False acceptance, also called a type II error, is a mistake occasionally made by biometric security systems. In an instance of false acceptance, an unauthorized person is identified as an authorized person.
Obviously, false acceptance is an undesirable event. One of the most important specifications in any biometric system is the false acceptance rate (FAR). The FAR is defined as the percentage of identification instances in which false acceptance occurs. This can be expressed as a probability. For example, if the FAR is 0.1 percent, it means that on the average, one out of every 1000 impostors attempting to breach the system will be successful. Stated another way, it means that the probability of an unauthorized person being identified an an authorized person is 0.1 percent.
Compare false rejection.