Large depth-of-field imaging system and iris recogniton system

• US 8,594,388 B2
• Filed: 02/29/2008
• Issued: 11/26/2013
• Est. Priority Date: 04/06/2007
• Status: Expired due to Fees

• Current Assignee: Global Bionic Optics Ltd
• Original Assignee: Fm-Assets Pty Ltd.
• Inventors: Mathieu, Gilles
• Primary Examiner(s): STREGE, JOHN B
• Application Number:
  US12/450,488
• Publication Number:
  US 20100110275A1
• Time in Patent Office: 2,097 Days
• Field of Search: 382/117, 382/298
• US Class Current: 382/117
• CPC Class Codes: A61B 3/14 Arrangements specially adapted for eye photography apparatus or arrangements for taking photographs per se G03B
• CPC Class Codes: A61B 5/1171 based on the shapes or appearances of their bodies or parts thereof
• CPC Class Codes: G02B 13/20 Soft-focus objectives
• CPC Class Codes: G02B 27/0025 for optical correction, e.g. distorsion, aberration
• CPC Class Codes: G02B 27/0075 with means for altering, e.g. increasing, the depth of field or depth of focus
• CPC Class Codes: G06V 40/19 Sensors therefor

Abstract

(A2) An extended depth of field (DOF) imaging system (10) is disclosed that has a corresponding extended depth of focus (DOF′) by virtue of its optical system (20) having a select amount of spherical aberration. The imaging system has an image processing unit (54) adapted to process the raw images and perform contrast enhancement to form processed images. The image processing includes restoring the defocused modulation transfer functions (MTFs) using a gain function (G) and the amount of defocus. The imaging system can include an illumination system (60) that illuminates the object being imaged to establish a distance (DH) between the optical system and the object, where distance DH is used in the restoring of the MTF. An iris-recognition (I-R) system based on the enhanced DOF imaging system is also disclosed.; Optical system embodiments for use in the DOF imaging system that can provide select amounts of spherical aberration—and thus select increases in DOF—without increasing the adverse impact of other aberrations on image formation are also disclosed.

Claims

1. An iris-recognition (I-R) system for capturing images of one or both irises of a person over a large depth of field (DOF), comprising:

an illumination system adapted to i) illuminate at an imaging wavelength λ

_IM a facial region of the person, wherein the facial region includes one or both eyes and at least a forehead portion of the person, and ii) to form a spot on the forehead portion;

an optical system having an object plane within the large depth of field DOF, an image plane within a correspondingly large depth of focus DOF'"'"' with an image sensor arranged thereat, and an amount of spherical aberration (SA) at an imaging wavelength λ

_IM such that the depth of field DOF increases by an amount between 50% and 500% as compared to the optical system being diffraction limited, the optical system configured to form on the image sensor an image of the facial region when the facial region is placed within the depth of field DOF, wherein said facial region image includes an image of the forehead spot; and

a controller electrically connected to the image sensor and to the illumination system, the controller adapted to control and coordinate the operation of the illumination system and the image sensor, and adapted to perform image-processing on the facial-region image to form an enhanced image of one or both irises based on an enhanced modulation transfer function (MTF) formed using forehead distance information obtained from the forehead spot.

2. The I-R system of claim 1, wherein the amount of spherical aberration SA satisfies at least one of the following relationships:

i) 0.2λ

_IM≦

SA ≦

5λ

_IM;

ii) 0.2λ

_IM≦

SA≦

0.9λ

_IM; and

iii) 0.2λ

_IM≦

SA≦

0.75λ

_IM.

3. The I-R system of claim 1, wherein the imaging wavelength λ

_IM satisfies the relationship 700 nm≦

λ

_IM≦

900 nm.

4. The I-R system of claim 1, wherein the image sensor is adapted to form an electrical signal SRI representative of the raw facial image formed on the image sensor.

5. The I-R system of claim 4, wherein the controller includes an image-processing unit adapted to receive and process the electronic signal SRI to form an enhanced image of one or both irises that has a higher contrast than the raw image contrast.

6. The I-R system of claim 5, further including a database unit electronically connected to the image processor and adapted to retrievably store on a computer-readable medium at least one enhanced-contrast image.

7. The I-R system of claim 1, wherein the illumination system includes a first light source adapted to illuminate the facial region, and a second light source adapted to form the forehead spot.

8. The I-R system of claim 1, wherein the optical system consists of a single optical element and an aperture stop located at a position different from the single optical element.

9. The I-R system of claim 8, wherein the single optical element is a plano-convex lens wherein the plano surface faces objectwise.

10. The I-R system of claim 8, wherein the aperture stop is arranged imagewise of the lens in a position that minimizes comatic aberration.

11. The I-R system of claim 1, wherein the optical system includes an afocal or substantially afocal front lens group.

12. The I-R system of claim 1, wherein the optical system is configured so that the amount of spherical aberration can be adjusted by adjusting one or more surface radii of a single optical element in the front lens group.

13. A method of forming an enhanced image of at least one iris of a person, comprising:

forming a raw image of a facial region of the person that includes at least one eye of the person and a forehead portion of the person using an optical system having an amount of spherical aberration that increases a depth of field (DOF) of the optical system by an amount between 50% and 500% as compared to the optical system being diffraction limited;

using an image sensor, electronically capturing the raw image to form a digitized raw image, the raw image having a corresponding raw modulation transfer function (MTF);

establishing an amount of defocus in the digitized raw image based on a position of the person relative to the optical system including determining the amount

of defocus by forming a spot formed on the person'"'"'s forehead portion and establishing the amount of defocus based on the location of the spot in the raw image;

forming an enhanced MTF from the raw MTF by multiplying the raw MTF by a gain function, wherein the enhanced MTF is a function of the amount of defocus; and

applying the enhanced MTF to the digitized raw image to obtain the enhanced image.

14. The method of claim 13, wherein the optical system consists of a single optical element and an aperture stop.

15. The method of claim 13, including increasing the DOF by up to an additional 20% by forming the single optical element from a glass type that adds chromatic aberration and while only reducing the average MTF by no more than about 25%.

16. The method of claim 13, including forming the single optical element from fused silica.

17. The method of claim 14, wherein the single optical element is one of a) a plano-convex lens, with its plano surface facing objectwise, b) a meniscus lens and c) a biconvex lens.

18. The method of claim 17, wherein the aperture stop is arranged imagewise of the lens in a position that minimizes comatic aberration.

19. The method of claim 13, wherein the optical system includes an afocal or a substantially afocal front lens group.

20. The method of claim 19, wherein the optical system is configured so that the amount of spherical aberration can be adjusted by adjusting one or more surface radii of a single optical element in the front lens group.

21. The method of claim 13, further including:

illuminating the face portion with a first illumination beam that forms an illumination region corresponding to the face portion and a second illumination beam that forms that spot.

22. The method of claim 13, further including capturing a number N of raw images and analyzing said raw images to establish whether one or both eyes had eye motion and/or eye blinking.

23. The method of claim 22, including tracking said eye motion and accounting for said eye motion to identify regions of said image sensor to be used for forming the enhanced image.

24. The method of claim 22, including averaging the N images so as to reduce noise in the enhanced image.

25. The method of claim 13, including:

measuring a distance between the person and the optical system; and

wherein forming the enhanced MTF includes applying a two-dimensional linear digital filter to the raw MTF according to the measured distance.

26. The method of claim 24, including measuring the raw MTF at different sampled distances that overlap the depth of field DOF by using a slanted edge with uniform incoherent backlighting.

27. An enhanced depth of field (DOF) imaging system for

forming an enhanced contrast image of an object, comprising:

an optical system having an amount of spherical aberration (SA) at an imaging wavelength λ

_IM such that the depth of field DOF increases by an amount between 50% and 500% as compared to the optical system being diffraction limited, the optical system configured to form an image of the object at an image plane when the object is within the DOF and at a distance away from the optical system;

an image sensor located at the image plane and adapted to form an electrical signal representative of the image as a raw image;

a controller electrically connected to the image sensor and adapted to receive the electrical signal and perform image-processing on the raw image to form an enhanced image using an enhanced modulation transfer function (MTF) formed based on said object distance further including an

illumination system operably coupled to the controller and adapted to form a mark on the object that shows UP in the raw image, and wherein the controller is configured to determine the object distance based on a position of the mark in the raw image.

28. The imaging system of claim 27, wherein the controller is adapted to average N enhanced images to form a final enhanced image having reduced noise as compared to any of the N enhanced images.

29. The imaging system of claim 27, wherein the optical system consists of a single optical element and an aperture stop located at a position different from the single optical element.

30. The imaging system of claim 29, wherein the single optical element is one of a) a plano-convex lens wherein the plano surface faces objectwise, b) a meniscus lens, and c) a biconvex lens.

31. The imaging system of claim 29, wherein the aperture stop is arranged imagewise of the lens in a position that minimizes comatic aberration.

32. The imaging system of claim 27, wherein the optical system includes an afocal or substantially afocal front lens group.

33. The imaging system of claim 32, wherein the optical system is configured so that the amount of spherical aberration can be adjusted by adjusting one or more surface radii of a single optical element in the front lens group.

34. The imaging system of claim 27, wherein the mark is a spot.

35. The imaging system of claim 27, wherein:

the image sensor is adapted to switch between a low-resolution and a high-resolution image-capture mode; and

wherein the controller is configured to track object motion by processing the low-resolution-mode raw image.

36. The imaging system of claim 35, wherein the object motion tracking is performed by locating at least one peak of a correlation function of the low-resolution-mode raw image relative to a reference location.