IMAGE ACQUISITION SYSTEM

• US 20100033677A1
• Filed: 08/08/2008
• Published: 02/11/2010
• Est. Priority Date: 08/08/2008
• Status: Active Grant

• Current Assignee: Gentex Corporation
• Original Assignee: Honeywell International Inc.
• Inventors: Jelinek, Jan
• Granted Patent: US 8,090,246 B2
• Time in Patent Office: Days
• US Class Current: 351/208
• 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; and

an optics assembly coupled to the image sensor; and

wherein;

the optics assembly is for conveying images of a subject to the image sensor with a varying focus; and

the image sensor is for acquiring a sequence of images from the optics assembly of the subject during the varying focus by the optics assembly.

2. The system of claim 1, further comprising a window within the mages for capturing a target on the subject.

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

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

5. The system of claim 1, wherein:

the sequence of images is acquired at a frame rate;

the frame rate is independent of the rate at which the sequence of images is processed for determining a best focused image in the sequence of images; and

the determining a best focused image is a based on a level of contrast or more generally, spatial frequency content relative to that of other images 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 a 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 the images to capture a target on the subject.

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 target within the window in the image limited primarily to an eye of the person.

14. An iris image acquisition system comprising:

a camera; and

a variable focus mechanism coupled to the camera; and

wherein;

the camera is for acquisition of images; and

the variable focus mechanism is for varying a focus distance of a subject during the acquisition 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 an acquisition 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 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:

T_F≦

Δ

D/V_F;

Δ

D is a depth of field of focus;

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

T_F is a frame time between acquisitions of images.

19. The system of claim 14, wherein:

the images comprise an area of interest on the 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 which includes the area of interest; and

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

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