Method and system for determining optical distortion in a transparent medium

• US 7,423,673 B1
• Filed: 03/18/2005
• Issued: 09/09/2008
• Est. Priority Date: 03/18/2004
• Status: Active Grant

• Current Assignee: Elbit Systems Limited
• Original Assignee: Elbit Systems Limited
• Inventors: Efrat, Ilan, Tell, Eliyahu, Shamir, Hanan
• Primary Examiner(s): Hannett; James M
• Application Number:
  US11/084,820
• Time in Patent Office: 1,271 Days
• Field of Search: 348/222.1
• US Class Current: 348/222.1
• CPC Class Codes: G02B 2027/011 comprising device for correcting geometrical aberrations, distortion
• CPC Class Codes: G02B 2027/0138 comprising image capture systems, e.g. camera
• CPC Class Codes: G02B 2027/014 comprising information/image processing systems
• CPC Class Codes: G02B 27/01 Head-up displays
• CPC Class Codes: G06T 7/80 Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
• CPC Class Codes: H04N 23/81 for suppressing or minimising disturbance in the image signal generation
• CPC Class Codes: H04N 5/33 Transforming infrared radiation cameras or camera modules for generating image signals from infrared radiation H04N23/20; circuitry of SSIS for transforming infrared radiation into image signals H04N25/20

Abstract

System for determining a distortion model for an optically distorting medium, the system including an image sensor located at an image acquisition position behind the optically distorting medium, and an image processor coupled with the image sensor, the image sensor acquiring an obstructed image of an object located substantially at infinity, when the optically distorting medium is located between the image sensor and the object in a line of sight connecting the image sensor and the object, the image sensor acquiring an unobstructed image of the object, when the optically distorting medium is absent from the line of sight, the image processor receiving information respective of the obstructed image and the unobstructed image from the image sensor, the image processor identifying a plurality of features in the unobstructed image, and a respective feature in the obstructed image for each of the identified features in the unobstructed image, thereby determining pairs of associated features, the image processor determining a distortion parameter for each of the pairs of associated features, the image processor determining a reference distortion model for the optically distorting medium, respective of the image acquisition position, according to the distortion parameters.

Claims

1. System for determining a distortion model for an optically distorting medium, the system comprising:

a first image sensor located at a first image acquisition position behind said optically distorting medium, said first image sensor acquiring a first obstructed image of an object located substantially at infinity, when said optically distorting medium is located between said first image sensor and said object in a line of sight connecting said first image sensor and said object, said first image sensor acquiring an unobstructed image of said object, when said optically distorting medium is absent from said line of sight; and

an image processor coupled with said first image sensor, said image processor receiving information respective of said first obstructed image and said unobstructed image from said first image sensor, said image processor identifying a plurality of first features in said unobstructed image, and a respective first feature in said first obstructed image for each of said identified first features in said unobstructed image, thereby determining pairs of associated features, said image processor determining a first distortion parameter for each of said pairs of associated features, said image processor determining a reference distortion model for said optically distorting medium, respective of said first image acquisition position, according to said first distortion parameters.

2. The system according to claim 1, further comprising a storage unit coupled with said image processor.

3. The system according to claim 1, further comprising a position indicator coupled with said image processor, said position indicator indicating when said optically distorting medium is present in said line of sight, and when said optically distorting medium is absent from said line of sight.

4. The system according to claim 1, further comprising an analog to digital converter coupled with said first image sensor and with said image processor, said analog to digital converter converting a signal produced by said first image sensor, from analog format to digital format.

5. The system according to claim 1, wherein said image processor comprises an image recognition module for identifying said first pairs of associated features.

6. The system according to claim 1, wherein said optically distorting medium is selected from the list consisting of:

canopy of an aircraft;

windshield of a ground vehicle;

observation window of a marine vehicle;

observation window of an observation post;

observation window of a spacecraft; and

observation window of a spaceship.

7. The system according to claim 1, wherein said first image sensor is selected from the list consisting of:

near infrared image intensifier tube;

digital camera;

charge-coupled device; and

light sensitive film.

8. The system according to claim 1, wherein said first image sensor is located at an eye position of a pilot of an aircraft, within a motion box of a cockpit of said aircraft.

9. The system according to claim 1, wherein said image processor operates in a mode selected from the list consisting of:

on-line; and

off-line.

10. The system according to claim 1, wherein said first distortion parameter is selected from the list consisting of:

displacement; and

angle of deviation.

11. The system according to claim 1, wherein said first image acquisition position is substantially within a motion box of an operator of a vehicle.

12. The system according to claim 1, further comprising a second image sensor located at a second image acquisition position behind said optically distorting medium, said image processor determining a relative position between a first image acquisition position of said first image sensor and a second image acquisition position of said second image sensor, said first image sensor acquiring a second obstructed image while said optically distorting medium is in a selected physical condition, said second image sensor acquiring a third obstructed image while said optically distorting medium is in said selected physical condition, both said second obstructed image and said third obstructed image being acquired respective of the same object, said image processor identifying a plurality of second features in said second obstructed image, said image processor determining the apparent location of each of said second features according to said reference distortion model, said image processor identifying a plurality of third features in said third obstructed image for respective ones of said second features, said image processor determining a second distortion parameter according to each of said apparent locations, respective ones of said third features, and said relative position, said image processor determining a dynamic distortion model for said optically distorting medium, respective of said second image acquisition position, according to said second distortion parameters.

13. The system according to claim 12, wherein said second image acquisition position is substantially within a motion box of an operator of a vehicle.

14. The system according to claim 12, wherein said selected physical condition exhibits at least one item selected from the list consisting of:

velocity;

acceleration;

pressure; and

temperature.

15. The system according to claim 1, further comprising:

a first position detector coupled with said first image sensor, said first position detector detecting said first image acquisition position; and

a second position detector coupled with said second image sensor, said second position detector detecting said second image acquisition position.

16. Method for determining a distortion model for an optically distorting medium, the method comprising the procedures of:

identifying a plurality of first features of an object in a first obstructed image of said object, said first obstructed image being acquired from a first image acquisition position behind said optically distorting medium, said object being located substantially at infinity, in front of said optically distorting medium;

identifying a respective first feature in an unobstructed image of said object for each of said identified first features, thereby determining pairs of associated features, said unobstructed image being acquired from said first image acquisition position;

determining a first distortion parameter for each of said pairs of associated features; and

determining a reference distortion model for said optically distorting medium respective of said first image acquisition position, according to said first distortion parameters.

17. The method according to claim 16, further comprising a preliminary procedure of acquiring said first obstructed image.

18. The method according to claim 16, further comprising a preliminary procedure of acquiring said unobstructed image.

19. The method according to claim 16, further comprising a preliminary procedure of acquiring said first obstructed image and said unobstructed image.

20. The method according to claim 16, wherein said procedures of identifying said first features, identifying said respective first feature, determining said first distortion parameter, and determining said reference distortion model, are performed in a mode selected from the list consisting of:

on-line; and

off-line.

21. The method according to claim 16, further comprising a preliminary procedure of converting a signal produced by an image sensor acquiring an image of said object, from analog format to digital format.

22. The method according to claim 16, further comprising the procedures of:

identifying a plurality of second features of an object in a second obstructed image of said object, said second obstructed image being acquired from said first image acquisition position;

determining the apparent location of each of said second features according to said reference distortion model;

identifying a plurality of third features in a third obstructed image for respective ones of said second features, said third obstructed image being acquired from a second image acquisition position behind said optically distorting medium, said third obstructed image being respective of said object;

determining a second distortion parameter according to each of said apparent locations, respective ones of said third features, and a relative position between said first image acquisition position and said second image acquisition position; and

determining a dynamic distortion model for said optically distorting medium respective of said second image acquisition position, according to said second distortion parameters.

23. The method according to claim 22, further comprising a preliminary procedure of acquiring said second obstructed image.

24. The method according to claim 22, further comprising a preliminary procedure of acquiring said third obstructed image.

25. The method according to claim 22, further comprising a preliminary procedure of determining said relative position.

26. The method according to claim 25, further comprising a preliminary procedure of determining said first image acquisition position.

27. The method according to claim 25, further comprising a preliminary procedure of determining said second image acquisition position.

28. Method for determining a distortion model for an optically distorting medium, the method comprising the procedures of:

detecting a first obstructed image of a first feature in a first known target located in front of said optically distorting medium, at a first location, a first finite distance apart from an image sensor located behind said optically distorting medium;

determining an obstructed viewing direction respective of said first feature, relative to a horizontal axis, according to said first obstructed image;

detecting a second obstructed image of a second feature on a second known target, at substantially the same obstructed viewing direction, said second known target being located at a second location a second finite distance apart from said image sensor, in front of said optically distorting medium;

determining a feature pair direction between said first feature and said second feature, according to coordinates of said first feature and said second feature, said first finite distance and said second finite distance; and

determining said distortion model, according to said obstructed viewing direction and said feature pair direction.

29. The method according to claim 28, further comprising a preliminary procedure of placing said optically distorting medium in front of said image sensor.

30. The method according to claim 28, wherein said second known target is obtained by moving said first known target from said first location to said second location.

31. The method according to claim 28, wherein said coordinates of said second feature are determined by interpolating between a plurality of predetermined coordinates in said second known target.