Non-parametric score normalization for biometric verification systems

In this paper we study the problem of score normalization in biometric verification systems. Specifically, we introduce a new class of normalization techniques, which unlike the commonly used parametric score normalization techniques, such as z- or t-norm, make no assumptions regarding the shape of the underlying score distribution. The proposed class of normalization techniques first estimates the relevant score distribution in an impostor-centric manner using kernel density estimation and then maps the estimated distribution to a common one. Our experimental results obtained on the FRGCv2 face database show that the proposed non-parametric score normalization techniques consistently outperform their parametric counterparts when the target distribution takes a log-normal form and that all assessed techniques, i.e., z-, t-, zt- and tz-norms, improve upon the setting where no score normalization is used.