Automatic exposure calibration and compensation for machine vision
View Patent ↗An automatic process for calibrating the optical image exposure value to compensate for changes in machine vision systems to maintain optimal exposure of captured images and adjust for the different characteristics among cameras components, such as imaging sensor sensitivity, LED strength, external lighting, reflective functions of any background material, different external lighting conditions, possible changes or updates of the systems over the years, such as LED changes, and even the aging of camera sensor and LEDs over time. A series of images of a target are captured with a predetermined sequence of exposure values, the saturation exposure percentage of a region of interest is calculated in each of the images, and the saturation exposure percentages are compared to determine the exposure value that has a saturation exposure percentage that varies the least from the saturation exposure value of the preceding and following exposure values.
1. A method for automatically setting the optimal exposure for capturing a digital image, comprising the steps of:
capturing a series of digital images having an array of pixels, wherein each image is captured using a different exposure value;
determining a set of boundaries defining a region of interest that includes at least of a portion of a target image and a background region;
calculating a background saturation percentage for each image based upon the percentage of pixels falling within the background region that exceed a saturation threshold relative to the total number of pixels falling within in the background region;
identifying the exposure value associated with the image having a converged background saturation percentage.
2. The method of claim 1 , wherein the different exposure values fall within a predetermined range and are evenly incremented.
3. The method of claim 2 , wherein the step of determining a set of boundaries defining a region of interest that includes at least of a portion of a target image and a background region is performed on the digital image having one of the highest exposure values.
4. The method of claim 3 , wherein the step of determining a set of boundaries defining a region of interest that includes at least of a portion of a target image and a background region is performed on the digital image having the highest exposure value.
5. The method of claim 3 , wherein the saturation threshold is 250.
6. The method of claim 5 , wherein the converged background saturation percentage comprises the exposure value having a background saturation percentage that varies the least from adjacent exposure values.
7. A machine vision system having an automatic exposure control, comprising:
an optical image for capturing digital images having an array of pixels;
a microcontroller associated with the optical imager;
wherein the microcontroller is programmed to capture a series of digital images having an array of pixels, wherein each image is captured using a different exposure value, determine a set of boundaries defining a region of interest in one of said images that includes at least of a portion of a target image and a background region, calculate a background saturation percentage for each image based upon the percentage of pixels falling within the background region that exceed a saturation threshold relative to the total number of pixels falling within in the background region, identify the exposure value associated with the image having a converged background saturation percentage.
8. The system of claim 7 , wherein the different exposure values fall within a predetermined range and are evenly incremented.
9. The system of claim 8 , wherein the step of determining a set of boundaries defining a region of interest that includes at least of a portion of a target image and a background region is performed on the digital image having one of the highest exposure values.
10. The system of claim 9 , wherein the step of determining a set of boundaries defining a region of interest that includes at least of a portion of a target image and a background region is performed on the digital image having the highest exposure value.
11. The system of claim 9 , wherein the saturation threshold is 250.
12. The system of claim 11 , wherein the converged background saturation percentage comprises the exposure value having a background saturation percentage that varies the least from adjacent exposure values.
13. A method for automatically setting the optimal exposure for capturing a digital image, comprising the steps of:
capturing a first image at a first exposure value;
capturing a second image at a second exposure value;
calculating a first median pixel intensity in at least a portion of said first image;
calculating a second median pixel intensity in a corresponding portion of said second image;
determining whether the said first median pixel intensity and said second median pixel intensity satisfy a predetermined condition;
changing said first predetermined exposure value and said second predetermined exposure value if said different exceeds said threshold.
14. The method of claim 13 , wherein said predetermined condition comprises said first median pixel intensity and said second median pixel intensity falling below a threshold.
15. The method of claim 14 , wherein said predetermined condition comprises the difference between said first median pixel intensity and said second median pixel intensity falling below a threshold.
16. The method of claim 14 , wherein said predetermined condition comprises the difference between said first median pixel intensity and said second median pixel intensity falling below a threshold and said first median pixel intensity and said second median pixel intensity falling below a threshold.