It's used in high security applications like air traffic control rooms and automated passport gates.
The eye looks into an infrared camera.
The infrared light reflected back is captured by a CCD.
You must position yourself correctly since otherwise the image can't be taken. The camera can focus automatically to compensate for this.
The light photons are converted into electrical signals corresponding to how much colour has been captured. These are sent to a ADC so the computer can retrieve the binary codes for the colours of the image.
An algorithm is applied to an image locating the areas of the eye, such as the centre of the pupil, the edge of the pupil, the edge of the iris and the eyelids and eyelashes. The algorithm ensures that only the iris is used for validation. A code is created typically of 512 bytes. This is compared to a series of iris codes stored in a database.
Practical: the chance of mistaking an iris code for another is 10^78:1
It's also significantly fast, much more so than a retina scanner.
However, people can fake the scan by using contact lenses.
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