January 2009

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


Training can also be adjusted to meet the specific need of a requesting agency or association. We conduct one and two day training courses for both coroner and medical examiner associations for annual re-training and other state agencies having a specific training need in the field of homicide investigation.


          | Home | About Us | Contact Us | Classes | Calendar | Host A Class |