Method and apparatus providing pixel array having automatic light control pixels and image capture pixels
A pixel array uses two sets of pixels to provide accurate exposure control. One set of pixels provide continuous output signals for automatic light control (ALC) as the other set integrates and captures an image. ALC pixels allow monitoring of multiple pixels of an array to obtain sample data indicating the amount of light reaching the array, while allowing the other pixels to provide proper image data. A small percentage of the pixels in an array is replaced with ALC pixels and the array has two reset lines for each row; one line controls the reset for the image capture pixels while the other line controls the reset for the ALC pixels. In the columns, at least one extra control signal is used for the sampling of the reset level for the ALC pixels, which happens later than the sampling of the reset level for the image capture pixels.
1. An image processing system, comprising:
an imager structure including an array of pixels arranged in rows and columns, at least one row in the array comprising:
a plurality of pixels for providing image capture signals, each pixel in the plurality of pixels configured to transfer charge to a corresponding charge storage region after an integration period, and
at least one automatic light control (ALC) pixel for providing a light control pixel signal, the at least one ALC pixel configured to transfer charge to a corresponding charge storage region during charge integration; and
at least one processor configured to—
for each individual column of the pixel array including one or more ALC pixels, compare the light control pixel signal to a predetermined voltage level, and
determine a time interval for the integration period based, at least in part, on a comparison of the light control pixel signal to the predetermined voltage level.
2. The image processing system of claim 1 , wherein the at least one ALC pixel and each pixel in the plurality of pixels comprises (i) a corresponding photosensor for providing charge to the corresponding charge storage region and (ii) a corresponding reset transistor, and wherein each pixel in the plurality of pixels further comprises a corresponding transfer transistor for selectively transferring charge from the corresponding photosensor to the corresponding charge storage region.
3. The image processing system of claim 2 , wherein the at least one ALC pixel further comprises a corresponding transfer transistor for transferring charge from the corresponding photosensor to the corresponding charge storage region, and wherein the corresponding transfer transistor of the ALC pixel is always activated.
4. The image processing system of claim 1 , wherein the array of pixels further comprises (i) a first reset line for resetting the plurality of pixels and (ii) a second reset line for resetting the at least one ALC pixel.
5. The image processing system of claim 4 , wherein the array of pixels further comprises a control circuit for providing a first reset signal on the first reset line and for providing a second reset signal on the second reset line.
6. The image processing system of claim 5 , wherein the control circuit is configured to provide the first reset signal on the first reset line at a different time than providing the second reset signal on the second reset line.
7. The image processing system of claim 1 , wherein the at least one ALC pixel is configured to continuously transfer charge to the corresponding charge storage region during charge integration.
8. The image processing system of claim 1 , wherein the array of pixels is an array of color pixels arranged in a Bayer pattern.
9. The image processing system of claim 8 , wherein a subset of pixels in the array of pixels are used for automatic light control, and wherein the subset of pixels correspond to one color.
10. The image processing system of claim 9 , wherein the color is red.
11. A method of controlling an imager comprising an array of pixels arranged in rows and columns, at least one row in the array including a plurality of pixels having four-transistor pixel characteristics and at least one automatic light control (ALC) pixel having three-transistor pixel characteristics, the method comprising:
operating the plurality of pixels to obtain a set of image pixel signals;
operating the at least one ALC pixel to obtain a light control pixel signal, wherein operating the at least one ALC pixel includes connecting a photosensor of the at least one ALC pixel to a respective charge storage region of the at least one ALC pixel during an integration period;
for each individual column of the array including one or more ALC pixels, comparing the light control pixel signal to a predetermined voltage level; and
determining a time interval for the integration period based, at least in part, on a comparison of the light control pixel signal to the predetermined voltage level.
12. The method of claim 11 , wherein operating the plurality of pixels includes transferring charge from a photosensor of each pixel in the plurality of pixels to a respective charge storage region of each pixel in the plurality of pixels after the integration period.
13. The method of claim 11 , wherein operating the at least one ALC pixel further includes continuously activating a transfer transistor of the at least one ALC pixel.
14. The method of claim 11 , wherein operating the at least one ALC pixel further includes continuously transferring charge to the charge storage region of the at least one ALC pixel during the integration period.
15. The method of claim 11 , wherein operating the at least one ALC pixel includes operating the at least one ALC pixel to obtain the light control pixel signal at a different time than operating the plurality of pixels to obtain the set of image pixel signals.
16. The method of claim 11 , wherein operating the at least one ALC pixel includes operating the at least one ALC pixel to obtain the light control pixel signal at the same time as operating the plurality of pixels to obtain the set of image pixel signals.
17. The method of claim 11 further comprising:
resetting the plurality of pixels via a first reset line electrically coupled to a respective reset transistor of each pixel in the plurality of pixels; and
resetting the at least one ALC pixel via a second reset line electrically coupled to a reset transistor of the at least one ALC pixel.
18. The method of claim 17 , wherein resetting the at least one ALC pixel includes resetting the at least one ALC pixel at a different time than resetting the plurality of pixels.
19. The method of claim 11 further comprising:
resetting the plurality of pixels via a first reset line electrically coupled to a respective reset transistor of each pixel in the plurality of pixels; and
using an estimated reset level for the at least one ALC pixel, wherein the estimated reset level is extracted from an extra pixel row of the array of pixels.
20. The method of claim 11 , wherein operating the plurality of pixels to obtain the set of image pixel signals includes performing correlated double sampling.