Unknown Fingerprints (Commonly Referred to as Latent Prints)
Fingerprints are the most common form of physical evidence at crime scenes. Conversely, in some instances, items touched by an individual may not produce latent fingerprints. Fingerprints are extremely fragile and susceptible to damage, so caution needs to be exercised to prevent damage or loss through carelessness. The components of latent fingerprints make them extremely fragile. Latent fingerprints are deposited from a transfer medium in the form of perspiration exuded through the pores and deposited on the friction ridges. Perspiration consists of 95 to 98 percent water and 2 to 5 percent of other chemicals in the body such as chloride, amino acids, and lipids. Most fingerprints developed at crime scenes are partial fingerprints. That fact does not lessen the strength of the evidence. So, whenever fingerprints are developed at the crime scene, it is necessary to obtain elimination fingerprints from all of those who had a legitimate purpose at that scene. What type of fingerprints may be found at a crime scene?
- 1. Latent Print: A chance impression caused by perspiration through the sweat pores on the friction ridges of the skin being transferred to another surface. (In the strictest sense, latent means hidden, and the prints must be chemically enhanced to make them usable and identifiable.) A short note regarding the placement of latent prints is in order at this time. One must consider various factors involved that will dictate if a sufficient latent print will be deposited on a surface. Three things must be present for a quality latent print to be deposited, a sufficient transfer medium, a suitable substrate or surface, and appropriate handling of the object. Oftentimes, a true latent print may be enhanced through the use of reflective or oblique lighting. This process will sometimes cause the latent to glow somewhat to the point of being visible. The necessary development technique, such brush and powder, may then be applied to render the print usable.
- 2. Visible/Patent Print: An impression caused by ridge detail being transferred in blood, grease, or other foreign substance adhering to the surface or substrate.
- 3. Plastic Impression: An impression resulting from the pressure of the print into a soft surface such as wax, paint, putty, and the like. Ridges will be reversed. The valleys will actually be the ridges, and vice versa. These prints should be photographed or cast and the entire item(s) should be taken as evidence.
Note: Plastic and patent prints are visible to the naked eye and need no enhancement.
What information can be revealed from the presence of fingerprints at the crime scene? What are the limitations of latent fingerprints?
The age of the placement of a latent fingerprint may only be estimated by relationship to other events such as washing, known handling, weather, and the like. The darkness or ready development of a latent print will not be indicative of the age of the print. The latent print itself will not yield the age, sex, or race of the person who deposited the print. An educated estimate may be made by investigators based on their knowledge, training, and experience. An occupational endeavor may affect the appearance of a latent print. Various other conditions can affect not only the deposit but also the appearance of latent prints. General guidelines are listed below. But remember, for every rule, there are always exceptions.