Digital exposure circuit for an image sensor

• US 8,040,394 B2
• Filed: 12/14/2009
• Issued: 10/18/2011
• Est. Priority Date: 04/23/1998
• Status: Expired due to Fees

• Current Assignee: Round Rock Research LLC
• Original Assignee: Round Rock Research LLC
• Inventors: Fossum, Eric R., Clark, Christopher, Krymski, Alexander I., Panicacci, Roger A.
• Primary Examiner(s): Tran; Nhan T
• Application Number:
  US12/637,224
• Publication Number:
  US 20100134653A1
• Time in Patent Office: 673 Days
• Field of Search: 348/229.1, 348/362, 348/221.1, 348/297, 358/443, 358/482
• US Class Current: 348/229.1
• CPC Class Codes: H04N 23/71 Circuitry for evaluating the brightness variation
• CPC Class Codes: H04N 23/72 Combination of two or more compensation controls

Abstract

Automatic exposure adjusting device considers the image on a pixel-by-pixel basis. Each pixel is characterized according to its most significant bits. After the pixels are characterized, the number of pixels in any particular group is counted. That counting is compared with thresholds which set whether the image is over exposed, under exposed, and can optionally also determine if the image is seriously over exposed or seriously under exposed. Adjustment of the exposure is carried out to bring the image to a more desired state.

Claims

1. A device comprising:

an analog to digital converter for producing a plurality of digital outputs comprising a plurality of bits, each of said digital outputs indicative of an output of at least one pixel of an image sensor;

a first counter which counts a number of said digital outputs indicating overexposure during an image frame;

a second counter which counts a number of said digital outputs indicating underexposure during said image frame;

at least one threshold detector for comparing counting results of said first and second counters with desired thresholds; and

a decision element for making a decision to either increase, decrease, or maintain an exposure of a next frame of said image sensor based on comparison results from the at least one threshold detector.

2. A device as in claim 1, wherein said at least one threshold detector is configured to compare said counting results on a frame by frame basis.

3. A device as in claim 1, wherein said at least one threshold detector is configured to compare said counting results for a group of frames.

4. A device as in claim 1, further comprising first and second characterization elements configured to review only a predetermined number of most significant bits of each of said plurality of digital outputs and provide a signal to a respective one of said first and second counters according to said most significant bits.

5. A device as in claim 1, wherein said first counter counts a number of digital outputs having most significant bits including a plurality of ones, and said second counter counts a number of digital outputs having most significant bits including at least one zero.

6. A device as in claim 5, wherein said second counter counts a number of digital outputs having most significant bits including a plurality of zeros.

7. A device as in claim 6, wherein said number of most significant bits is two.

8. A device as in claim 1, wherein said desired thresholds include a first value indicative of what percentage of said plurality of digital outputs can indicate overexposure and a second value indicative of what percentage of said plurality of digital outputs can indicate underexposure, and

wherein said decision element is configured to decrease said exposure of said next frame if said number counted by said first counter exceeds said first threshold, and to increase said exposure of said next frame if said number counted by said second counter exceeds said second threshold.

9. A device as in claim 1, further comprising a third counter which counts a number of seriously underexposed pixels, wherein said at least one threshold detector further compares counting results of said third counter with said desired thresholds.

10. A device as in claim 9, wherein said first counter counts a number of said digital outputs having most significant bits including a plurality of ones, said second counter counts a number of said digital outputs having a most significant bit of zero, and said third counter counts a number of said digital outputs having most significant bits including a plurality of zeros.

11. A device as in claim 9, wherein said desired thresholds including a first value indicative of what percentage of said plurality of digital outputs, when counted by said first counter, indicate serious overexposure, a second value indicative of what percentage of said plurality of digital outputs, when counted by said first counter, indicate moderate overexposure, a third value indicative of what percentage of said plurality of digital outputs, when counted by said third counter, indicate serious underexposure, and a fourth value indicative of what percentage of said plurality of digital outputs, when counted by said second counter, indicate moderate underexposure.

12. A device as in claim 11, wherein said decision element is configured to:

decrease said exposure of said image sensor by a first amount if said count of said first counter exceeds said first value;

decrease said exposure of said image sensor by a second amount if said count of said first counter exceeds said second value;

increase said exposure of said image sensor by a third amount if said count of said third counter exceeds said third value; and

increase said exposure of said image sensor by a fourth amount if said count of said second counter exceeds said fourth value.

13. A device as in claim 1, wherein said decision element is configured to increase or decrease said exposure by adjusting at least one of:

a shutter width of said image sensor; and

a gain of said image sensor.

14. A method of controlling exposure comprising:

obtaining from a current frame a plurality of digital values representing values of pixels of an image sensor;

reviewing only most significant bits of each of said digital values from the current frame to determine if each digital value indicates an overexposed pixel or an underexposed pixel;

comparing results of said determination with desired thresholds; and

adjusting exposure of a next frame of said image sensor by;

decreasing said exposure if a number of overexposed pixels is greater than or equal to a first threshold; and

increasing said exposure if a number of underexposed pixels is greater than or equal to a second threshold.

15. A method as in claim 14, further comprising comparing said results of said determination on a frame-by-frame basis.

16. A method as in claim 14, further comprising comparing said results of said determination for a group of frames.

17. A method as in claim 14, wherein said step of reviewing further comprises determining that a digital value indicates:

an overexposed pixel if said most significant bits of said digital value are “

11”

; and

an underexposed pixel if a most significant bit of said digital value is “

0”

.

18. A method as in claim 14, said step of reviewing further comprising reviewing to determine if each digital value indicates a seriously underexposed pixel.

19. A method as in claim 18, said step of adjusting said exposure of said next frame further comprising:

if said number of overexposed pixels is greater than or equal to a third threshold, decreasing said exposure of said image sensor by a greater amount than if said number of overexposed pixels is greater than or equal to said first threshold but less than said third threshold; and

if a number of seriously underexposed pixels is greater than or equal to a fourth threshold, increasing said exposure of said image sensor by a greater amount than it said number of underexposed pixels is greater than or equal to a second threshold but said number of seriously underexposed pixels is less than said fourth threshold.

20. A method as in claim 19, said step of reviewing further comprising determining that a digital value indicates:

an overexposed pixel if said most significant bits are “

II”

;

an underexposed pixel if a most significant bit is “

0”

; and

a seriously underexposed pixel if said most significant bits are “

00”

.

21. A method as in claim 14, said step of adjusting said exposure comprising adjusting at least one of:

a shutter width of said image sensor; and

a gain of said image sensor.

22. A device comprising:

an analog to digital converter for producing a plurality of digital outputs, each of said digital outputs indicative of an output of at least one pixel of an image sensor;

a first counter which counts a number of said digital outputs indicating overexposure during at least one image frame;

a second counter which counts a number of said digital outputs indicating underexposure during said at least one image frame;

at least one comparator for comparing counting results of said first and second counters; and

a decision element for making a decision to either increase, decrease, or maintain an exposure of a next frame of said image sensor based on factors including comparison results from the at least one comparator.

23. The device of claim 22, wherein each of said digital outputs comprises a plurality of bits, and wherein said first and second counters count said digital outputs indicating overexposure and said digital outputs indicating underexposure according to a predetermined number of most significant bits of each of said plurality of digital outputs.

24. The device of claim 22, wherein said decision element makes a decision to either increase, decrease, or maintain an exposure of a next frame of said image sensor.

25. The device of claim 22, wherein said comparator compares a count of said digital outputs indicating overexposure and a count of said digital outputs indicating underexposure.

26. The device of claim 22, wherein said comparator compares a percentage of said digital outputs indicating overexposure and a percentage of said digital outputs indicating underexposure.

27. The device of claim 22, wherein said comparator compares counting results of said first and second counters for a plurality of frames.

28. A method of controlling exposure comprising:

obtaining a plurality of digital values from a current frame representing values of pixels of an image sensor;

reviewing each of said digital values to determine if each digital value indicates an overexposed pixel or an underexposed pixel;

comparing a number of digital values determined to indicate overexposed pixels and a number of digital values determined to indicate underexposed pixels in said determination with desired thresholds; and

adjusting exposure of a next frame of said image sensor according to factors including said comparison.

29. The method of claim 28, wherein each of said digital outputs comprises a plurality of bits, said step of reviewing each of said digital values further comprising reviewing only a predetermined number of most significant bits of each of said plurality of digital outputs.

30. The method of claim 28, wherein said step of adjusting exposure of a subsequent frame includes adjusting exposure,of a next frame of said image sensor.

31. The method of claim 28, wherein said step of comparing includes comparing a count of digital values determined to indicate overexposed pixels with a count of digital values determined to indicate underexposed pixels.

32. The method of claim 31, further comprising accumulating said count of digital values determined to indicate overexposed pixels and said count of digital values determined to indicate underexposed pixels over a plurality of frames.

33. The method of claim 28, wherein said step of comparing includes comparing a percentage of digital values determined to indicate overexposed pixels with a percentage of digital values determined to indicate underexposed pixels.

34. The method of claim 33, further comprising accumulating said percentage of digital values determined to indicate overexposed pixels and said percentage of digital values determined to indicate underexposed pixels over a plurality of frames.

35. A device comprising:

an analog to digital converter for producing a plurality of digital outputs comprising a plurality of bits, each of said digital outputs indicative of an output of at least one pixel of an image sensor;

a first counter which counts a number of said digital outputs indicating overexposure during an image frame;

a second counter which counts a number of said digital outputs indicating underexposure during said image frame;

a third counter which counts a number of seriously underexposed pixels, wherein said at least one threshold detector further compares counting results of said third counter with desired thresholds;

at least one threshold detector for comparing counting results of said first and second counters with said desired thresholds; and

a decision element for making a decision to either increase, decrease, or maintain an exposure of a next frame of said image sensor based on comparison results from the at least one threshold detector wherein said desired thresholds including a first value indicative of what percentage of said plurality of digital outputs, when counted by said first counter, indicate serious overexposure, a second value indicative of what percentage of said plurality of digital outputs, when counted by said first counter, indicate moderate overexposure, a third value indicative of what percentage of said plurality of digital outputs, when counted by said third counter, indicate serious underexposure, and a fourth value indicative of what percentage of said plurality of digital outputs, when counted by said second counter, indicate moderate underexposure.

36. A device as in claim 35, wherein said decision element is configured to:

decrease said exposure of said image sensor by a first amount if said count of said first counter exceeds said first value;

decrease said exposure of said image sensor by a second amount if said count of said first counter exceeds said second value;

increase said exposure of said image sensor by a third amount if said count of said third counter exceeds said third value; and

increase said exposure of said image sensor by a fourth amount if said count of said second counter exceeds said fourth value.