Stephen Mason, an Australian optometrist, has discovered a new way to use scans of people’s tears as passwords which he calls “the world’s first one-time biometric pin”.

He has focused on the cornea, rather than the iris, which is the norm in most optical scanners, because cyber criminals cannot copy the unique way tears change our eyes.

The scanner can recognise a person because each cornea has a unique map. But if a criminal was to steal and try to use the data from the last time someone logged in, the machine would find it invalid because it expects the data to change slightly each time.