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The common procedures now employed for proving one’s identity—the use of passwords, personal identification numbers, picture IDs and so forth—have given way in many situations to automated biometric analysis. One such system takes advantage of the detailed patterns within a person’s iris, which make it possible to identify someone using nothing more than an infrared image of the eye and a suitably programmed computer to process the information. The author describes how his algorithm for iris recognition discerns whether two images taken at different times are of the same iris. His scheme encodes iris patterns compactly, so that comparisons can be made extremely quickly and tests against large numbers of candidate images can be performed in a reasonable time when searching for a match.