Forensic Fingerprint Identification – Part I

A. History and Background

Fingerprint comparison is the most venerable of forensic identification tools in use today. The admission of fingerprint comparison testimony indicating that an unknown fingerprint left at a crime scene matches that of a suspect was first approved in the United States in 1911 in the case of People v Jennings (Ill 1911) 96 NE 1088. In just a few years, fingerprint comparison became a widespread forensic identification technique in the United States. In the spread of forensic identification technique in the United States. In the past 10 years, however, in nearly every state, the scientific basis of fingerprint comparison has been challenged in Daubert and Frye hearings. The majority of courts have upheld the 100 years of precedent but without responding to the Daubert issues raised.

B. Terminology and Definitions.

§9.2 1. Properties of Fingers

The surface skin of fingers, palms, and feet consists of intricate patterns of ridges and grooves that develop between that 11th and 20th week of fetal gestation. This patterned skin, called “friction ridge skin” (because it assists with grasping), arises as a combination of genetic and random environmental factors.

Fingers are three-dimensional, their outer skin is flexible and stretchy, and along the ridges of their skin are irregularly spaced pores that exude substances loosely referred to as sweat. Oil in skin combines with these substances so that when the friction ridges of a finger touch a surface, they are likely to leave behind an image of themselves known as a fingerprint.

§9.3 2. Fingerprints

“Fingerprints” are images of the friction ridge skin on fingers. Fingerprint evidence introduced in court is based on images produced by fingers. These images are two-dimensional and therefore distort the patterns of three-dimensional fingers in much the same way that a map of earth distorts the shape of countries found on a globe. The patterns of friction ridge are further distorted by changes in direction and pressure each time the flexible skin of a finger touches a surface. As a consequence of the strechiness of the skin, no two touches made by the finger are ever identical. It is impossible to lay one image of a finger on top of a second image of the same finger and find a perfect overlap.

Fingerprint images may be separated into two types: exmeplars (known prints acquired under controlled conditions) and latent (unknown prints, typically left inadvertently). The finger that made the print is called the “donor finger.”

§9.4   a. Exemplars

Exemplar fingerprint can be recorded on a standard paper card using either ink or chemicals or, digitally, using a scanning device. A scanner captures images of the fingers when the fingers are pressed lightly against a glass plate. Saferstein, Criminalistics: An introduction to Forensic Science, chap 16 (10th Ed 2011). Each fingerprinting method is intended to capture the maximum amount of clear detail of the friction ridge patterns in the most information-rich part of a finger’s surface. FBI, Taking Legible Fingerprints, available at http://ww.fbi.gov?hq/cjisd/takingfps.html (visited May 31, 2011).

With either method, first each finger in succession is rolled from one side to the other creating so-called “rolled” impressions, and then all five digits from one hand and then the other hand are pressed down simultaneously to create a “plain” or “flat” impression. Rolling the finger captures more detail across the surface of the friction ridge skin but creates distorted images of detail rich centers of the fingers and are used to verify the sequence and accuracy of the rolled impressions. When all ten fingers are imaged on the same fingerprint card, the collective image is called a “ten-print.”

The Morales Law firm would like to thank Scientific Evidence in California Criminal Cases for sharing this information with us.