From Halopedia, the Halo wiki
A retinal scanner is a human security device which scans the retina of an eye for identity verification and authentication. Individuals attempting to acquire confidential information or gain access to a secured locale must often undergo a retinal scan and a fingerprint or voiceprint scan. They are utilized aboard UNSC starships, space stations, and in various other military complexes. They are also commercially available and can be equipped on the doors of residential homes.
List of appearances