Single-sensor hand and footprint-based multimodal biometric ...

2 Biometric system design
When designing biometric systems, a number of questions need to be addressed, such as
comparison mode (verification vs. identification), operational mode (automatic vs. semiautomatic),
the selection of one or many modalities according to the needs of the application
and hardware for sensing these modalities [21]. The awareness of the target application
is crucial for the design of biometric systems. In the case of hand- and footprint-based
systems, it is important to notice that evidently their target applications may be quite
different. While multimodal hand-based biometrics perfectly fits into the mainstream of
authentication applications, foot biometrics is too time-consuming in environments where
users have to take off shoes first. Thus, the explicit definition of a target application for
foot biometrics is needed. A good way to identify target applications is to formulate conditions
on application domains. When the domain is found, it can be classified according
to characteristics introduced by Wayman [40] (namely cooperative vs. non-cooperative,
overt vs. covert, habituated vs. non-habituated, attended vs. non-attended, standard vs.
non-standard, public vs. private, and open vs. closed), that further help to justify the
selection of specific features.
The first question arising naturally when dealing with new modalities, such as footprints,
is “Why do we need a new modality?”. There are many accurate techniques and foot
biometry has largely been neglected so far, so the answer to this question is not obvious.
However, when privacy interests of users are considered, footprint-based authentication
might be the right choice, since it is not employed in high-security applications. Second,
foot biometrics is closely related to hand geometry, palmprint and fingerprints. Many of
the biometric techniques using the human hand or parts thereof can be translated to this
new domain. In contrast to hand geometry, a footprint-based identification system may
be implemented in covert mode (i.e. without awareness of the user that the biometric is
taken [40]), if certain prerequisites are met (such as in environments where users walk on
bare feet). It is, in fact, the only known covert biometric without being captured at a
distance. To identify target application domains for footprint-based authentication, the
following prerequisites should be met:
• low throughput should be avoided by choosing environments, where users already
walk on bare feet;
• unhygienic recording conditions should be avoided;
• large space for the sensor has to be provided, i.e. foot biometric sensors can not
be integrated into small devices (such as PDAs, notebooks);
• privacy interests should justify the selection of footprints as the identifier of choice.
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