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  • Light Sources Help Fight Crime

Photonics Spectra
Apr 2009
David L. Shenkenberg, Features Editor,

Like the actors on the television show CSI, real-life crime scene investigators and policemen often use special light sources to reveal hidden trace evidence.

In fact, forensic light sources can reveal fingerprints, hairs and fibers, bone fragments, chemical accelerants and biological fluids such as urine, semen, blood and saliva. The light sources either make the evidence fluoresce or enhance the contrast of the evidence against the background.

The Rofin Polilight Flare systems are portable flashlights.

According to an article written for Police Magazine by David Spraggs, a detective with the Boulder, Colo., police department, newer LED light sources are becoming more popular, but most law enforcement agencies around the country use xenon lamp-based systems such as CrimeScopes from the Spex Forensics division of Horiba Jobin Yvon, Polilights from the Rofin Forensic division of Rofin Australia and the OmniChrome OmniPrint from CVI Melles Griot.

However, lasers such as the Coherent TracER can reveal more detail than lamps and LEDs. “I felt that the TracER consistently showed me more ridge detail than either lamp or LED forensic light sources,” Spraggs said. “There’s just no comparison,” said C. Jason Guffey, a former CSI (crime scene investigator) who now sells software to CSIs.

Jason Guffey examines crime scenes.

The laser can reveal more trace evidence than a lamp or LED light because the laser emission is brighter and more coherent than those light sources. A CSI operating the 532-nm green laser wears orange goggles that block nonfluorescent light, enabling him or her to see only the fluorescent light. Lasers also can scan a larger area than LED systems.

Pros and cons

Whereas laser emission comes in a single color, the Rofin Polilight Flare LED flashlights come with interchangeable heads of various colors. Likewise, forensic lamp systems such as the CrimeScope emit white light and come with several filters that change the color of the emission. With no filters or heads to change, a CSI using the laser can process a crime scene faster and with little to no experience or training as to which filters to use and which stains to use with those filters or heads.

Lasers have a narrower bandwidth than lamp systems, which improves their ability to help users see trace evidence.

On the other hand, different types of trace evidence will show up more clearly under different wavelengths of light. “That one color [that the laser emits] is pretty damn good, but it’s only one,” said Keith Breeding, business manager for Rofin Forensic in the US.

Coherent has developed prototype TracER systems of various colors. The Home Office Scientific Development Branch, an important national crime lab in the UK, analyzed these various lasers and found that a 577-nm yellow TracER prototype revealed the most fingerprints.

The Coherent TracER laser reveals more detail than lamps and LEDs.

Although experts say that, for revealing trace evidence, the TracER is most effective, lamp systems and LED flashlights have some advantages over lasers. LEDs are the most portable systems because they are flashlights. The disadvantages of the laser are its weight and cost. Although the 50-lb TracER is the lightest forensic laser on the market, it still requires strength to carry, and its $50,000 price tag is cheap for a laser but still a lot of money to consider when the CrimeScope and Polilights can cost a fraction of that amount.

As George Setola, manager of Spex Forensics said, the lasers are the most powerful forensic light source but also the most costly, lamps are moderately priced and more powerful than LEDs, and LEDs are the least expensive.

Spex makes several versions of the CrimeScope, a popular lamp source used in crime labs. The Mini-CrimeScope is more portable and therefore used almost exclusively at crime scenes.

The TracER is a great improvement over the first forensic lasers, argon-ion types that require refills with water for cooling and an extra power supply that must be supported by a high-current three-phase outlet, like the electrical outlet that clothes dryers are plugged into. They can snake along an 8-ft optical table. In other words, they are not even close to being portable.

In contrast, the TracER can be carried in one hand, needs no water source for cooling and can run off a battery as well as through an electrical outlet, and it also is significantly less expensive than the argon-ion lasers. It is superior because it is based on an optically pumped semiconductor chip, which is compact, efficient and can be mass produced relatively cheaply.

There were lasers that came after the argon-ion lasers that were more portable. In 1997, Coherent introduced the Incriminator, which weighs 100 lb, and, in 2005, Spectra-Physics introduced the 82-lb Reveal thin-disk diode-pumped solid-state laser system.

Case studies with the laser

Guffey and Marcus Noble, who markets products on behalf of Coherent, noted that the latent fingerprints involved in the death of NFL star Sean Taylor, which happened in Miami, were revealed by the use of a TracER laser.

In an article that appeared in Forensic Magazine, Guffey described a child rape case in which he recovered hair, a semen stain and a fiber. The hair was partially bleached, or two-toned, which matched only one occupant of the house, whose fate was sealed later when DNA extracted from the semen matched the suspect perfectly. “The end result was a guilty plea, and the perpetrator is now serving a significant jail sentence,” he wrote.

At a breaking and entering crime scene, he used the laser to scan a broken window that appeared to be the point of entry. He found bloody prints on a black plastic trash bag that appeared to have been discarded hastily. The forensic evidence enabled a successful conviction.

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