Digital exposure circuit for an image sensor

• US 7,646,407 B2
• Filed: 05/05/2005
• Issued: 01/12/2010
• Est. Priority Date: 04/23/1998
• Status: Expired due to Fees

• Current Assignee: Round Rock Research LLC
• Original Assignee: Micron Technology, Inc.
• Inventors: Fossum, Eric R., Clark, Christopher, Krymski, Alexander I., Panicacci, Roger A.
• Primary Examiner(s): Tran; Nhan T
• Application Number:
  US11/121,956
• Publication Number:
  US 20050195314A1
• Time in Patent Office: 1,713 Days
• Field of Search: 348/229.1, 348/362, 348/297, 348/221.1, 358/443, 358/482, 250/208.1
• 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 camera comprising:

an image sensor having a plurality of pixels, and further having an exposure adjustment capability;

an analog to digital converter which produces a digital output comprising a plurality of bits, said digital output indicative of an output of each pixel of said image sensor;

at least one first counter which counts a number of overexposed parts of said digital output;

at least one second counter which counts a number of underexposed parts of said digital output;

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

an exposure decision element, which is capable of making a decision to either increase or decrease a next frame of exposure of said image sensor based on a detected relation of said counting results with said thresholds.

2. A camera as in claim 1, further comprising at least one coincidence detector, which reviews only a predetermined number of most significant bits of said digital output.

3. A camera as in claim 2, wherein said first counter counts a number of pixels whose most significant bits include ones.

4. A camera as in claim 3, wherein said second counter counts a number of pixels in which said most significant bits include a zero.

5. A camera as in claim 3, in which said second counter counts a number of pixels whose most significant bits include at least one zero.

6. A camera as in claim 5, wherein said at least one threshold detector includes values indicative of what percentage of the image can have underexposed or overexposed pixels, said decision element increasing or decreasing said next frame of exposure based on said percentages.

7. A camera as in claim 2, wherein said at least one coincidence detector further comprises two coincidence detectors representing relationships with two different thresholds, one of which is for an overexposed image and another of which is for an underexposed image.

8. A camera as in claim 7, wherein said decision element is configured to reduce said next frame of exposure when an overexposed image is detected and increase said next frame of exposure when an underexposed image is detected.

9. A camera as in claim 2, wherein said at least one coincidence detector further comprises at least three coincidence detectors each detecting a different one of:

an overexposed image;

an underexposed image;

a seriously underexposed image; and

a seriously overexposed image.

10. A camera as in claim 9, further comprising a threshold storing element for storing first and second increase and decrease increments, wherein said decision element is configured to:

increase said next frame of exposure by said first increment when an underexposed image is detected;

decrease said next frame of exposure by said first increment when an overexposed image is detected;

increase said next frame of exposure by said second increment when a seriously underexposed image is detected; and

decrease said next frame of exposure by said second increment when a seriously overexposed image is detected.

11. A camera as in claim 1, wherein said image sensor includes an active pixel sensor with a plurality of pixels of a CMOS image sensor, each pixel including an in-pixel buffer transistor and an in-pixel selection transistor.

12. A camera as in claim 1, wherein said desired thresholds comprise a first threshold for a seriously deficient image and a second threshold for a less seriously deficient image, said first and second thresholds collectively adding up to more than 100%.

13. A camera as in claim 1, wherein said exposure is one of a shutter width or a gain of the image sensor.

14. An automatic exposure adjusting CMOS image sensor device, comprising:

an image sensor, including a plurality of adjustable photoreceptors, each photoreceptor defining a pixel of the image, and said image sensor having an adjustable exposure which when increased, increases an amount of exposure for a next frame of the image, and when decreased, decreases an amount of exposure for said next frame of the image;

an analog to digital converter which obtains an analog output from said image sensor and produces a digital output indicative of said analog output to thereby produce a plurality of digital outputs for said plurality of pixels;

a pixel characterization element, representing certain most significant bits of at least a plurality of said digital outputs, to thereby characterize said pixel according to its exposure characteristic;

a counter element, which counts numbers of pixels characterized by said pixel characterization element and compares said count with certain thresholds; and

an image adjusting element, for adjusting said exposure based on said count.

15. A device as in claim 14, further comprising a memory storing said thresholds, and said memory is variable to change said thresholds.

16. A method of operating a camera, the method comprising:

obtaining a plurality of digital values, said plurality of digital values representing values of pixel signals of an image sensor;

setting a first variable threshold for overexposed pixels;

setting a second variable threshold for underexposed pixels;

characterizing said digital values according to a number of most significant bits of said pixels and not all bits of said pixels and investigating said most significant bits to determine if they represent overexposed pixels, normally-exposed pixels, or underexposed pixels;

counting said characterized digital values; and

if the number of overexposed pixels is greater than said first threshold, then decreasing an exposure for a next frame and if the number of under exposed pixels is greater than said second threshold then increasing the exposure for the next frame.

17. A method as in claim 16, wherein characterizing a digital value as an overexposed pixel includes determining that the two most significant bits are “

11”

.

18. A method as in claim 17, wherein characterizing a digital value as an underexposed pixel includes determining that at least one most significant bit is zero.

19. A method as in claim 16, further comprising detecting seriously overexposed and underexposed pixels as well as moderately overexposed and moderately underexposed pixels.

20. A method as in claim 19, wherein said increasing and decreasing an exposure comprises changing the exposure by one amount for seriously overexposed or underexposed pixels and changing the exposure by another amount for moderately underexposed or overexposed pixels.

21. A method as in claim 17, wherein said first threshold is initially set at about 30% of the total number of the pixels of said image sensor.

22. A method as in claim 18, wherein said second threshold is initially set at about 75% of the total number of the pixels of said image sensor.