General Description of Fingerprints

General Description of Fingerprints

The general classification of fingerprints used today came from the work of Sir Edward Henry, who published his book, Classification and Use of Fingerprints , in 1900. This work forms the basis for modern-day fingerprint forensics. Fingerprints are identified by both macro and micro features. The macro features of a fingerprint include:

• Ridge patterns

• Ridge pattern area

• Core point

• Delta point

• Type lines

• Ridge count

The micro features of a fingerprint are made up of minutia points. Minutia points are classified by:

• Type

• Orientation

• Spatial frequency

• Curvature

• Position

Let's examine the macro features, then the micro features of a fingerprint.

Macro Fingerprint Features

Macro fingerprint features are, as the name implies, large in size (Figure 5-1). In general, a feature is considered macro if it can be seen unaided by the human eye. The most visible macro feature seen is the ridge pattern. Others can be seen if the print has good ridge/valley definition, the lighting is good, and your eyesight is excellent !

Figure 5-1. Ridge patterns.

Ridge patterns

• Arch ” Arches account for approximately 5% of the ridge pattern in a given population. Arches are different from loops in that arches are more open curves. Arches can also form a subgroup called tented arches. In a tented arch, the arch angle is much more obtuse than in a normal arch.

• Loop ” Loops account for approximately 60% of the ridge patterns in a given population. Loops may slant left or right, or be presented as a double loop. A double loop has both a left and right loop, conforming to each other's outline.

• Whorl ” Whorls account for approximately 35% of the ridge patterns in a given population. Whorls are defined by at least one ridge making a complete circle.

Ridge pattern area

The ridge pattern area is the area in the print where all the macro features are found (Figure 5-2). It is normally defined by diverging ridge flows that form a delta.

Figure 5-2. Ridge pattern area.

Core point

The core point is found at the center of the finger image (Figure 5-3). It may or may not correspond to the center of the ridge pattern area. It is used as a reference point for measuring other minutia and also during classification. Classification is the organizing of prints based on their ridge pattern.

Figure 5-3. Core point.

Delta point

"The Delta is the point on the first ridge bifurcation , abrupt ending ridge, meeting of two ridges, dot, fragmentary ridge, or any point upon a ridge at or nearest the center of divergence of two type lines, located at or directly in front of their point of divergence . It is a definite fixed point used to facilitate ridge counting and tracing." ^[1] In Figure 5-4, the delta point has been magnified.

^[1] J. Edgar Hoover, Classification of Fingerprints (Federal Bureau of Investigation, Department of Justice, U.S. Government Printing Office), 1939.

Figure 5-4. Delta point.

Type lines

Type lines are the two parallel innermost ridges that define the ridge pattern area. In Figure 5-5, the type lines are the two slightly darker ridge lines in the enlarged section.

Figure 5-5. Type lines.

Ridge count

Ridge count is the number of ridges that intersect a line drawn from a delta to the core. There could be more than one ridge count for each finger image. For each delta in the finger image, there will be a corresponding ridge count between it and the core. In Figure 5-6, the ridge count from the delta to the core is 12 ridges.

Figure 5-6. Ridge count.

Micro Fingerprint Features

As the name implies, micro fingerprint features cannot be seen unaided by the human eye. A number of the current fingerprint scanners on the market now have a high enough resolution that pores can be counted. What follows is a description of the minutia that make up the micro features:

• Type

• Orientation

• Spatial frequency

• Curvature

• Position

Type

There are a number of different types of minutia; the common ones are:

• Ridge ending

• Ridge bifurcation

• Ridge divergence

• Dot or island

• Enclosure or lake

• Short ridge

Ridge ending

A ridge ending is a gap in a ridge or a point where a ridge suddenly stops (Figure 5-7).

Figure 5-7. Ridge ending.

Ridge bifurcation

A ridge bifurcation occurs when a ridge splits into two or more new ridges (Figure 5-8).

Figure 5-8. Ridge bifurcation.

Ridge divergence

A ridge divergence occurs when two ridges running parallel suddenly diverge in opposite directions. In Figure 5-9, the two ridges diverge at the point where another ridge has a bifurcation.

Figure 5-9. Ridge divergence.

Dot or island

A dot or island occurs when a ridge is short enough to be perceived as a single point (dot) or straight line (island). In Figure 5-10, we can see a group of three dots or islands.

Figure 5-10. Dot or island.

Enclosure or lake

An enclosure (lake) minutia occurs when a ridge bifurcates and then rejoins itself. This then leaves a valley surrounded by the rejoined ridge. In Figure 5-11, the ridge has created two enclosures.

Figure 5-11. Enclosure or lake.

Short ridge

A short ridge is a ridge of short length, but not so short as to be considered an island or dot. In Figure 5-12, you can see two short ridges and two dots. Notice the short ridges have more of a linear look to them. The dots have more of a circular look to them.

Figure 5-12. Short ridge.

Orientation

Orientation refers to the general direction in which a minutia feature appears to be moving. In Figure 5-13, both scaled areas contain bifurcations, but their orientation is different. The bifurcation in the left-hand enlarged area would have a general slope of approximately 1. The bifurcation in the right-hand enlarged area would have a general slope of -1.

Figure 5-13. Orientation.

Spatial frequency

Spatial frequency can be viewed as the density of ridges around a given minutia point. In Figure 5-14, the spatial frequency is higher around the island at the top right than the bifurcation at the middle left.

Figure 5-14. Spatial frequency.

Curvature

Curvature is the rate of change of a ridge's direction. In the above left enlargement in Figure 5-15, the rate of change is lower, as the ridge curves are flatter. In the bottom right enlargement in Figure 5-15, the rate of change is higher, as the ridge curves are tighter.

Figure 5-15. Curvature.

Position

Position refers to the relative location of the minutia. References are normally made using a Euclidian grid, and having the origin either at the core point or at a delta. In Figure 5-16, the origin has been placed at the core point. The numbers at the end of each axis are at their maximum value. In our example, you can find a bifurcation at (1,-1) and a group of dots at (-1,1.5)

Figure 5-16. Position.