Thesis - Instituto de Telecomunicações

16 CHAPTER 2. STATE OF THE ARTwith an acceptable overall equal error rate (typically below 2%-5%). A soft-biometriccharacteristic is a human trait that alone, can’t serve to produce a biometric system.Examples of soft-biometric traits are the easily collectable body metrics such as height,weight, eye color, skin color, or some other new biometric traits that for intrinsic reasons(the trait does no have sufficient discriminability power [3]) or for immaturity, yet fail topresent good performance.The main causes of failure for the current biometric techniques can be enumerated asfollows [114]:1. Noise in sensed data. In some situations the acquisition process provides low qualitydata from where sufficient relevant data for classification purposes cannot be extracted.In some systems the quality of the acquisition is evaluated and if it is belowa predefined threshold, then the user is not accepted, with a failure to accept error.2. Intra-class variations. Some biometric traits (particularly in behavioral biometrictechniques) present significant differences from one acquisition to another. This canhappen to a specific user or group of users that are more inconsistent and bringdifficulties to the correct classification/verification process.3. Distinctiveness. Some of the biometric traits bear too little distinctive information tobe used in a large population [90].4. Non-universality. Some users fail to present the trait required for a specific biometricsystem.5. Spoof and mimicry attacks. The different systems for biometric identification havepotential weaknesses that can be exploited in the form of a fraudulent attack, by presentinga fake copy of the biometric trait. The term spoof is used in case of a physicalcharacteristic and a mimicry in the context of behavioral biometric system [123].2.2 Biometrics TechnologiesIt is easy for a human being to find characteristics that enable the recognition of anotherperson. Looking at a person’s face, or hearing a known voice, are everyday examples ofhuman identification techniques we use.