Camera system with minimal area monolithic CMOS image sensor
The disclosure extends to methods, systems, and computer program products for digitally imaging with area limited image sensors, such as within a lumen of an endoscope.
1. A method of calibrating an image sensor, comprising:
providing an image sensor comprising a pixel array, wherein the pixel array comprises an optical black portion and an active pixel portion;
sampling one or more pixels in the optical black portion;
wherein sampling the one or more pixels in the optical black portion includes determining a median black offset measurement of each of the one or more pixels in the optical black portion;
wherein the median black offset measurements for each one of the one or more pixels in a line of the optical black portion are averaged to provide a final line offset estimate;
determining, in response to sampling one or more pixels in the optical black portion, a black level for the optical black portion;
comparing the determined black level to a target black level by subtracting the target black level from the determined black level to determine a difference between the determined black level and the target black level for the optical black portion;
comparing the difference to a threshold value; and
calibrating a black clamp voltage of the image sensor based on the comparison of the difference to the threshold value;
wherein, when the difference is greater than the threshold value, a first adjustment amount is applied to the black clamp voltage in the image sensor, and
wherein, when the difference is less than the threshold value, a second adjustment amount is applied to the black clamp voltage in the image sensor.
2. The method of calibrating the image sensor of claim 1 , wherein the first adjustment amount is larger than the second adjustment amount.
3. The method of calibrating the image sensor of claim 1 , wherein the second adjustment amount is one digital-to-analog converter count.
4. The method of calibrating the image sensor of claim 3 , wherein the first adjustment amount is more than one digital-to-analog converter count.
5. The method of calibrating the image sensor of claim 1 , wherein the second adjustment amount is more than one digital-to-analog converter count.
6. The method of calibrating the image sensor of claim 5 , wherein the first adjustment amount comprises more digital-to-analog converter counts than the second adjustment amount.
7. The method of calibrating the image sensor of claim 1 , wherein the image sensor is a CMOS image sensor.
8. The method of calibrating the image sensor of claim 1 , wherein calibrating the image sensor includes calibrating line noise for each column of pixels in the optical black portion.
9. The method of calibrating the image sensor of claim 1 , wherein the median black offset for a pixel of the one or more pixels is determined by finding a median of a group of pixels including the pixel and a plurality of pixels neighboring the pixel in the optical black portion.
10. The method of calibrating the image sensor of claim 1 , wherein sampling the one or more pixels in the optical black portion includes reading out a plurality of pixels a plurality of times.
11. The method of calibrating the image sensor of claim 10 , wherein reading out the plurality of pixels a plurality of times causes a delay equal to at least the total number of optical black pixels per analog-to-digital converter channel, per row of pixels.
12. The method of calibrating the image sensor of claim 11 , wherein the total number of optical black pixels per analog-to-digital converter channel affected by the delay are ignored.
13. The method of calibrating the image sensor of claim 1 , further comprising:
determining an overall frame black level by incrementally adjusting a running estimate of line offset estimates for each line of the image sensor when a new pixel sample for a line is available.
14. The method of calibrating the image sensor of claim 13 , wherein each time the running estimate is incrementally adjusted, the new pixel sample is multiplied by a first factor before being added to a previous estimate of the running estimate.
15. The method of calibrating the image sensor of claim 14 , wherein each time the running estimate is incrementally adjusted, the previous estimate is multiplied by a second factor before the new pixel sample multiplied by the first factor is added to the previous estimate.
16. The method of calibrating the image sensor of claim 1 , wherein the black clamp voltage is applied when optical black pixels from the optical black portion are in a digital readout path for the image sensor.
17. The method of calibrating the image sensor of claim 16 , wherein the black clamp voltage applied to the optical black pixels in the digital readout path reduces line noise.