Abstract

To satisfy the need for fingerprint sensors in security applications, we have developed a compact sensor only 22 mm thick. First, we studied a holographic fingerprint sensor¹ consisting of a flat glass plate with a hologram. Unfortunately, the sensor needed a thermally controlled laser diode for its light source, which is relatively large. Thus, to develop a compact low-cost illuminator, we investigated a compact optics system, using four light-emitting diodes, that does not require a hologram. The focusing optics consists of a flat glass plate attached to the aperture and a short-focal-length lens that shortens the optical path. The optical aberration can be reduced by setting the aperture at the center of the lens curvature, so that only the light incident perpendicular to the lens surface is transmitted. A mirror was fabricated on an incline of the glass plate opposite the lens to internally reflect the light toward the lens. This enabled us to reduce the sensor size. We were able to produce a prototype sensor with a CCD driver and an AD converter that is no larger than a cigarette case (80 by 55 by 22 mm).

© 1991 Optical Society of America