The retina scanner, a machine that scans the web of tiny blood vessels in the retina, stores information about the pattern formed by the blood vessels  This information allows it to recognize any pattern it has previously scanned  No two eyes have identical patterns of blood vessels in the retina
A retina scanner can ttherefore be used successfully to determine for any person whether it has ever scanned a retina of that person before
diseases of the human eye do not alter the pattern of blood vessels in the retina in ways that would make the pattern unrecognizable to the retina scanner