobe Photoshop CS3 software (and later), to enhance
images of fingermarks left on multicoloured substrates (345). The B&W option
provides the ability to work on a monochromatic representation of an image while still
having the full access to the individual colour channels. Compared to the “channel
mixer” function, which works on three colour channels (i.e., red, green, blue), the
B&W option allows to work on six colour channels (i.e., red, green, blue, cyan,
magenta, yellow), providing fine image tuning capabilities such as multicoloured
background removal.
2.3.3 Chemical imaging using Fourier transform infrared (FTIR) and Raman
FTIR
Vibrational spectroscopic techniques, such as Fourier transform infrared
(FTIR) or Raman, represent a non-invasive way to investigate the
chemical composition of latent fingermarks left on non-porous (348, 349)
or on difficult substrates (350, 351), as well as to monitor the presence of
exogenous substances, such as cosmetics (352), drugs (352-356) or
explosives (320-322, 353, 357). But they could also be used to image
superimposed fingermarks (357). Nevertheless, the sensitivity of these
spectroscopy-based methods (to image fingermarks or to identify
contaminants) strongly depends on the differentiation between the
spectrum of the material from the spectral response of the substrate and
of the secretion. Moreover, the issues related to the size of the imaged
area, the difficulty to localize a mark on an entire item prior to any imaging
attempt, and to the scanning time still constitute limiting aspects that are
not always sufficiently emphasized in the contributions.
The conjunction of attenuated total reflection microscopy with Fourier transform
infrared spectroscopy (ATR-FTIR) provided the possibility to image gelatine-lifted
(untreated) fingermarks to study the chemical composition of various components
contained in adult fingermarks (348, 349). The advantage of ATR-FTIR, compared to
conventional FTIR, mainly lies in the possibility to reduce the interference of the
substrate (the gel lifter in this case) that could cover and hinder the spectral features
of the latent secretions. In a first study, sebum-rich fingermarks were left on various
substrates (e.g., door handle, mug handle, curved glass surface, computer screen)
before being lifted using two commercial gel lifters (i.e., BVDA Gelatine Lifter and
Dycem Gel Print Lifter) and analysed by ATR-FTIR. Imaging of sebaceous-rich
regions of the lifted-fingermarks showed that the gels could spectrally dominate and
hinder the latent secretion spectra, making difficult the investigation of the chemical
composition of the fingermarks. By changing the angle of incidence, the interference
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