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The iris is the coloured part of the eye located behind the cornea. The hole at its centre is the pupil. By reflex, it contracts and expands to let in more or less light, like the diaphragm of the camera. The colour of the iris is partly determined by a pigment called melanin, which is responsible for hair colour too.
The iris has a unique pattern of tiny blood vessels. The colour of the iris can change over time but not these characteristic patterns. They are unique not only to each person, even twins, but also to each eye. Some biometrics systems use iris recognition to identify people and control access to areas or to information. It is not, however, a very widespread method because iris image acquisition today can be somewhat inconvenient and is not always well accepted by users. Producing a correct analysis requires users to keep still with their eye very close to the sensor during acquisition.