A CSI professional describes how the latest portable lasers allow even non-experts to find more evidence in less time and generate higher quality data including trace and latent prints, compared to any other technology.

Sweeping crime scenes for various types of forensic evidence is a time and cost sensitive proposition, not least because most crime scenes degrade over time, particularly in exposed or highly trafficked locations, or where evidence is chemically/physically unstable. Visual and photographic scene processing using an ALS (alternative light source) is time consuming work involving multiple separate sweeps using different filters. Moreover, to get the best out of this equipment requires a high level of training and expertise, and is generally unsuited to use by typical “first on scene” personnel. In contrast, the author has now accumulated several months CSI (and lab) time experience with a new type of lightweight, portable green laser. This experience confirms that a single sweep with such a laser can capture all possible evidence types (serological, prints, fibers, accelerants, etc.) with greater sensitivity and efficacy than multiple conventional sweeps with filters. Together with the fact that this new type of laser requires virtually no operational or optimization training, this means that crime scenes can be processed sooner, faster, and more economically than ever before, by even small law enforcement teams with low staffing and budgets. This article discusses how the laser delivers these benefits and shows some crime scene reconstruction data representative of two recent cases—a juvenile rape case as well as a breaking and entering case.

A REVIEW OF FORENSIC LIGHT SOURCES
Special light sources have a long history in CSI forensic work. The intent is to reveal evidence that is not visible to the naked eye under ambient illumination. The same light source may be used to photograph the evidence or just to indicate its location for physical retrieval such as print lifting or recovering hairs and fibers. Such evidence may be “invisible” for several reasons. For instance, there may be only a trace amount, such as a very small drop of blood or a single hair. The material may be colorless, such as a semen stain or trace of fire accelerant. Other difficult examples include prints on reflective surfaces such as aluminum foil.

Light sources can be used to reveal this evidence by either fluorescence or enhanced contrast. The idea in both cases is to enhance evidence visibility and/or render the background darker when the scene is swept with the light source.

Fluorescence occurs when a material absorbs light of one color and then re-emits it virtually simultaneously as a different color. The re-emitted color is always more toward the red end of the color spectrum than the illumination. Because of this, a material can be illuminated in the blue or green and then viewed with filter goggles that only pass orange or red. Thus, the reflected and scattered light from non-fluorescent material is almost completely blocked, allowing the fluorescent objects to glow with enhanced visibility. (Figure1)

The first forensic light source for fluorescent sweeping was the so-called “blue light” which passed light from a bulb through a filter that allowed only ultraviolet (e.g. black light) and violet light to reach the scene. Then came the ALS (alternative light source) which was basically a white light source emitting all through the near-ultraviolet and visible. It was supplied with a few different filters (up to eight are now available) as well as several different viewing filters. These catapulted fluorescence detection into the mainstream, and virtually every law enforcement department got into fluorescence sweeping of crime scenes. More recently these lamp-based systems have been challenged by second-generation ALS systems based on LEDs (light emitting diodes). Each LED in these systems can be turned on or off as needed and emits over a specific color band.

But CSI professionals have long known that the ultimate light source for fluorescent detection is the laser. Their main advantage over the ALS and other sources is shown in Figure 2. Other forensic sources emit a broad range of colors, but all the laser’s intensity is produced at a single color. Since fluorescence detection is based on color selectivity, the end result is that just a few watts of laser power can give much better results than a kilowatt of lamp power. And in the case of faint or particularly difficult situations, such as prints on crumpled aluminum foil, the laser can reveal undeveloped prints that an ALS simply cannot deliver. Until recently however, lasers were too large, heavy, expensive, and altogether too complex, for widespread use in forensics. They also required line power and often a source of cooling water. This situation has completely changed with the advent of a new generation of compact green lasers using OPS (optically pumped semiconductor) technology. These produce intense green light, are hands-free, battery-powered tools, and are as easy to operate as a flashlight. In some models, the battery can even be re-charged from a car cigarette lighter on the way to a crime scene.