Thesis - Instituto de Telecomunicações

2.4. MULTIMODAL BIOMETRICS 312.4 Multimodal BiometricsThe fusion of several biometrics sources to produce a classification, called multibiometrics,is a trend that is promoting performance increase in the area [27]. The modalities thatare being used in combination are typically the current hard-biometric traits [64, 30]. Thegradual increments in performance of current biometric techniques have been minor. Thereasons are mainly related to the quantity of information a biometric trait has [181, 47].Even if new, and more robust algorithms appear, they tend to require new and more expensivehardware to be able to perform only for a marginal performance gain. The currenttrend is to capture more information from the user in order to fuse the multimodal formatsacquired from the user, using the hard-biometric traits with new soft-biometrics traits [102].One of the first attempts on using more than one biometric trait for user authenticationwas performed by Brunel and Falavigna [30].Information fusion can be performed in several points in the information path of abiometric system [204]:1. Feature extraction, features from several sources are combined to provide more informationto the classifier/matcher. Although it is considered the best strategy, it candamage the performance via the dimensionality increase of the feature vector (see thepaper on the “curse of dimensionality” [12, 115]).2. Matching Score, the score of each of the biometric systems is combined via an informationfusion rule such as the product or sum of all the scores [203].3. Decision, each system proposes a decision and a majority vote is used to produce thefinal decision.This multimodal approach is more robust to spoofing, given that the user has to beprepared to present several distinct traits in a singular order specified by the system. Thepreparation to present several examples of finger prints in an unimodal fingerprint systemis easier to break than a system that asks for several finger prints, the voice, the palm andagain a fingerprint.