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Abstract

A curved cylindrical segment platen has a concave touching surface. An interrogating slit light beam is scanned around the platen to scan a finger held against the touching surface. An optical imaging system images the zone illuminated onto an array of photo-transducers to provide a signal representing the fingerprint. The optical system is rotated around the platen. The axis of the interrogating light beam is substantially normal to the touching surface. The axis of the imaging system is off normal by about 56 degrees, an amount which is greater than a critical angle equal to the maximum angle of refraction in the platen. This critical angle is approximately 41 degrees. The axis of the imaging system is also off normal to the non-touching surface by an angle that is less than this critical angle of 41 degrees and thus less than the angle of total internal reflection. As a consequence, the image at the photo-transducers provides a completely dark valley zone and a sharply contrasting light image ridge zone. In a second embodiment, a thick flat platen has a flat touching surface and an opposed parallel flat surface through which an illuminating light beam is applied to scan across a finger applied to the touching surface. The illuminating beam is normal to the touching surface. The axis of the imaging system is at an angle of approximately 50 degrees to the normal, an angle that is thus well past 41 degrees, the maximum angle of refraction. The platen has a side face which is at an angle of 45 degrees to the touching surface. The imaging axis emerges from this sloping side face at an angle to the normal that is substantially less than the angle of total internal reflection.

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First Claim