Increasing Dynamic Range Using Multisampling

• US 20140267884A1
• Filed: 03/15/2013
• Published: 09/18/2014
• Est. Priority Date: 03/15/2013
• Status: Active Grant

• Current Assignee: Rambus Inc.
• Original Assignee: Rambus Inc.
• Inventors: Shen, Jie, Xue, Song, Smirnov, Maxim
• Granted Patent: US 9,319,605 B2
• Time in Patent Office: Days
• US Class Current: 348/362
• CPC Class Codes: H04N 23/684 performed by controlling the image sensor readout, e.g. by controlling the integration time
• CPC Class Codes: H04N 23/73 by influencing the exposure time
• CPC Class Codes: H04N 25/533 by using differing integration times for different sensor regions
• CPC Class Codes: H04N 25/589 with different integration times, e.g. short and long exposures
• CPC Class Codes: H04N 25/745 Circuitry for generating timing or clock signals

Abstract

Methods and systems for increasing the effective dynamic range of an image sensor are disclosed. Each pixel in the sensor is exposed for a respective first exposure time. Each pixel'"'"'s response to the respective first exposure is measured and compared to threshold values. Based on the pixel'"'"'s response to the respective first exposure time, an optimal exposure is calculated for each pixel. The optimal exposure time is applied to each pixel by utilizing row-enabled and column-enabled signals at each pixel within the sensor.

Claims

1. A method for operating an image sensor, said method comprising:

exposing each of a plurality of pixels in said image sensor for a respective first exposure time;

measuring a respective response of each of said plurality of pixels to said respective first exposure time; and

exposing each of said plurality of pixels for a respective second exposure time that depends for each pixel on said respective measured response of each of said plurality of pixels, wherein, for a group of pixels read using a common row read signal, the respective second exposure times end at a common time.

2. The method of claim 1 further comprising:

comparing said measured response of each of said plurality of pixels to at least one threshold value; and

determining said second exposure time for each of said plurality of pixels based on said comparing.

3. The method of claim 2, wherein for a given pixel, the second exposure time is for a current frame and the first exposure time comprises an exposure from a previous frame, and wherein determining said second exposure time comprises determining whether to decrease, increase, or maintain an exposure time used for the previous frame.

4. The method of claim 1 further comprising:

encoding each of said measured responses of each of said plurality of pixels as a digital word.

5. The method of claim 4 further comprising:

saving each of the bits of the digital word in a corresponding bitplane, each bitplane corresponding to a sensor reset time.

6. The method of claim 1, wherein exposing each of said plurality of pixels for a respective second exposure time that depends for each pixel on said respective measured response of each of said plurality of pixels comprises, for a sub-plurality of pixels sharing a common row reset signal:

asserting the row reset signal a plurality of times during an exposure period; and

for each of the pixels in the sub-plurality, preventing the row reset signal from causing a reset at any of the plurality of times that occur closer to the end of the exposure period than the respective second exposure time.

7. The method of claim 6, wherein preventing the row reset signal from causing a reset comprises setting a column signal to disable a reset operation.

8. The method of claim 6, wherein the plurality of times are spaced substantially exponentially.

9. The method of claim 8, wherein at least some of the substantially exponentially spacings are each adjusted in length from an exact exponential spacing by a corresponding fraction of a minimum exposure time.

10. The method of claim 9, wherein the fraction of a minimum exposure time is selected such that shorter spacings are adjusted by a smaller fraction of the minimum exposure time than longer spacings.

11. The method of claim 1, wherein exposing each of a plurality of pixels in said image sensor for a respective first exposure time comprises:

at a plurality of intervals after the beginning of the first exposure time for a given one of the pixels, comparing an output signal of the given pixel to a threshold to produce a corresponding plurality of comparison results;

for each of the comparison results that indicates that the given pixel has passed the corresponding threshold, using that comparison result to reset the given pixel at a time interval before the end of the second exposure time; and

when none of the comparison results indicate that the given pixel has passed the corresponding threshold, including the first exposure time in the second exposure time.

12. An image sensor, said sensor comprising:

logic for exposing each of a plurality of pixels in said image sensor for a respective first exposure time;

logic for measuring a respective response of each of said plurality of pixels to said respective first exposure time; and

logic for exposing each of said plurality of pixels for a respective second exposure time that depends for each pixel on said respective measured response of each of said plurality of pixels, wherein, for a group of pixels read using a common row read signal, the respective second exposure times end at a common time.

13. The image sensor of claim 12 further comprising:

logic for comparing said measured response of each of said plurality of pixels to at least one threshold value; and

logic for determining said second exposure time for each of said plurality of pixels based on said comparing.

14. The image sensor of claim 13, wherein said logic for determining said second exposure time determines, for a given pixel, whether the second exposure time is for a current frame and the first exposure time comprises an exposure from a previous frame, and wherein said logic for determining said second exposure time comprises logic for determining whether to decrease, increase, or maintain an exposure time used for the previous frame.

15. The image sensor of claim 12 further comprising:

logic for encoding each of said measured responses of each of said plurality of pixels as a digital word.

16. The image sensor of claim 15 further comprising:

logic for saving each of the bits of the digital word in a corresponding bitplane, each bitplane corresponding to a sensor reset time.

17. The image sensor of claim 12, wherein said logic for exposing each of said plurality of pixels for a respective second exposure time that depends for each pixel on said respective measured response of each of said plurality of pixels further comprises logic that, for a sub-plurality of pixels sharing a common row reset signal:

asserts the row reset signal a plurality of times during an exposure period; and

for each of the pixels in the sub-plurality, prevents the row reset signal from causing a reset at any of the plurality of times that occur closer to the end of the exposure period than the respective second exposure time.

18. The image sensor of claim 17, wherein said logic for preventing the row reset signal from causing a reset comprises logic for setting a column signal to disable a reset operation.

19. The image sensor of claim 17, wherein the plurality of times are spaced substantially exponentially.

20. The image sensor of claim 19, wherein at least some of the substantially exponentially spacings are each adjusted in length from an exact exponential spacing by a corresponding fraction of a minimum exposure time.

21. The image sensor of claim 20, wherein the fraction of a minimum exposure time is selected such that shorter spacings are adjusted by a smaller fraction of the minimum exposure time than longer spacings.

22. The image sensor of claim 12, wherein the logic for exposing each of the plurality of pixels for a respective first exposure time comprises logic to, at a plurality of intervals after the beginning of the first exposure time for a given one of the pixels, compare an output signal of the given pixel to a threshold to produce a corresponding plurality of comparison results.

23. A high dynamic range image sensor comprising:

a pixel array, each pixel in said array comprising logic for receiving a plurality of row-enabled signals effective to reset said pixel and a plurality of column-enabled signals effective to mask at least one of said plurality of row-enabled signals; and

exposure logic coupled to said pixel array, said exposure logic configured to;

expose each of said plurality of pixels in said pixel array for a respective first exposure time;

measure a response of each of said plurality of pixels to said first respective exposure time; and

expose each of said plurality of pixels for a second respective exposure time that depends for each pixel on said respective measured response of each of said plurality of pixels.

24. The high dynamic range image sensor of claim 23, wherein said exposure logic is further configured to:

encode each of said measured responses of each of said plurality of pixels as a digital word.

25. The high dynamic range image sensor of claim 23, wherein said exposure logic, for a sub-plurality of pixels sharing a common row reset signal, is further configured to

assert the row reset signal a plurality of times during an exposure period;

and

for each of the pixels in the sub-plurality, prevent the row reset signal from causing a reset at any of the plurality of times that occur closer to the end of the exposure period than the respective second exposure time.

26. The high dynamic range image sensor of claim 24, wherein said exposure logic is further configured to:

expose each of said plurality of pixels for a second exposure time that is associated with said digital word.

27. The high dynamic range image sensor of claim 23, wherein said exposure logic is further configured to:

save each of the bits of the digital word in a corresponding bitplane, each bitplane corresponding to a sensor reset time.