Biometric devices identify people by their physical characteristics. Some examples include fingerprint scanners and other devices that can recognize a particular voice, hand, or signature. The subject of science fiction for decades, the retinal scanner is a recent addition to this family of machines. It is a shoebox-size device that can recognize the unique pattern of blood vessels in the back of each person's eye. The pattern is so complex that even identical twins do not have the same blood vessel configuration. Its proponents claim it has an error rate of only one in a million.

A retinal scanner uses infrared light for its mapping. As a person looks into the eyepiece, an invisible beam of low-energy infrared light traces a circular path on the retina, located at the back of the eye. Because blood-filled capillaries absorb more of the infrared light than the surrounding tissue, there is...