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法医光源的应用

发布时间:2016-06-16 00:00:00      发布人:   浏览量:92

Forensic Light Source Applications

The value that a forensic light source will bring to your department will be measured by the significant increase in the number of suspects apprehended and successfully prosecuted.

What is a Forensic Light Source?

A forensic light source is a crime scene investigator's and lab technician's tool for enhancing observation, photography and collection of evidence including latent fingerprints, body fluids, hair and fibers, bruises, bite marks, wound patterns, shoe and foot imprints, gun shot residues, drug traces, questioned documents, bone fragment detection, etc.. It provides more sensitivity than traditional methods thus increasing the amount of evidence uncovered and the quality of the evidence photographed and collected.

A forensic light source is made up of a powerful lamp containing the ultra-violet, visible and infrared components of light. It then filters down the light into individual color bands (wavelengths) that enhance the visualization of evidence by light interaction techniques including fluorescence (evidence glows), absorption (evidence darkens), and oblique lighting (small particle evidence revealed).

Utilizing forensic light source techniques allows the latent print to be detected with much more sensitivity (10-100 times more!) than the conventional method of black powder dusting and lifting.

Latent Fingerprint Detection

The primary application of a forensic light source is for enhancing the detection of latent fingerprints. The use of fluorescent enhancement processes that compliment a light source greatly increases the types of surfaces from which a latent fingerprint can be detected. Consider the difficulties of dusting and lifting a print off of the following surfaces: thin plastic bags, rigid duct tape, thin aluminum foil, heavily grained wood, concrete wall, brick, printed glossy magazine pages, paper products, etc. Using traditional methods, fingerprint evidence on these and other types of surfaces may go undetected or even dismissed because they could not be detected with enough detail. Forensic light source techniques have been successfully utilized for revealing latent prints on these and many other types of textured surfaces, backgrounds which mask ridge detail, fragile surfaces, and contaminated surfaces. Different color bands (wavelengths) are required for processing different types of surfaces making a forensic light source with tunable or multiple color bands (wavelengths) a coveted tool for any crime scene investigator. In many cases the background surface will also glow under light source illumination. In these cases it is necessary to tune to a color band (wavelength) of light that causes the print to glow and not the background. The quality and quantity of evidence revealed is proportional to the output power and the extent of color tunability of the light source. This ability is exclusive to a forensic light source, UV lights or Blue lights cannot offer this selectivity due to their limited number of color bands (wavelengths) and low power.

Since fluorescent techniques are very sensitive, only trace amounts of fluorescent powder are required when dusting for prints at a crime scene. It therefore leaves the scene much cleaner than when using black powder.

Body Fluids

Since body fluids like semen, saliva, and vaginal fluids are naturally fluorescent, the use of a light source offers a unique method for locating them. A crime scene investigator can narrow down the specific locations of stains for collection instead of testing entire, large pieces of evidence such as a mattress, a carpet, a sheet, an article of clothing, etc.. The dried body fluids will actually glow under the light source illumination. Although the body fluids will fluoresce under an ordinary UV black light, many articles on which you would find them including clothing and sheets will also glow and deter their detection. It is therefore necessary to tune to visible color bands (wavelengths) to eliminate the background interference. Considering you will be searching for body fluids on high profile, capital crime cases, the more body fluid evidence you can reveal the better. Again, the more powerful and more tunable your light source, the more evidence you will uncover. Although blood does not glow in the visible range, it has a unique color band (wavelength) under which the blood stain will darken to enhance its contrast by approximately 4 times. This is most effective in photographing blood prints because more of the detail of the blood print will be revealed by the enhanced contrast.

Many background surfaces glow under UV light and therefore a simple UV Black light will not yield the quality and the quantity of evidence that can be achieved with a tunable or multiple color band forensic light source.

Hair & Fibers

Two light illumination methods can be employed to locate hair and fibers with a forensic light source. First, oblique or parallel lighting of a surface such as a floor or carpet with a strong white light (the more powerful the better) will reveal small particles like hair and fibers for collection. Second, some hair and fibers will also glow under UV or visible light and will stand out strongly for collection. For the best chance of collecting the maximum number of hair and fibers at the crime scene, you need a powerful light source that offers strong White light illumination as well as strong UV and visible color bands (wavelengths); all of which are available only in a forensic light source.

A blue filtered flash light is limited by a single blue color band and even more severely by the limited power output of a flash light.

Bruises / Bite Marks / Pattern Wounds

A tunable or multiple color band (wavelength) forensic light source reveal bruise and patterned wound details that are invisible under normal white light illumination. Details of a bruise pattern in a suspects palm can link a suspect to a weapon. Furthermore, details of a bruise on a victim, for instance, a bite mark or a shoe mark, can link a suspect to the victim. Multiple color bands (wavelengths) are necessary because different colors penetrate to different depths within the skin and therefore depending on the depth of the bruise or wound you will need to vary the color band (wavelength) of the instrument. Deep wounds may require infrared illumination to get enough skin penetration.

Only a forensic light source gives you the versatility of UV, visible, and IR color bands for enhancing bruise or wound pattern detail.

Questioned Documents

Inks have different formulations, even within the same apparent color type. A tunable forensic light source can be used to identify slight variations in ink type by viewing ink responses as the color of the light is tuned through the visible and infrared regions. Regardless of the skill of the forger, this examination would reveal that 2 different pens were used on the document.

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