Alternate Light Source or a forensic light source are used in crime scene
investigation to identify, photograph and collection of many forms of physical
evidence including latent fingerprints, hair and fibers, bruises, bite marks,
shoe and foot prints, gunshot residues etc. due to their natural characteristic
of light absorption or florescence effect Physiological fluids such as semen,
saliva and urine are identified by using UV light. In any scene of crime the
fast and accurate detection of physical evidence is very
important.
The forensic scientists were used visible light sources for the examination of
crime scenes. Some investigators were used ultraviolet (UV Black Light) at
crime scene but white light was the mainstay. In 1970s the Royal Canadian
Mounted Police (RCMP) added a new dimension. They used argon-ion lasers
to forensic examination of physical evidence, but these expensive, bulky machines
were not transported to the crime scenes, so their use was limited to the crime lab.
In the late 1980s Portable lasers appear in the marketplace. They were capable of
delivering a single wavelength of light. In the 1990s, the development of a high-intensity
incandescent bulb revolutionized alternate light source and resulted in a wider range
of wavelengths, from the ultraviolet range of the light spectrum to the near infrared.
These machines were less than 20 pounds(9 kg) and they were providing an intense
light beam that passed through various filters covering from visible green, blue,
yellow and red light.
An alternate light source consists a laser or incandescent bulb, a filter or combination
of filters that filters down the light into individual color bands (wavelengths) and
enhance the evidence visualization by making evidence glow (fluorescence), evidence
darkens (Absorption) by light
interaction technique.
For different types of surfaces Different wavelengths (color
bands) are required. Sometimes the background surface also glows under light
source illumination. In this case, a wavelength of light is necessary that
causes the processed print to glow (fluoresce) and not the background. The
output power and the extent of wavelength tunability of the light source are
proportional to the quality and quantity of evidence revealed. This is the
ability of a forensic light source; UV lights cannot offer this selectivity due
to its limited wavelength and low power.
Fluorescence is the key source of alternate light. One
wavelength of light absorbs and the emission of light at a longer wavelength
takes place. By using the screening filters emitted light can be detected and
filters block out the other wavelengths of light.
Fluorescence -
Fluorescence is occur when light of a certain frequency and color strikes on an object, and light of a different color and frequency returns by the object. This fluorescence is very weak in intensity as compared to the light beam that created it, for visualizing this fluorescence, we must use filtration that blocks the visible light but passes the fluorescence. The UV light is invisible but it creates visible fluorescence from certain substances and objects. To view UV fluorescence no filtration is required. Clear glass or plastic lenses is recommended for eye protection.
For fingerprints detection, fluorescence is visible without the addition of any powders or dyes. EX- When light (blue-green) from a laser or incandescent source directed towards the organic materials in the fingerprint, it fluoresce yellow. It is difficult to lift prints from surfaces such as thin plastic bags, thin aluminum foil, rigid duct tape, concrete wall, heavily grained wood brick, printed glossy magazine pages, paper products, etc. by using traditional powder technique but by using Forensic Light Source techniques we have successfully seen and lift latent prints on these and many other types of textured surfaces. For processing of different types of surfaces different wavelengths are required and here the Forensic Light Source fulfilled the requirement having tunable or multiple wavelengths. We have faced problem when the background surface glow under light source illumination. To solve this, we need a wavelength of light that makes the print to glow and not the background. This means that different organic materials absorb light and they fluoresce at different wavelengths. So, fingerprint or a bite mark detected on materials is as diverse as on skin, paper, rubber, and cloth fabric. Two things are important first illuminating light is very critical to elicit the maximum fluorescence from the evidence, second the selection of the filter(s) is important.
Under room light Finger and palm prints are invisible on metal
surfaces such as a gun or knife but prints can be seen when illuminated by
green light and observed through orange filter. Apart from that there are fluorescent
chemicals or powders used for latent fingerprint development. These powders
have the ability that makes the latent residue to be fluorescing on a surface. For
developing latent prints on porous surfaces such as paper and cardboard fluorescent
liquids are used. After superglue fuming, fluorescent dyes are used to enhance developed
latent prints
Body fluids - Body fluids saliva, urine stains, vaginal
secretions and semen stains can be detected using alternate light sources. Some
narcotics substances will also fluoresce. Under the light source illumination
dried body fluids will glow. The body fluids will also fluoresce under UV black
light but on many articles like clothing and sheets, these articles will also
glow and hinder the evidence detection. It is advisable to use longer
wavelengths to eliminate the background interference (background rejection). If
your light source is powerful and you have more wavelengths then you will
uncover the more evidence. Blood is not fluorescing under UV or visible blue light;
it has luminesced by spraying some chemicals on it such as BlueStar and luminol.
Hair and Fibers
There are two light illumination methods to locate hair and
fibers by using a Forensic Light Source. Lighting of a surface (floor or carpet)
by applying oblique or parallel strong white light reveals small particles like
hair and fibers. Under UV or Blue light hair and fibers will also glow.
Bruises/Bite Marks/Pattern Wounds
Forensic Light Sources may reveal bruise and patterned wound
details that are invisible under normal white light illumination. Depending on
the depth of the wound or bruise the wavelength of the instrument vary as
different colors penetrate to different depths within the skin. Illumination is
required for deep wounds to get enough skin penetration.
Gun Shot Residues/Explosive Residues
Gun Shot residues/Explosive residues are found in ammunition
and explosives and are readily observed by using the appropriate illumination.
Questioned Documents
Document forgery detected when we used alternate forensic
light sources. Inks have different formulations and composition even within the
same apparent, visible, color type and the ink in contact with the paper will
produce detectable responses as the color of the light is tuned through the
visible and infrared regions. IR sensitive film or a digital camera having IR
sensitivity is required for visualization of evidence in IR lightening.
UV (ultraviolet) lights are used for examination of scene of
crime to reveal evidence not visible to the eye under normal lighting. UV light
reveals hidden blood, fibers and fingerprints, subcutaneous bruises on living
and dead bodies. It’s also used on authenticating paintings, authenticating
signatures, analyzing questioned documents at crime scenes and search for trace
evidence on clothing etc. Blood on dark and patterned clothing is used to
enhance through Infrared photography.
The installation of 1 and 3 watt Light Emitting Diodes (LEDs)
in small, hand-held flashlights further advanced evidence recovery and their
performance is remarkable. Precautions are always needed as Exposure to
Alternate Light Source of radiation can result in serious, and sometimes
irreversible, eye and skin injuries. Doesn’t look directly into the light,
always wear protection (filters) on your eyes.