Patent US 4,428,670

The fingerprint sensor derives an electric output signal in accordance with the topographic relief of the finger under investigation. The sensor incorporates a contact body which is formed at least in part by a light-transparent elastic material. The contact body, preferably a flat sensor plate made of said elastic material and attached to a flat supporting plate, has a light receiving surface and a contact surface for receiving a contact pressure by the finger. Before an investigation process is started, the contact surface is smooth. By pressing the finger against the contact surface, the contact surface will attain a shape conforming to the fingerprint pattern of the finger. The sensor also incorporates a light source, a photodetector and an optical system. The light source obliquely passes light to the contact surface which reflects a light beam modulated in accordance with the fingerprint. The optical system passes the reflected light beam to the light sensitive area of the photodetector. In the optical path between finger and photodetector is included a light stop for blocking reflected light which does not contain fingerprint information.