Finger imaging apparatus using curved platen

In a fingerprint processing apparatus having an interrogating light beam for interrogating a finger having ridge and valley zones to provide a reflected reading light beam and having imaging means including photo-electric transducers coupled to said reading light beam, to provide an electrical signal at the transducers representative of a fingerprint, the improvement comprising:

• a curved platen positioned in the interrogating light beam, said platen having a first curved surface as a touching surface, a second curved surface as a non-touching surface spaced from said touching surface, a finger placed on said touching surface being illuminated by said interrogating light beam, the optical axis of said interrogating light beam having an internal angle to the normal at said touching surface that is less than the maximum angle of refraction for said platen,
• the optical axis of said imaging means having a first internal angle to the normal at said touching surface and a second internal angle to the normal at said non-touching surface, said first angle being greater than the maximum angle of refraction for said platen and said second angle being less than the angle of total internal reflection for said platen,
• the axis of said interrogating light beam intersecting said non-touching surface.

The method of optically imaging a finger held against the surface of a platen onto a photo-electric transducer comprising the steps of:

• scanning an interrogating light beam across a finger held against a concave curved touching surface of a curved platen, said light beam having an angle to the normal within said platen at said curved touching surface which is less than the critical angle, to provide a modulated reflected light beam which emerges from a convex curved non-touching surface of the platen,
• imaging the modulated reflected light beam onto the photo-electric transducer along an axis that has an angle to the normal within said platen at said touching surface which is greater than the critical angle and an angle to the normal within said platen at the non-touching surface which is greater than the critical angle.