The Power Of The
Forensic (Alternate) Light Source
It is often times the
most challenging phase of any homicide or suspicious death
investigation: the evaluation phase. If we look at the
proper order in which events occur during a homicide
investigation it is the evaluation phase which sets the tone
for not only the immediate direction and leads of the
investigation, but also the forensic footprint for that
particular scene. It sets in motion a chain of events where
the ultimate question is, how do we link the suspect back to
what he or she did? Through evaluation we establish motive
and through motive we establish and identify the actions of
the suspect at the crime scene and determine the best way to
identify, document, collect and have analyzed the physical
evidence. One of the greatest tools often times over looked
and not utilized enough is the forensic light source.
Although it is normally used after the evidence collection
process, it may be implemented during the evaluation phase
to avoid unnecessary destruction and contamination of
physical evidence.
Often
times when scene investigators arrive there is already
information which has been gathered from patrol officers,
witnesses and victim’s family members. This information may
direct to a degree the areas where physical evidence can be
examined or searched for during the evaluation phase. If the
information received points in the direction of sexual
assault or rape, or the surface of the crime scene may be
suitable for pattern impression evidence, the forensic light
source can be an invaluable tool. It can immediately
indicate red flag areas where delicate trace evidence may
exist. Trace evidence such as hairs and fibers are often
times displaced (indirect transfer) and moved to a different
location or carried away from the scene altogether. When
dealing with smooth surfaces at your crime scene this can be
especially challenging. A general rule that should be
followed during the evaluation of a crime scene is to travel
along the path of least destruction. This means to avoid
areas of the crime scene or articles the suspect may have
had contact with whether it is an item touched or a surface
walked on. Staying close to the walls and away from these
areas is important. The use of the alternate light source at
this point can be an outstanding tool when searching for
trace.
From my
personal experience and also through outside agency
interviews, too few law-enforcement agencies employ this
tool at the crime scene at all much less during the
evaluation phase of the investigation. One of the problems
falls back on budget restrictions. A decent portable
forensic light source is not cheap, averaging over ten
thousand dollars for the basic set-up. The other problem is
the lack of knowledge as far as what can be searched for
when using the alternate light source. Some evidence that
can be uncovered has been discovered either by accident or
by careful research, or in the course of searching for other
physical sources of evidence. Bruising beneath the skin for
example is one of the uses of the forensic light that was
discovered while searching for other types of physical
evidence related to a sexual assault. As a result, it is now
an important tool when searching for bruising beneath the
skin in cases involving potential gentle homicide and
unexplained infant death.
The
forensic alternate light source takes the three components
of light; ultra violet, infrared and visible light and
filters them down into individual color bands or
wavelengths. When the color bands interact with evidence it
will do one of three things: fluoresce, absorb or reveal
small particles, hairs, fibers and impression evidence. The
later would require a white or oblique light on a smooth or
carpeted surface that hopefully has not been contaminated by
witnesses and first responders. This is where the traveling
along the path of least destruction comes into play.
It is
important also to understand what type of evidence can be
searched for during the course of evaluation. The scene
investigator should not be searching for latent prints on
smaller items here, which would come later. What you should
be covering is broader areas such as carpets, bed sheets,
smooth surfaces, clothing, walls, etc. Body fluids, small
particles, shoe impression evidence, hairs & fibers and GSR
are just a few items of physical evidence that can be
searched for on larger surfaces. Holes in walls made by
gunfire have been spackled, painted over and later
discovered with the use of the alternate light. If any
evidence is discovered during this phase it would then
become important to red flag these areas and make the
determination to deal with it then, or have the area marked
and return to and dealt with later. Often times it is better
to simply make note of the potential area of evidence,
complete the scene evaluation and then return to those areas
when the evidence collection process begins.
During
the search for evidence it may also become necessary to
increase the wavelength being used to block out or eliminate
the background the evidence may be on in order to show
contrast. Some types of carpet, bed sheets, and articles of
clothing, fibers with certain synthetic values or painted
surfaces may also fluoresce making it difficult to
distinguish the evidence that may be present.
For
agencies that do not have a decent alternate light source
simply using oblique lighting or a white light may do.
Simply darken the setting of the indoor crime scene and use
a low level white light to examine smooth surfaces and even
carpeted areas. What can be detected are small particles,
hairs, fibers and impression evidence. Why this is important
is the scene investigators should always be able to show a
judge or a jury the efforts that were made to search for and
discover evidence at the scene. Unsuccessful attempts to
locate it should always be documented. It looks better to
have searched for and not found than to never have searched
at all.
What can
be detected by using an alternate light source? Included
here are list of items of physical evidence that can
hopefully link the suspect back to the victim or the scene.
1) Latent
Fingerprints: Several surfaces when using simply black
powder techniques will not show enough detail for a latent
print to be of value. Once latent prints have been detected,
they can be enhanced and photographed for comparison.
2) Body Fluids:
Since most body fluids
glow, the alternate light source is a must at crime scenes
where sexual assault may be suspected. Semen, saliva &
vaginal secretions are a few examples, all of which can be
sources of DNA. Blood does not glow but absorbs light
instead and darkens. Again it sometimes becomes important to
eliminate the background to show contrast between what is
glowing and absorbing.
3)
Hairs and fibers: Oblique lighting is the best method on
smooth and carpeted surfaces to detect this type of
evidence. Human hairs do not glow or fluoresce unless they
are silver in color or have been treated with certain types
of dye. Some fibers do glow if they are made up of certain
synthetic values. This type of lighting is also good for
small particles such as soils, glass, paint fragments, etc.
4) Bruises:
Although not well known, bruising beneath the skin caused by
impacts or bites are also visible by the use of a forensic
light. This type of evidence will not glow but absorb or
darken showing the pattern not visible to the naked eye. How
well the bruise shows up depends on how deep beneath the
skin it is. Infrared settings will penetrate the skin for
deeper bruising. The more recent the bruise, the better off
you will be. Often time’s livor mortis becomes very visible
as soon as 4 hours after death, so the sooner used the
better. It works on both living and deceased victims.
Potential SIDS cases or unexplained infant death may reveal
possible bruises around the face area suggesting smothering.
5) Questioned
Documents & Forgeries:
The type of ink or two different pens may be detected by the
use of the ALS. The same blue or black ink to the naked eye
may show obvious differences when viewed with an alternate
light.
6) Gun Shot Residue:
Different wavelengths again can be used based on the type of
ammunition or explosives used. Small particles of gunshot
residue can be detected on clothing, skin and various
surfaces.
Goggles or a filter must
always be utilized when working with an alternate light. The
filter must be such that the visible light cannot be seen
when they are worn. One simple way to check and see if the
correct filter is being used is to place the light source to
the filter and see if the light penetrates. If it does and
the light is visible, a different colored filter must then
be used. The operator will understand through practice what
filter to use with the various wavelengths, and the
appropriate wavelength setting when searching for various
types of evidence.
The following is the
basic order or protocol by which a crime scene should be
approached and processed. It is during two phases of this
protocol where it is appropriate to use an alternate light
source. Keep in mind that after a scene has been made safe,
most crime scenes need only three members for the walk
through and evaluation process. This normally includes two
case investigators and one forensic investigator. As soon as
evaluation is complete, other forensic applications may be
then introduced to the scene.
1) Establishing Scene
Boundaries and Scene Safety.
2) Scene
Evaluation – Alternate Light Source
3) Scene
Documentation
4) Evidence
Collection
5) Search for
Latent and Trace – Alternate Light Source
6) Scene Final
Walk Thru and Release.
Like
DNA, the applications of an alternate light source at crime
scenes are ever changing. Its use in searching for drugs and
its value at arson scenes is also becoming more evident. It
remains a powerful tool that can simplify the work of the
scene investigators and make the detection of trace evidence
a much easier and more efficient task.
Investigator David
Newman
Inside the Tape Homicide Investigation & Crime Scene
Management Training
January 2009