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Biometrics refers to methods for uniquely recognizing humans based upon one or more intrinsic physical or behavioral traits. Biometric characteristics can be divided in two main classes:

Physiological are related to the shape of the body. Examples include, but are not limited to fingerprint, face recognition, DNA, hand and palm geometry, iris recognition, which has largely replaced retina, and odor/scent.

Behavioral are related to the behavior of a person. Examples include, but are not limited to typing rhythm, gait, and voice. Some researchers have coined the term behaviometrics for this class of biometrics. Strictly speaking, voice is also a physiological trait because every person has a different vocal tract, but voice recognition is mainly based on the study of the way a person speaks, commonly classified as behavioral.

It is possible to understand if a human characteristic can be used for biometrics in terms of the following parameters:

• Universality – each person should have the characteristic.
• Uniqueness – is how well the biometric separates individuals from another.
• Permanence – measures how well a biometric resists aging and other variance over time.
• Collectability – ease of acquisition for measurement.
• Performance – accuracy, speed, and robustness of technology used.
• Acceptability – degree of approval of a technology.
• Circumvention – ease of use of a substitute.

A biometric system can operate in the following two modes:

• Verification – A one to one comparison of a captured biometric with a stored template to verify that the individual is who he claims to be. Can be done in conjunction with a smart card, username or ID number.
• Identification – A one to many comparison of the captured biometric against a biometric database in attempt to identify an unknown individual. The identification only succeeds in identifying the individual if the comparison of the biometric sample to a template in the database falls within a previously set threshold.

The first time an individual uses a biometric system is called an enrollment. During the enrollment, biometric information from an individual is stored. In subsequent uses, biometric information is detected and compared with the information stored at the time of enrollment. Note that it is crucial that storage and retrieval of such systems themselves be secure if the biometric system is to be robust. The first block (sensor) is the interface between the real world and the system; it has to acquire all the necessary data. Most of the times it is an image acquisition system, but it can change according to the characteristics desired. The second block performs all the necessary pre-processing: it has to remove artifacts from the sensor, to enhance the input (e.g. removing background noise), to use some kind of normalization, etc. In the third block features needed are extracted. This step is an important step as the correct features need to be extracted in the optimal way. A vector of numbers or an image with particular properties is used to create a template. A template is a synthesis of the relevant characteristics extracted from the source. Elements of the biometric measurement that are not used in the comparison algorithm are discarded in the template to reduce the filesize and to protect the identity of the enrollee. If enrollment is being performed the template is simply stored somewhere (on a card or within a database or both). If a matching phase is being performed, the obtained template is passed to a matcher that compares it with other existing templates, estimating the distance between them using any algorithm (e.g. Hamming distance). The matching program will analyze the template with the input. This will then be output for any specified use or purpose (e.g. entrance in a restricted area).

Biometric verification is any means by which a person can be uniquely identified by evaluating one or more distinguishing biological traits. Unique identifiers include fingerprints, hand geometry, earlobe geometry, retina and iris patterns, voice waves, DNA, and signatures. The oldest form of biometric verification is fingerprinting. Historians have found examples of thumbprints being used as a means of unique identification on clay seals in ancient China. Biometric verification has advanced considerably with the advent of computerized databases and the digitization of analog data, allowing for almost instantaneous personal identification.

The following are used as performance metrics for biometric systems:

•  False accept rate or false match rate (FAR or FMR) – the probability that the system incorrectly matches the input pattern to a non-matching template in the database. It measures the percent of invalid inputs which are incorrectly accepted.
• False reject rate or false non-match rate (FRR or FNMR) – the probability that the system fails to detects a match betweenthe input pattern and a matching template in the database. It measures the percent of valid inputs which are incorrectly rejected.
• Receiver operating characteristic or relative operating characteristic (ROC) – The ROC plot is a visual charactization of the trade-off between the FAR and the FRR. In general, the matching algorithm performs a decision based on a threshold which determines how close to a template the input needs to be for it to be considered a match. If the threshold is reduced, there will be less false non-matches but more false accepts. Correspondingly, a higher threshold will reduce the FAR but increase the FRR. A common variation is the Detection error trade-off (DET), which is obtained using normal deviate scales on both axes. This more linear graph illuminates the differences for higher performances (rarer errors).
• Equal error rate or crossover error rate (EER or CER) – the rate at which both accept and reject errors are equal. The value of the EER can be easily obtained from the ROC curve. The EER is a quick way to compare the accuarcy of devices with different ROC curves. In general, the device with the lowest EER is most accurate. Obtained from the ROC plot by taking the point where FAR and FRR have the same value. The lower the EER, the more accurate the system is considered to be.
• Failure to enroll rate (FTE or FER) – the rate at which attempts to create a template from an input is unsuccessful. This is most commonly caused by low quality inputs.
• Failure to capture rate (FTC) – Within automatic systems, the probability that the system fails to detect a biometric input when presented correctly.
• Template capacity – the maximum number of sets of data which can be stored in the system.. As the sensitivity of the biometric device increaes, the FAR decreases but the FRR increases.