Image acquisition system

• US 8,090,246 B2
• Filed: 08/08/2008
• Issued: 01/03/2012
• Est. Priority Date: 08/08/2008
• Status: Active Grant

• Current Assignee: Gentex Corporation
• Original Assignee: Honeywell International Inc.
• Inventors: Jelinek, Jan E.
• Primary Examiner(s): BLACKMAN, ROCHELLE ANN J
• Application Number:
  US12/188,561
• Publication Number:
  US 20100033677A1
• Time in Patent Office: 1,243 Days
• Field of Search: 396/89, 396/18, 396/80, 396/93, 396/102, 396/104, 348/296, 348/345, 351/206
• US Class Current: 396/18
• CPC Class Codes: G06V 40/19 Sensors therefor
• CPC Class Codes: H04N 23/61 based on recognised objects
• CPC Class Codes: H04N 23/611 where the recognised objects include parts of the human body
• CPC Class Codes: H04N 23/673 based on contrast or high frequency components of image signals, e.g. hill climbing method
• CPC Class Codes: H04N 23/959 by adjusting depth of field during image capture, e.g. maximising or setting range based on scene characteristics

Abstract

A system having a sensor and variable focus lens for iris image standoff acquisition. The sensor may capture a sequence of images at a high frame rate of a person for getting an eye or an iris in a window within the images. Even if the eye moves around in the image, the window may stay on the eye. During this capture, the focus of the lens may be changed, with a best focus situated somewhere in between the end focus positions of the lens. The sensor may be an infrared (IR) sensor and an IR illuminator or flash may provide light for the capture of images. An intensity variance indicator may be incorporated to select an in-focus image of the sequence. Processing of the images may be subsequent to the capture of images, thus not hindering the frame rate of the system.

Claims

1. A standoff image acquisition system comprising:

an image sensor;

an optics assembly coupled to the image sensor; and

a processor coupled to the image sensor and the optics assembly;

wherein;

the optics assembly is configured to convey images of a subject suitable for biometric imaging to the image sensor with a varying focus distance while the standoff image acquisition system is disposed at a standoff distance from the subject; and

the processor is programmed to control the image sensor to acquire a sequence of images from the optics assembly of the subject and control the optics assembly to vary the focus distance monotonically during acquisition of the sequence of images.

2. The system of claim 1, wherein at least one image of the sequence of images acquired comprises a window within the at least one images enclosing a biometric target on the subject, the window enclosing an area of the image substantially less than the total area of the image.

3. The system of claim 1, wherein the focus distance of the optics assembly is varied in a continuous manner during the conveying of images.

4. The system of claim 1, the focus distance of the optics assembly is varied in a discrete manner during the conveying of images.

5. The system of claim 1, wherein:

the processor is programmed to determine a best focused image based on spatial frequency content relative to that of other images in the sequence of images; and

the sequence of images is acquired at a frame rate independent of the rate at which the processor processes the images for determining a best focused image in the sequence of images.

6. The system of claim 1, wherein:

a frame period between exposures for images is less than or equal to a depth of field of focus divided by focus velocity; and

the focus velocity is a change in focus distance relative to the optics assembly per unit time.

7. The system of claim 1, wherein:

the image sensor is for capturing infrared images; and

an exposure time of the image sensor for an image is equivalent to a duration of illumination of an infrared illuminator.

8. The system of claim 1, further comprising:

a shutter; and

wherein;

the shutter of the image sensor is electronic; and

an exposure time of the image sensor for an image is less than 100 milliseconds.

9. The system of claim 1, wherein:

an estimate of an in-focus distance of the subject is provided to the optics assembly; and

the focus of the optics assembly is set to vary between a first distance behind and a second distance in front of the estimate of the in-focus distance.

10. A method for iris image acquisition comprising:

providing a sensor for capturing images;

conveying a plurality of images with a lens onto the sensor;

varying a focus with the lens of at least two of the plurality of images;

selecting an image from at least two of the plurality of images having a sufficient focus on a subject; and

providing a window within at least one of the plurality of images to capture an eye target on the subject, wherein the window encloses an area of the at least one of the plurality of images substantially less than the total area of the image.

11. The method of claim 10, wherein:

a rate of processing images is independent of a rate of capturing;

processing images comprises selecting an image out of at least two images of the plurality of images, as having a good focus.

12. The method of claim 10, wherein capturing images comprises providing an illuminator flash for freezing motion of a subject of the images.

13. The method of claim 11, wherein:

an item of the images is at least a portion of a person; and

the sufficient focus is of the iris target within the window in the at least one of the plurality of images.

14. An iris image acquisition system comprising:

a camera configured to acquire a plurality of images; and

a variable focus mechanism coupled to the camera, the variable focus mechanism configured to vary, during the acquisition of the plurality of images, a focus distance of the images;

wherein;

T<sub>F≦

Δ

D/VF</sub>;

Δ

D is a depth of field of focus;

V_F is a velocity at which the focus distance changes during the acquisition of the plurality of images; and

T_F is a frame time between acquisitions of images of the plurality of images.

15. The system of claim 14, wherein:

an estimate of a correct focus distance is determined;

the focus mechanism is set at the estimated focus distance offset by a first distance;

during acquisition of the plurality of images, the focus mechanism is varied from the estimated focus distance plus the first distance to the estimated focus distance minus a second distance; and

the first and second distances are either plus or minus distances.

16. The system of claim 15, wherein during or after the acquisition of the plurality of images, an image having a sufficient focus is selected from the images.

17. The system of claim 16, wherein an image having the sufficient focus is selected according to an image intensity variance of the image.

18. The system of claim 14, wherein:

the images comprise an area of interest on a subject; and

a field of view of the camera is sufficiently larger than that of the area of interest to minimize pointing of the camera to capture an image that includes the area of interest; and

wherein the area of interest is delineated by a window in the image.

19. The system of claim 18, wherein the area of interest comprises an eye and/or an iris.