SYSTEM AND METHOD FOR ADAPTIVE NON-UNIFORMITY COMPENSATION FOR A FOCAL PLANE ARRAY

• US 20080056606A1
• Filed: 08/29/2006
• Published: 03/06/2008
• Est. Priority Date: 08/29/2006
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Kilgore, Patrick M.
• Granted Patent: US 7,684,634 B2
• Time in Patent Office: Days
• US Class Current: 382/275
• CPC Class Codes: F41G 7/2213 maintaining the axis of an orientable seeking head pointed at the target, e.g. target seeking gyro
• CPC Class Codes: F41G 7/2253 Passive homing systems, i.e. comprising a receiver and do not requiring an active illumination of the target
• CPC Class Codes: F41G 7/2293 using electromagnetic waves other than radio waves
• CPC Class Codes: G06T 2207/20032 Median filtering
• CPC Class Codes: G06T 5/002 Denoising; Smoothing noise processing or correction adapted to be used in an image pickup device containing and electronic image sensor H04N23/81, H04N25/60 - H04N25/67
• CPC Class Codes: G06T 5/005 Retouching; Inpainting; Scratch removal detecting, correction, reducing or removing defects, e.g. non-responsive pixels of solid state image sensors H04N25/68, scratch removal for cinematographic films scanned by electronic image sensor H04N5/253
• CPC Class Codes: G06T 5/20 using local operators
• CPC Class Codes: G06T 5/70 Denoising; Smoothing
• CPC Class Codes: G06T 5/77 Retouching; Inpainting; Scratch removal
• CPC Class Codes: G06V 10/30 Noise filtering
• CPC Class Codes: H04N 25/674 based on the scene itself, e.g. defocusing
• CPC Class Codes: H04N 25/68 applied to defects
• CPC Class Codes: H04N 25/683 by defect estimation performed on the scene signal, e.g. real time or on the fly detection

Abstract

A method of reducing an amount of fixed pattern noise from an image signal generated by an image sensor. The method includes, for each operational pixel in the image signal, applying a recursively updated offset term to generate a corrected image signal. The offset correction terms are recursively updated by spatially filtering the corrected image signal for a current frame of the image signal; comparing the filtered corrected image signal of the current frame with a spatially filtered corrected image signal of a preceding frame of the image signal; and updating the offset correction terms with terms generated as a function of the comparison.

Claims

1. A method of reducing an amount of fixed pattern noise from an image signal generated by an image sensor, comprising:

for each operational pixel in the image signal, applying a recursively updated offset term to generate a corrected image signal; and

recursively updating the offset correction terms by;

spatially filtering the corrected image signal for a current frame of the image signal;

comparing the filtered corrected image signal of the current frame with a spatially filtered corrected image signal of a preceding frame of the image signal; and

updating the offset correction terms with terms generated as a function of the comparison.

2. The method of claim 1, wherein the spatial filtering includes applying an anti-means filter to pixels of the frame of the image signal.

3. The method of claim 2, wherein the spatial filtering includes applying a median filter to pixels of the frame of the image signal and the anti-means filter is applied to the output of the median filter.

4. The method of claim 3, wherein the median filter has an arrangement of pixel elements centered on a pixel to be filtered, the median filter selects the value of the middle pixel element in terms of magnitude as an output of the median filter for the pixel to be filtered.

5. The method of claim 4, wherein a value for a defective pixel in the pixel elements of the filter is replaced with a replacement value prior to selection of the output of the median filter.

6. The method of claim 5, wherein the replacement value is a maximum possible pixel value or a minimum possible pixel value.

7. The method of claim 3, wherein the anti-means filter has an arrangement of pixel elements centered on a pixel to be filtered, the anti-means filter averages values of the pixel elements to generate an output of the spatial filtering.

8. The method of claim 7, wherein the anti-means filter averages pixels neighboring the pixel to be filtered to generate the output of the spatial filtering.

9. The method of claim 1, further comprising moving the image sensor so that the current frame and the preceding frame have different perspectives of a scene imaged by the image sensor.

10. The method of claim 1, further comprising identifying defective pixels in the corrected image signal and replacing a value for each defective pixel.

11. The method of claim 10, wherein the replacement value for each defective pixel is generated by applying a median filter to the defective pixel.

12. The method of claim 11, wherein the median filter has an arrangement of pixel elements centered on the defective pixel, the median filter selects the value of the middle pixel element in terms of magnitude as the replacement value for the defective pixel.

13. The method of claim 12, wherein during median filtering a value for each defective pixel in the pixel elements of the filter is replaced with a replacement value selected from a maximum possible pixel value or a minimum possible pixel value.

14. The method of claim 11, wherein the defective pixels are selected from factory dead pixels, drifters, blinkers, and combinations thereof.

15. The method of claim 1, wherein generation of the offset correction terms as a function of the comparison includes:

generating an error value for each pixel, the error value being the smaller of the relative magnitudes of the spatially filtered pixel value for the current frame and the spatially filtered pixel value for the preceding frame;

modifying the error value by generating a fraction of the error value, the fractional size based on the magnitude of the error value; and

applying a term decay to the modified error values.

16. The method of claim 1, further comprising freezing the recursive updating of the offset correction terms an end of an initiation period and correcting the image signal with the frozen correction terms after the initiation period.

17. The method of claim 1, wherein the offset correction term is not updated for a pixel if a difference between the current spatial filter value for the pixel and the preceding filter spatial filter value for the pixel is greater than a predetermined threshold.

18. The method of claim 1, further comprising outputting the corrected image signal to a target tracking system of a missile.

19. An imaging system comprising:

a focal plane array that generates an image signal; and

a video processing assembly that reduces an amount of fixed pattern noise from the image signal by applying a recursively updated offset term to each operational pixel in the image signal to generate a corrected image signal, wherein the video processing assembly recursively updates the offset correction terms by;

spatially filtering the corrected image signal for a current frame of the image signal;

comparing the filtered corrected image signal of the current frame with a spatially filtered corrected image signal of a preceding frame of the image signal; and

updating the offset correction terms with terms generated as a function of the comparison.

20. The imaging system of claim 19, further comprising a gimbal to move the focal plane array so that the current frame and the preceding frame have different perspectives of a scene imaged by the focal plane array during updating of the offset correction terms.

21. The imaging system of claim 19, wherein the video processing assembly identifies defective pixels in the corrected image signal and replaces a value for each defective pixel.

22. The imaging system of claim 19, wherein the spatial filtering includes applying a median filter to pixels of the frame of the corrected image signal and applying an anti-means filter to the output of the median filter.