Today, much of the public know about DNA evidence, because of shows such as CSI. Many jurors who know about DNA evidence have unrealistic expectations regarding the production of such evidence in court. But, there are many other types of evidence, which, while not as “sexy,” can be instrumental in placing a person at the scene of a crime. One of these is fingerprint evidence.
Fingerprints can be found on many types of surfaces. Basically, prints are divided into three main categories determined by the type of surface on which they’re found: porous, non-porous, and semi-porous. All result from the human body’s production of various secretions, such as sweat and oils, on the skin.
Prints deposited on a surface when materials such as dirt, blood, or ink have adhered to the skin because of the natural oils found there, they’re called patent prints. Not surprisingly, they can be readily seen. But, if prints are simply the result of the body’s own oils being deposited on another surface, they’re called latent prints, which aren’t easily observed. In order to discern them, they must be processed, which can be done by utilizing various materials such as dusting powders, lasers, alternate light sources, and chemical reagents, such as acetone, ethanol and petroleum ether. As one would expect, the smoother a surface is, the more likely it is that latent prints can be detected and processed.
Prints are collected in a variety of ways, with the simplest being photography. Patent prints are usually photographed with a measuring device, or forensic measurement scale, to show their relative size. Prints that aren’t readily observable can be shown with greater clarity by employing the use of dusting powders, which adhere to oils deposited on a surface by the skin. Once prints are made visible with dusting powder, they can be photographed and “lifted,” with tape. The tape is fastened to a card, which is kept as physical evidence. But, since dusting powders can also contaminate print evidence, very often investigators turn to alternate light sources before using powders. Alternate light sources can be LEDs or lasers. And, another technique often used before turning to dusting powders is the use of super glue (cyanoacrylate) and fumes, which will adhere to any prints left on an object. These can then be seen with specific light sources.
Of course, procedures must be followed and best practices employed in order to obtain prints that can be used as evidence. However, even if procedures are scrupulously carried out and excellent evidentiary specimens are obtained, the integrity of the chain of custody must be maintained. And, this is where the importance of evidence keeping procedures and practices comes into play. Because prints are photographed, increasingly, they’re kept as digital evidence. Therefore, an excellent digital evidence management system is paramount.