READ: Fingerprints

Fingerprint Identification Systems

Fingerprints are one of the best types of individual evidence because they are totally unique to each person. No two people have the exact same pattern of minutiae in their fingerprints, so it is very important that any fingerprints found at a crime scene are detected and analyzed! While a bloody patent print may be easy to see, latent prints can be quite difficult to find without special tools! Let's go over a few of the most common fingerprint detection and development techniques.

Latent Prints Methods

Powders - About 70% of fingerprint detection at crime scene is done using this method. Fingerprint powders come in a variety of colors but black is the most common for most light colored surfaces. On a darker surface, a lighter or fluorescent colored powder may be used for optimal detection. A fine brush is used to lightly dust an area with the fingerprint powder. Once on the surface, the powder reacts and sticks to any oils left behind. This method works best on hard, non-absorbent surfaces such as glass, tile, mirrors, and painted wood. Prints can then be "lifted" using clear tape. The developed print will be the same color as the applied powder

Cyanoacrylate Vapor - Commonly referred to as super glue fuming, this method is used on plastics, metals, glass, and skin (of cadavers). The object with the suspected fingerprint is placed into a fuming chamber with a small amount of cyanoacrylate. The cyanoacrylate is heated to release a vapor. The vapor adheres to the oil left behind by the fingers as they touched the object. Once dried, the print looks like a white or light colored three dimensional fingerprint on the object. This method is often successful when other attempts to obtain a latent print have been unsuccessful. The print is generally observable with the naked eye.

Ninhydrin - Ninhydrin is used to develop latent prints on paper, tissue or clothing. The object is sprayed, dipped, or "painted" with ninhydrin and left to dry for about 24 hours before the print can be visualized. The developed print will be purple due to a reaction of the Ninhydrin to amino acids and salts in the sweat and oil left behind on the paper. Once the Ninhydrin prints develop the characteristic purple color, they can be photographed for documentation.

Silver Nitrate - If a fingerprint is present on a wood or Styrofoam object, it can be visualized after it is sprayed with silver nitrate. When exposed to ultraviolet (UV) light, silver nitrate reacts with the salt in sweat to form a black/brown compound. The print will have a brown appearance when visualized under an ultraviolet (UV) light.

This type of fingerprint development is best used on porous surfaces, like paper or drywall.

Iodine Fuming - Iodine crystals are heated and through sublimation turn from solid to vapor (gas) without entering into a liquid phase. The vapor then adheres to the fingerprint and makes it visible in a characteristic brown colored print. This method will develop a latent print left on paper, cardboard, or an unpainted surface, however, the print will only be visible for a short period of time. When using this technique the print must be photographed as soon as possible.

Alternate Light Sources - Latent fingerprints can sometimes be located and visualized using alternate light sources such as UV light or special fluorescent lighting. Once they are visualized with the alternate lighting source, they are photographed and further processing such as dusting with fingerprint powder may then be completed.

Patent Print Methods

Photography - All patent prints are visible and are therefore photographed and documented visually as thoroughly as possible.

Amido Black - Bloody fingerprints found on a body are usually enhanced with Amido Black. This is a stain that reacts with any protein it encounters. Investigators use this stain to enhance bloody fingerprints on a body for better documentation and further analysis.

Plastic Print Methods

Photography - Because they are visible, plastic prints are generally photographed and collected for submission to the crime lab for further analysis when possible.

Fingerprint Analysis Process: ACE-V

Forensic Scientists follow a specific process in order to analyze fingerprints known as ACE-V. It is an acronym which represents the ordered steps in the process: Analysis, Comparison, Evaluation and Verification.

1) Analysis: The first step is to analyze the prints for comparison qualities. If a print is incomplete, too faint or does not display enough minutiae points to compare to another fingerprint, it is dismissed as unsuitable. If a fingerprint is found to be suitable for comparison to another fingerprint, it is documented with the most significant minutiae, as well as any scars or other markings which can help to identify the print. Whereas the Henry Classification System was once the main way in which fingerprints were differentiated for documentation, today they are are classified by pattern and minutiae points on each print within a massive fingerprint database maintained by the FBI known as IAFIS.

2) Comparison: A print from a person of interest, person from the crime scene or victim is compared to known prints to verify any matching sets. The examiner will look for matching minutiae points or other identifying characteristics. Matches can also be made through searching computer databases of fingerprints such as IAFIS.

3) Evaluation: This is the point in the process where the examiner decides if the prints match, do not match or are inconclusive. A print may be inconclusive for any of several reasons such as a poor quality print or an incomplete print without enough minutiae points.

4) Verification: In this step, the prints are evaluated by another examiner to verify the findings of the first examiner. If both concur, the prints are finished with the analysis process. If both examiners do not agree on the print, the process must be repeated or the fingerprint will be considered inconclusive and unusable.

An example of an exclusion; the print on the left does not match the known print on the right:

An example of how minutiae points are matched in a fingerprint during comparison and evaluation:

Georgia Virtual, Fingerprints, CC BY-NC-SA 3.0