Optimum sweeping of a crime scene with an ALS requires an experienced operator or some evidence may be missed. These systems are not suited for use by “first on scene” law enforcement personnel for example, as they usually don’t have the training or experience.

Plus using all these filters takes time and patience. With any light source, sweeping an extended scene, such as entire bedroom, the illumination from the handpiece is usually set to a spot size of a few inches. The operator sweeps the spot side to side, slowly moving forward after each lateral sweep. It can take many minutes (even hours) to sweep a room in this grid-like manner. Specifically, most operators automatically start with the ALS set for blue output and view through an orange filter. This will generally reveal reasonable fluorescence images of strongly fluorescent materials, such as body fluids, combustion accelerants, and dyes, albeit with less clarity than a laser. The experienced operator may then choose to optimize the ALS to another filter before capturing digital images. This may be accompanied by treating the evidence with dusting compounds. For example, crystal violet is usually needed to find and lift bloody prints, but these can still be hard to see on dark surfaces with an ALS.

Unfortunately, all these single color sweeps will miss trace evidence with very low fluorescence, such as hairs and fibers. So as an absolute minimum, the operator must follow up with a white light sweep of the scene without the use of a viewing filter. Here the goal is to use full color vision to spot evidence based on reflections, scatter, edge contrast, or anything else that enables the operator to better see the evidence. To the uninitiated, this is like spotting a tiny reflective object by catching a view at the right angle. Patience and experience are key elements in this.

With a laser however, the super bright and single color illumination means that even very weakly fluorescent materials and objects can be spotted in the same sweep used to look for fluids and so forth. This means one single sweep with the laser will retrieve the same and often more evidence than multiple filter and white light sweeps with an ALS system. (For example, the laser can readily spot a red hair on a red carpet, which would otherwise be virtually impossible to see.)

This is a revolutionary step forward for several reasons. First, the entire scene can be fully processed much faster than ever before. Just as important, there are no decisions to be made on color selection and filters. Any semi-skilled operator can don the goggles, turn on the laser with a single button, and perform the same level of CSI study as a seasoned expert. In conjunction with the economic pricing of these new lasers (they are now comparable to a top of the line ALS), the author believes this has significant implications for trace evidence recovery. In the past, an expert would be called in to process a crime scene, which often limited their use to serious crimes or well-funded law-enforcement departments. For serious crimes in small towns, considerable time elapses before an expert can be brought in from a neighboring or federal agency. In the future, more and more departments will have a portable laser, ready to go immediately after a crime has been reported.

From: Laser Technology: Revolutionizing CSI Work by Christopher J. Guffey