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Master Thesis - Department of Computer Science

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can address the problem <strong>of</strong> noisy sensor data, but all other potential problems<br />

associated with unimodal biometric systems remain.<br />

• Single Biometry, Multiple Instances: The same biometric unit is acquired<br />

several times by a same sensor and combined to complete and improve the<br />

recognition process (e.g. multiple face images <strong>of</strong> a person obtained under dif-<br />

ferent pose/lighting conditions).<br />

• Single Biometry, Multiple Units: The same biometric, but different units<br />

are acquired and combined to complete and improve the recognition process<br />

(e.g. left and right iris images). This is a recognition system that works on<br />

multiple units <strong>of</strong> the same biometric measurements (e.g. left middle finger<br />

followed by a right thumb).<br />

• Single Biometry, Multiple Representations: The same biometric is ac-<br />

quired once by a single sensor and different approaches <strong>of</strong> feature extraction<br />

and matching are combined to complete and improve the recognition process<br />

(e.g. multiple face matcher like PCA and LDA).<br />

• Multiple Biometrics: Different biometrics <strong>of</strong> the same person are acquired<br />

and combined to complete and improve the recognition process (e.g. face,<br />

fingerprint and iris). This approach is the only well-used multimodal biometric<br />

fusion scenario.<br />

Although the first four methods improve the recognition performance <strong>of</strong> a system,<br />

they still suffer from some <strong>of</strong> the problems faced by unimodal systems. A multimodal<br />

systems based on different traits seems to be more robust to noise, address the prob-<br />

lem <strong>of</strong> non-universality, provide reasonable protection against spo<strong>of</strong> attacks and also<br />

improve matching accuracy.<br />

2.3.3 Fusion Levels and Methods <strong>of</strong> Integration<br />

Fusion in multimodal biometric systems can take place at three major levels, namely,<br />

sensor level, feature level and decision level. These three levels can be broadly clas-<br />

sified into fusion prior to matching and fusion after matching [108]. Decision level<br />

fusion can be divided into three classes based on the type <strong>of</strong> the output <strong>of</strong> biometric<br />

35

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