Scaleable, Compact, High Resolution Optical Fingerprint Reader

• US 20120092474A1
• Filed: 10/17/2010
• Published: 04/19/2012
• Est. Priority Date: 10/17/2010
• Status: Active Grant

• Current Assignee: Southwest Research Institute
• Original Assignee: Southwest Research Institute
• Inventors: Mitchell, Joseph N., McFadden, Michael J.
• Granted Patent: US 8,564,653 B2
• Time in Patent Office: Days
• US Class Current: 348/77
• CPC Class Codes: G06V 40/1324 by using geometrical optics, e.g. using prisms G06V40/1312 takes precedence

Abstract

A fingerprint reader for acquiring a fingerprint image has a platen with a micro prism sheet on its underside. An arrangement of fold mirrors is used to receive light from the platen and fold the optical path twice before reaching the imagers. The reader has multiple imagers, each imager having at least an imaging lens and an image sensor. Each imager is configured so that the image plane of the image sensor is tilted relative to the optical axis of the imaging lens, such that keystone distortion in the fingerprint image is corrected.

Claims

1. A fingerprint reader for acquiring a fingerprint image, comprising:

a housing, generally open at the top, and having two sides, two ends, and a bottom;

a transparent platen that fills the top of the housing;

wherein the platen has a flat side facing out of the housing and a micro prism sheet side facing into the housing;

one or more fold mirrors in the housing configured to receive light transmitted through the platen;

at least one imager, wherein each imager is configured to receive light from a portion of the platen via an associated fold mirror or mirrors;

wherein each imager has at least an imaging lens and a image sensor, the imaging lens having an optical axis and the image sensor operable to detect light incident on an image plane; and

wherein the imager is configured so that the image plane of the image sensor is tilted relative to the optical axis of the imaging lens, such that keystone distortion in the fingerprint image is reduced.

2. The reader of claim 1, wherein the optical axis of the imaging lens is substantially parallel to the platen and the image plane is tilted back.

3. The reader of claim 1, wherein the reader has multiple imagers and each imager acquires a sub-image of the fingerprint image, each sub-image representing a portion of an active area of the platen.

4. The reader of claim 3, wherein the platen is inscribed with fiducial marks outside the active area.

5. The reader of claim 4, wherein at least some of the fiducial marks are visible to more than one imager.

6. The reader of claim 3, wherein a portion of each sub-image overlaps an adjacent sub-image.

7. The reader of claim 1, wherein the imager sensor is mounted on a circuit board, and the circuit board is tilted.

8. The reader of claim 1, wherein the reader has four imagers, one each for imaging a quadrant of the platen.

9. A method of correcting keystone distortion in a fingerprint image acquired by a prism-based fingerprint reader, the fingerprint reader having at least a platen, a prism or prism sheet, fold mirrors, and at least one imager, comprising:

receiving light transmitted through the platen and reflected by the fold mirrors at the imager;

wherein each imager has at least an imaging lens and an image sensor, the imaging lens having an optical axis and the image sensor having an image plane; and

tilting the image plane relative to the optical axis.

10. The reader of claim 9, wherein the imager sensor is mounted on a circuit board, and the circuit board is tilted.

11. The reader of claim 9, wherein the reader has multiple imagers, each receiving light from a different area of the platen.

12. A fingerprint reader for acquiring a fingerprint image, comprising:

a housing, generally open at the top, and having two sides, two ends, and a bottom;

a transparent platen that fills the top of the housing;

wherein the platen has a flat side facing out of the housing and a micro prism sheet side facing into the housing;

one or more fold mirrors in the housing configured to receive light transmitted through the platen;

a set of multiple imagers, wherein each imager is configured to receive light from a portion of the platen via an associated fold mirror or mirrors;

wherein each imager has at least an imaging lens and a image sensor, the imaging lens having an optical axis and the image sensor operable to detect light incident on an image plane.

13. The reader of claim 12, wherein the fingerprint reader has four imagers, such that each imager receives light from an associated quadrant of the platen.

14. The reader of claim 12, wherein each imager receives a sub-image that overlaps the sub-image of one or more of the other imagers.

15. The reader of claim 12, wherein the platen is inscribed with fiducial marks outside the active area.

16. The reader of claim 15, wherein each imager receives light from a fiducial mark that is also viewed by at least one other imager.

17. The reader of claim 12, wherein each imager has an associated optical path from the platen to the imager, and wherein the fold mirrors are configured such that the optical path of an imager crosses the optical path of another imager.