Vignetting compensation

• US 20040155970A1
• Filed: 03/04/2003
• Published: 08/12/2004
• Est. Priority Date: 02/12/2003
• Status: Active Grant

• Current Assignee: Gula Consulting Limited Liability Company (Intellectual Ventures LLC)
• Original Assignee: Dialog Semiconductor GmbH (Renesas Electronics Corporation)
• Inventors: Johannesson, Anders, Larsson, Ingemar
• Granted Patent: US 7,692,700 B2
• Time in Patent Office: Days
• US Class Current: 348/231.6
• CPC Class Codes: H04N 25/61 the noise originating only from the lens unit, e.g. flare, shading, vignetting or "cos4"

Abstract

A method used for the compensation of vignetting in digital cameras has been achieved. The compensation for vignetting is done in two steps. The first step is done during production of the camera unit and involves taking and analyzing an image of a test screen, preferably a gray test screen. This results in a set of e.g. 5×5 coefficients describing a polynomial surface. The second step is done for each image that is taken by the camera and involves calculating and applying a gain-table based on polynomial coefficients to all pixels of the image to compensate vignetting.

Claims

1. A method to compensate vignetting in digital cameras comprising:

providing a lens system, an array of pixel-based image sensors, a non-volatile memory, a processing device, a test chart, and a camera unit;

calibrate camera unit for vignetting deficiencies;

store calibration results in a non-volatile memory;

retrieve said calibration results and prepare said data for vignetting compensation; and

apply said calibration results to all pixels of each image taken to compensate vignetting.

2. The method of claim 1 wherein said calibrations results are represented as a polynomial surface function describing the vignetting deficiencies.

3. The method of claim 2 wherein said polynomial surface function is following the equation:

4. The method of claim 3 wherein said factor k_i,j is determined by using by a least squares fit to the measured data.

5. The method of claim 1 wherein said calibration results are stored and retrieved using a matrix of polynomial coefficients.

6. The method of claim 5 wherein said matrix of polynomial coefficients are derived from a normalized surface function describing the vignetting deficiencies.

7. The method of claim 6 wherein the brightness of the center of the image is used for said normalization.

8. The method of claim 1 wherein said compensation of vignetting is performed by multiplying pixel-by-pixel a normalized polynomial function with the raw pixel values of each picture taken.

9. The method of claim 1 wherein a test chart is used to calibrate the camera unit.

10. The method of claim 9 wherein said test chart is a gray test chart.

11. The method of claim 1 wherein said non-volatile memory is an EPROM.

12. The method of claim 1 wherein said non-volatile memory is an EEPROM.

13. The method of claim 1 wherein said non-volatile memory is a FLASH-memory.

14. The method of claim 1 wherein said array of pixel-based sensors are CMOS sensors.

15. The method of claim 1 wherein said array of pixel based sensors are CCD sensors

16. A method to compensate vignetting in digital cameras comprising:

providing a non-volatile memory, a lens system, a processing device, a RAM, a test chart, an array of pixel-based sensors, and a camera unit;

take a picture of a test chart;

analyze said test image;

fit test image with a polynomial surface function;

insert the coefficients of said polynomial function in a matrix;

store said matrix in a non-volatile memory;

retrieve said matrix from said non-volatile memory;

construct a polynomial surface function using coefficients retrieved;

build a gain table using said polynomial surface for all pixels of the image; and

apply values of said gain table to all pixels of the image taken to compensate vignetting.

17. The method of claim 16 wherein said polynomial surface function is following the equation:

18. The method of claim 17 wherein said factor k_i,j is determined by using by a least squares fit to the measured data.

19. The method of claim 16 wherein said matrix of polynomial coefficients are derived from a normalized surface function describing the vignetting deficiencies.

20. The method of claim 19 wherein the brightness of the center of the image is used for said normalization.

21. The method of claim 16 wherein said compensation of vignetting is performed by multiplying pixel-by-pixel a normalized polynomial function with the raw pixel values of each picture taken.

22. The method of claim 16 wherein said test chart is a gray test chart.

23. The method of claim 16 wherein said non-volatile memory is an EPROM.

24. The method of claim 16 wherein said non-volatile memory is an EEPROM.

25. The method of claim 16 wherein said non-volatile memory is a FLASH-memory.

26. The method of claim 16 wherein said matrix is a 5×

5 matrix.

27. The method of claim 16 wherein said array of pixel-based sensors are CMOS sensors.

28. The method of claim 16 wherein said array of pixel-based sensors are CCD sensors.

29. A system to compensate vignetting in digital cameras comprising:

a test chart;

a lens system;

a pixel-based image sensor array;

a non-volatile memory;

a processing device; and

an array of pixel-based image sensors.

30. The system of claim 29 wherein said array of image sensors array comprises CMOS image sensors.

31. The system of claim 29 wherein said array of image sensors array comprises CCD image sensors.

32. The system of claim 29 wherein said non-volatile memory is a FLASH-memory.

33. The system of claim 29 wherein said non-volatile memory is an MRAM.

34. The system of claim 29 wherein said non-volatile memory is an EPROM.

35. The system of claim 29 wherein said non-volatile memory is an EEPROM.

36. The system of claim 29 wherein said processing device is a microprocessor.

37. The system of claim 29 wherein said test chart is a grey test chart.

38. The system of claim 29 wherein said lens system comprises one lens.

39. The system of claim 29 wherein said lens system comprises more than one lens.