• Zack Kowalske

Alternative Light Sources: The Mechanics and Application within Forensic Investigations

Identification of Evidence

The successful investigation and prosecution of a criminal case can hinge upon the quality of evidence presented. Thus, the identification and collection of all available physical evidence contained within a crime scene is crucial. However, quite often it is not as easy as photographing and packaging an expended cartridge case. Latent evidence, physical material left behind by a perpetrator that may not be readily visible, often yields the most critical information. This may include fingerprints, trace hairs and fibers, or biological fluids, such as urine, seamen, or blood evidence potentially posing DNA evidence. Alternative Light Sources (ALS) are forensic investigation instruments, which when in application project various wave lengths of light to yield latent trace and biological evidence in criminal investigations.

Dynamics of Light in CSI

An Alternative Light Source “provides more sensitivity than traditional methods thus increasing the amount of evidence uncovered and the quality of the evidence photographed and collected” (Horiba Scientific, n.d.). A Forensic Light Source employs the application of a powerful lamp that filters light into individual color bands (wavelengths) contained within the ultra-violet, visible, and infrared spectrums. Through utilization of the light interaction techniques including fluorescence, light absorption, and oblique lighting the identification of stains and trace evidence may be determined (Spex Forensics, n.d.). This works off of the principal that when the various wavelengths are emitted upon the material, the material will either absorb or reflect that specific wavelength, and when viewed through a color spectrum filter the material will fluoresce or darken.

Applications within Investigations

Latent Fingerprints: Instruments such as the Reflective Ultra-Violet Imaging System (RUVIS), can be implemented in a crime scene search to locate undeveloped latent fingerprints. This is accomplished by casting short wave UV light upon a suspected area, in which the light reflects off of the material of the prints and the RUVIS device filters the reflected light identifying the detail of the reflection. Further wavelengths may be used to enhance the detail of chemically processed prints, such as those exposed to ninhydrin, cyanoacrylate, or small particle reagent. Tech such as RUVIS has evolved to applications such as the Full Spectrum Imaging System by Arrowhead Forensics or the Fuji XT1-IR Mirrorless camera. I use the Fuji in case work with incredible results.

Body Fluids: Biological fluids such as blood, urine, saliva, and semen are naturally florescent fluids, thus the use of an ALS can be optimal in locating bodily fluids. “Fluorescence is a two-step process: first, light is absorbed; then light emission follows, instantaneously…where the organic material itself absorbs the light and re-emits it at a longer wavelength.” (Masters, 2006). With this, ALS can be used to locate stains, and presumptive field test kits can then be employed to identify the substance.

Trace Evidence: Trace evidence can be difficult to detect due to the lack of substantial material to observe. “More correctly, there may be enough evidence to see, but the image is so weak that it is overwhelmed by ambient background light…Thus, the key to finding and imaging trace evidence is by maximizing contrast in order to enhance the visibility of the evidence relative to the background surface or scene.” (Masters, 2006). Therefore by casting an ALS over an area suspected of yielding trace evidence, the filter eliminates all background light, allowing only visualization of trace evidence fluorescing the emitted ALS color band.

Blood and GSR: As the examiner climbs into the infrared wavelength of the spectrum, the power of alternate light sources is really absorbed. With the advancement of imaging systems, such as the Fuji XT-1 IR Camera or Arrowhead Forensic’s FSIS, investigators can utilize the IR spectrum for the visualization of gunshot residue patterns and bloodstain patterns that would have otherwise been latent to the unaided eye. Through casework examples these systems have shown a portability to any scene or the addition to the 21st century lab.


As is always the greatest downside within public safety and investigation fields, is that of pricing. Most quality forensic light sources will cost an agency the upwards of a few thousand dollars, and that is just the LED lighting, once you breach into the world of laser technology the price tag becomes closer to fifty thousand. The other downside can be the bulk of the instruments, the size of a briefcase, not exactly as ease as a flashlight. And with that in mind, there are compromises that can be made, such as small LED flashlight which emit singular wavelengths. That being stated, an alternative light source and an assortment of filters to cover the light spectrum is an invaluable asset to the modern day criminal investigator, as it shines light on evidence that would otherwise have been unseen.


Horiba Scientific. (n.d.). Forensic light source applications. Retrieved from

Masters, A. (2006, December 01). New lasers speed evidence recovery. Forensic Magazine, Retrieved from

Spex Forensics. (n.d.). Forensic light source applications: Wavelengths and uses. Retrieved from

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