INTRODUCTION
The latent fingermark is visible and hence, for its exploitation, it
must be given characteristics which differentiate it from the
surface on which it is found. Traditionally, this differentiation is
obtained in the form of a coloured material, applied to the print by
either a physical or chemical process. In order for a latent
fingermark to be detected, a contrast must be produced between the
print and its support that exceeds a certain threshold corresponding
to the sensitivity of the eye or the photosensitive detector
utilised. The minimum amount of latent residue required to observe a
coloured print by chemical processing is approximately 100 to 200 ng
(1 ng = 1 nanogram = 0.000000001 g ), while 500 to 1000 ng is
necessary for the satisfactory adherence of a powder. When the
quantity of residue in the fingerprint deposit is below such levels,
the developed print is normally weak and/or partial, and may be
unsuitable for identification purposes regardless of the
colorimetric technique employed and the precautions taken to
optimise development.
Among the range of techniques based on an interaction between
light and matter, those which are followed by an emission of
electromagnetic radiation (in the present case, light) permit a
detection sensitivity considerably superior to that obtained by
absorption techniques (10 to 100 times higher - i.e., a valid result
can be obtained with 10 to 100 times less sample). Photoluminescence
is such a technique. It has allowed a sensitivity in fingerprint
detection that was not possible 20 years ago. Photoluminescence is
the emission of light by certain chemical species after exposure to
a flux of light energy of a given wavelength, known as the
excitation wavelength. Photoluminescence techniques can sometimes
detect latent prints directly, but are generally more successful
when used in conjunction with specific chemical processes as
detailed later.
The combination of optical methods, (diffusion, luminescence, UV
absorption and reflection), physical methods (powdering, small
particle reagent, vacuum metal deposition), physico-chemical methods
(physical developer, multimetal deposition, iodine, cyanoacrylate)
and chemical methods (ninhydrin and its analogues, metal
complexation after ninhydrin treatment, DFO, silver nitrate,
etc...), permits a rational and highly efficient processing of the
secretions deposited by the fingers on various surfaces. It is the
treatment of these natural secretions, exposed to the assault of
time and the environment, that permits the detection and development
of latent fingerprints.
The first step in fingerprint detection at a crime scene is to
examine all surfaces and objects and photograph or collect all
visible fingermarks. Some laboratories suggest the use of laser or
other high-intensity light source in the search for luminescent
fingerprints. Then arises the choice of development techniques for
the detection of latent fingermarks - the choice is not always easy
given the wide range of procedures currently available. The most
efficient approach is to allow for a sequence of techniques which
complement one another but are not mutually exclusive. To the extent
that is possible, all objects or pieces of evidence that can be
taken from the scene should be removed, with all the normal
precautions, for optimum fingerprint treatment in the laboratory. On
non-transportable objects having smooth, non-absorbent surfaces, the
traditional development technique is powdering.
For a given set of circumstances, the choice of the best
detection techniques, or sequence of techniques, would depend on
several factors that would include the following:
- the nature of the surface - eg, porous, non-porous, rough or
smooth
- the presence of any particular contaminants - eg, blood
- environmental factors - eg, whether or not the surface is or
had been wet
- the likely age of any evidential fingermarks.
One notion that is often ignored or misunderstood is that,
depending on the history of the evidential object, it may be useless
to proceed with certain examinations. For example, it is futile
applying a technique which relies on the detection of eccrine
secretions eg, amino acid detection using ninhydrin, if the object
has been wet since this component of the deposit is water-soluble
and would no longer be present!
If the correct choice of technique, or sequence of techniques is
made at the start, the chance of revealing any latent print that may
be present on an object is optimised. On the other hand, the
application of the wrong technique can ruin any possibility of
fingerprint detection.
FORENSIC LIGHT SOURCES
The efficient optical detection of both latent fingermarks and
prints treated by physical or chemical processes is dependent on the
availability of a suitable high intensity light source capable of
operating at a range of different wavelengths. A Forensic Light
Source (FLS) is defined as a high intensity light source that has
been specifically designed for forensic applications that may
include crime scene examination, fingerprint detection, and
questioned document analysis.
To obtain the best contrast from a luminescent fingerprint, it is
important that the incident light (excitation) is at the wavelength
of maximum absorption, and the luminescence observation is in the
spectral region of maximum emission. The required wavelength for the
incident light (the excitation) may be obtained by filtering a broad
band light source or by using a light source which is monochromatic
at that wavelength, i.e., a laser.
Observation should be made at the wavelength of maximum
photoluminescence emission. This emission, which occurs at a longer
wavelength than the excitation light i.e., lower energy, is observed
through a filter which transmits all, or the majority, of the light
emitted, but blocks or absorbs, the incident light and/or other
undesirable radiation. In this manner, a bright luminescence against
a dark background can be observed, giving the maximum possible
contrast between the luminescent mark and the support.
In practice,
these ideal conditions are rarely achieved since either the
substrate or environmental contamination can interfere with the
luminescence detection. In such cases, the fingerprint is only
visible if its luminescence is more intense or at a different
wavelength to that emitted by the background. The image obtained
using luminescence is always the reverse of the image obtained by
colorimetric methods since the image is observed as an emission of
light )luminescence) rather than an absorption of light (colour). As
a result, a fingerprint image obtained by luminescence has light
ridges whereas that obtained by conventional means shows dark
ridges.
LASERS
The word laser is an acronym derived from "light
amplification by stimulated emission of radiation". The main
physical differences between laser light and light produced by other
light sources is that the former is monochromatic, i.e., single
wavelength, coherent and very intense. The coherence of light is a
property which is not, at present, employed for the detection of
fingerprints. In contrast, the high intensity is necessary when the
excited compound exhibits a low quantum yield.
Monochromatic light, as supplied by a laser, is only an advantage
if its wavelength corresponds to, or approximates, the maximum
absorption of the compound under investigation. However, each type
of laser only operates at a limited number of wavelengths. The
selection of laser "lines" (specific monochromatic laser
emissions) is often such that their wavelength is far from being
optimum for the excitation process. A 50 per cent loss of efficiency
is not uncommon and it is only the power of the laser that can
compensate for this limitation in the selection of an excitation
wavelength. This high power can, however, introduce other problems
such as evidence destruction (holes burnt into dark surfaces, for
example). Lasers therefore suffer from a lack of flexibility which,
combined with their relatively high cost, has led to the development
of a number of "alternative" light sources based on high
intensity globes or arc lamps.
There are several classes of laser used in fingerprint detection
work. The argon ion laser is the most versatile laser and it can be
operated in two main modes: the "all lines lasing" mode
which produces a beam of light having light of different
wavelengths, or alternatively the laser can be adjusted to produce
individual lines of light, i.e., monochromatic.
NON-LASER LIGHT
SOURCES
A light source for fingerprint work should be capable of
giving a narrow band of intense light at any wavelength from and
including the UV through to at least 550nm. If the light source is
to be used for other applications such as document examination,
infrared light from around 700nm is an additional advantage. No one
laser can satisfy all of these requirements and few laboratories can
afford the purchase of several different lasers. For some
applications, such as the search for visible fingerprints or for the
detection of footwear marks on smooth surfaces, strong white light
is also a useful tool. This can never be obtained from a laser. As a
result, a number of non-laser light sources, or
"alternative" light sources (ALS) have been developed
specifically for fingerprint applications.
The versatility and relatively low cost of a conventional light
source compared to the laser has lead many laboratories to prefer
the former for fingerprint work. For example, the use of a modified
and filtered xenon arc lamp can give results comparable with those
obtained with a laser for a range of techniques. Better results can
be obtained with a filtered lamp if the required excitation
wavelength does not correspond to one of the available laser lines.
Another advantage, associated with the use of interference filters,
is the possibility of adjusting the bandpass (region of
transmission) by varying the angle of the filter in the light beam,
i.e., varying the angle of incidence - this "fine tuning"
of the excitation, or observation, can be used to optimise the
signal-to-noise ratio. The use of this phenomenon can give
remarkable results in the detection of luminescent fingerprints on
printed or multicoloured surfaces. The Polilight®, a forensic light
source produced in Australia, is the only commercial system offering
this fine tuning capability
