A Survey of Multibiometric Systems
  Most biometric systems deployed in real-world applications are unimodal. Using unimodal biometric systems have to contend with a variety of problems such as: Noise in sensed data; Intra-class variations; Inter-class similarities; Non-universality; Spoof attacks. These problems have addressed by using multibiometric systems, which expected to be more reliable due to the presence of multiple, independent pieces of evidence. 
Harbi AlMahafzah and Maen Zaid AlRwashdeh
cs.CV