First, the iris and the retina are not synonymous.
Second, while the iris can be used for biometric identification, so can the retina. People are identified by their blood vessels in their eyes. But there are complications, according to the Biometrics Institute:
“Retina recognition is one of the most accurate biometric applications but a number of common eye conditions and diseases (for example, cataracts, diabetes, glaucoma) can affect the arrangement of the blood vessels and consequently alter the pattern used for biometric recognition.”
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