Patterns and Impression Evidence

The Importance of Fingerprints

Today, the fingerprint is the pillar of modern criminal identification. With fingerprints, unequivocal identification is possible and they are useful for two major reasons:

1. Fingerprints are unique to each individual
2. Fingerprints do not change during a lifetime

What are Fingerprints

Fingerprints are the deposition of friction ridges. They appear of palms, soles, and the ends of fingers and toes. The ridges provide friction between the grasping mechanism and whatever it grasps. They have certain features:

• Ridge: a single curved segment
• Furrow: region between two adjacent ridges
• Pore: on the ridges

Fingerprints are produced by:

• The deposition of sweat
• by a finger contaminated with another material
• by a finger that has been pushed into a soft material such as putty

They are the stamp of the ridge detail on the skin’s surface. The age of a fingerprint is almost impossible to determine. A number of paremeters will determine how well a fingerprint is formed:

• The nature of a surface
• The pressure applied
• The time for which pressure is applied

Of all, contact time is the most important.

Types of Fingerprints

Visible Fingerprints

• Made by fingers touching a surface after the ridges have been in contact with colored colored materials (blood, paint, grease, ink, make-up, etc.)
• Patent: meaning obvious or evident; occurs when fingers are contaminated with something
• Plastic: made when a finger touches or presses against a soft-pliabe surface
• Plastic prints remain sas long as the impressed material is structurally intact
• Prints left ina medium, such as blood or dust, are fragile and do not last long

Latent Fingerprints

• Once the finger touches a surface, body persperation and / or oils present on the finger ridges are transferred to that surface, leaving an impression
• Prints deposited in this manner are invisible to the eye and are commonly referred to as latent or invisible fingerprints
• Friction ridge examination is the preeminent method of individualization in forensic science
• Latent prints in a proper environment can last for years

Fingerprint Classification

There are several types of whorls – they are the most complex

• Loop: a type of fingerprint pattern in which the ridge makes a curving path and then recurves back on itself to form a loop shape.
• Whorl: a type of fingerprint pattern in which the ridge makes two complete turns around a central point. Whorls typically have a circular or spiral shape.

In both cases, the patterns are unique to an individual and are used in forensic science for identification purposes.

Loops and whorls contain cores and deltas.

Core: approximation of the center; location of maximum curvature
Delta: point of triangulation; point of divergence

Once general patterns have been linked, it is the fingerprint minutiae that are used to individualize them. There are on average ~150 ridge characteristics, and about ~8 - 16 matching minutae are sufficient for ID.

Unusal fingerprints can be attributed to skin diseases and genetic diseases (naegeli syndrome, dermatopathia pigmentosa reticularis).

Fingerprint Enhancement

Procedure for Developing Latent Prints

Illumination Techniques

• Oblique lighting
• Expose fingerprint to UV or laser light
  • Oils and other foreign oils present in perspiration fluoresce when exposed to UV or laser light
  • Usually done after the application of a dye

Enhancement Techniques

• Enhancement techniques used to increase contrast between mark and background
• Contrast may be in color or brightness
• Techniques work by adding a ‘label’ to some compounds foud in a fingerprint (physical and chemical)
• Can use a number of enhancement techniques is succession

Powder Dusting

• Powders
  • Simplest method
  • Mechanical adherence of powder particles to the sweat components of the friction ridge deposits
  • Color used is determine by background
• Brushes
  • Applied with a soft fiberglass brush that has long, very fine bristles
  • Do not make contact with surface
  • Fiberglass, animal hair (camel, squirrel), synthetic, natural, magnetic

Chemical Enhancement

• Methods of visualizing prints from recovered objects; not conducted at the crime scene
• Process involves exposing the object to fumes
  • Carried out in a vented container
  • Ex: cyanoacrylate (superglue) fuming

Fingerprint Comparisons

ID Fingerprint to Class (Loop, Whorl, Arch) → ID Fingerprint Minutiae (Position, Type, Number, Orientation)

• Friction ridges are studied for the kind, number, and location of various ridge characteristics or minutiae
• Arrangement of characteristics create one-of-a-kind pattern
• A careful point-by-point study determines if enough significant minutiae present in the known print are present in the questioned print

Friction Ridge Analysis

Forensic scientists debate how many points of comparison are necessary and sufficient to reach a conclusion of identification

• Point counting standard dictated how many points of comparison were required before a positive conclusion could be reached (~8 - 16 points, with no statisstical basis)
• Most agencies use a “no-point” standard
  • Threshold is one of a sufficient number of characteristics necessary to make a conclusion of identification, however many that might be
• Experience and judgement are central to the process of quality examination
  • Requires proper training and practical experience
  • Experts must be able to articulate and support their findings
• Majority of prints are identified, resolved and compared are partial prints
  • Only a portion of the complete print pattern is represented
  • Scientist must determine if sufficient information is present to make a proper comparison
• Prints may be unidentifiable due to smudging, graininess, or size, it depends upon a scientist’s (experience, visual acuity, and judgement)

Automated Fingerprinting Identification System (AFIS)

AFIS has been around since the 1960’s. It as revolutionized law enforcement by computerizing fingerprint search and storage system.

Its applications include:

• Searching for a reference / inked prints
• Searching for single prints

It is an important and useful tool, as it places two images side by side and allows the examiner to chart the common features on both images simultaneously