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An article in Nature points at research claiming that human irises are not constant over time and instead change appreciably as we age. In a paper by Kevin Bowyer and Samuel Fenker, they assert that there is a noticeable change between iris scans taken only three years apart.
All iris-recognition systems have some margin of error, because there will always be slight differences between the original iris image — taken to create a digital template when people first enrol in iris-recognition schemes, for example — and those taken later to confirm a person's identity. If irises did not age, the false non-match rate would be expected to remain constant over time, but Bowyer says his results clearly show that this is not the case. “One iris biometric marketing claim has been that the iris allowed ‘a single enrolment for a lifetime’. This claim is now proven to be false,” he says.
The likelihood of software incorrectly matching two irises from different people is around 1 in 2 million (known as the false match rate). So in practical terms, Bowyer’s results suggest that the false match rate for a system would increase to 2.5 in 2 million after three years had elapsed. This rate sounds low, but the effect appears to be cumulative, says Bowyer: “So although you might not really notice the problem after one year or two years, after five or ten years it can become a huge problem,” he explains.
As the article further states, India's Aadhar project (with 200 million enrolments thus far) relies on iris scans as one of its biometric identifiers.