Retinal scans use infrared light to survey the unique pattern of blood vessels of the retina, which is the nerve tissue that lines the back of the eye.
The first company to exploit the idea, developed in the 1950s, was Eydentify, founded by Robert Hill in 1976. Hill was an electrical engineer who happened on the idea of using retinal scans as a form of identification when he was helping his father, an ophthalmologist, detect eye disease through photographs.
There is little chance that retinal patterns can be replicated or forged. Retinal scans are therefore considered to be among the least violable of biometric security measures. (Fingerprints, by comparison, are relatively easy to forge.)
Nevertheless, retinal scanners are not at present good candidates for widespread use. First they are expensive. Second, in order to work, users must permit light beams to be shone directly into their eyes for 10 to 15 seconds. The sensation is unpleasant and intrusive enough to make widespread acceptance among the general public unlikely. Additionally, diseases such as cataracts can cause the retina to change over time.