Multisystem Biometric Authentication: Optimal Fusion and UserSpecific Information
Type of publication:  Thesis 
Citation:  normanpoh:phdthesis:2006 
Year:  2006 
School:  École Polytechnique Fédérale de Lausanne 
Abstract:  Verifying a person's identity claim by combining multiple biometric systems (fusion) is a promising solution to identity theft and automatic access control. This thesis contributes to the stateoftheart of multimodal biometric fusion by improving the understanding of fusion and by enhancing fusion performance using information specific to a user. One problem to deal with at the score level fusion is to combine system outputs of different types. Two statistically sound representations of scores are probability and loglikelihood ratio (LLR). While they are equivalent in theory, LLR is much more useful in practice because its distribution can be approximated by a Gaussian distribution, which makes it useful to analyze the problem of fusion. Furthermore, its score statistics (mean and covariance) conditioned on the claimed user identity can be better exploited. Our first contribution is to estimate the fusion performance given the classconditional score statistics and given a particular fusion operator/classifier. Thanks to the score statistics, we can predict fusion performance with reasonable accuracy, identify conditions which favor a particular fusion operator, study the joint phenomenon of combining system outputs with different degrees of strength and correlation and possibly correct the adverse effect of bias (due to the scorelevel mismatch between training and test sets) on fusion. While in practice the classconditional Gaussian assumption is not always true, the estimated performance is found to be acceptable. Our second contribution is to exploit the userspecific prior knowledge by limiting the classconditional Gaussian assumption to each user. We exploit this hypothesis in two strategies. In the first strategy, we combine a userspecific fusion classifier with a userindependent fusion classifier by means of two LLR scores, which are then weighted to obtain a single output. We show that combining both userspecific and userindependent LLR outputs always results in improved performance than using the better of the two. In the second strategy, we propose a statistic called the userspecific Fratio, which measures the discriminative power of a given user based on the Gaussian assumption. Although similar class separability measures exist, e.g., the Fisherratio for a twoclass problem and the dprime statistic, Fratio is more suitable because it is related to Equal Error Rate in a closed form. Fratio is used in the following applications: a userspecific score normalization procedure, a userspecific criterion to rank users and a userspecific fusion operator that selectively considers a subset of systems for fusion. The resultant fusion operator leads to a statistically significantly increased performance with respect to the stateoftheart fusion approaches. Even though the applications are different, the proposed methods share the following common advantages. Firstly, they are robust to deviation from the Gaussian assumption. Secondly, they are robust to few training data samples thanks to Bayesian adaptation. Finally, they consider both the client and impostor information simultaneously. 
Userfields:  ipdmembership={learning}, 
Keywords:  multiple classifier system, pattern recognition, userspecific processing 
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