Advances in Fingerprint Technology.pdf

finger gets mapped onto the two-dimensional surface of the glass platen.
Because the finger is not a rigid object and because the process of projecting
the finger surface onto the image acquisition surface is not precisely controlled,
different impressions of a finger are related to each other by various
transformations. The most problematic of these projections appears to be
elastic distortions of the friction skin of the finger that displaces different
portions of the finger (ever so slightly) by different magnitudes and in different
directions (see Figure 8.14).
Non-uniform Contact
The ridge structure of a finger would be completely captured if ridges belonging
to the part of the finger being imaged are in complete physical/optical
contact with the image acquisition surface and the valleys do not make any
contact with the image acquisition surface (see Figure 8.6). However, the
dryness of the skin, shallow/worn-out ridges (due to aging/genetics), skin disease,
sweat, dirt, and humidity in the air all confound the situation, resulting
in a non-ideal contact situation. In the case of inked fingerprints, an additional
factor may include inappropriate inking of the finger; this results in “noisy,” lowcontrast
images, which leads to either spurious or missing minutiae.
Irreproducible Contact
Manual work, accidents, etc. inflict injuries to the finger, thereby changing
the ridge structure of the finger either permanently or semi-permanently.
Further, each impression of a finger may possibly depict a different portion
of its surface. This may introduce additional spurious fingerprint features.
Feature Extraction Artifacts
The feature extraction algorithm (see, for example, “Minutiae Feature Extraction”
section) is imperfect and introduces measurement errors. Various image
processing operations might introduce inconsistent biases to perturb the
location and orientation estimates of the reported fingerprint structures from
their gray-scale counterparts.
Sensing
The act of sensing itself adds noise to the image. For example, in the livescan
fingerprint acquisition method, residues from the previous fingerprint
capture may be left behind. A typical imaging system geometrically distorts
the image of the object being sensed due to imperfect imaging conditions.
In the Frustrated Total Internal Reflection sensing scheme (see “Fingerprint
Sensing” section), for example, there may be a geometric distortion because
the image plane is not parallel to the glass platen.