Calibration Guidelines
Here are some other important factors to consider when calibrating your image:
- When calibrating, the set-up between the In-Sight vision system and lens, and the physical relationship between the sensor and the scene being acquired, must be the same for both the calibration and run-time operation. If any of these items is altered, the system must be re-calibrated. Therefore, it is recommended that the same sensor and lens be used to both calibrate and process the run-time images.
- The sensor and lens must also retain their original physical set-up and calibration settings. For example, changing the acquisition format (by altering the resolution) or moving the sensor will invalidate the computed 2D transformation that maps pixel to real-world coordinates.
- For the highest degree of accuracy and to get the largest area of the image calibrated, ensure that the image area is sufficiently covered by the grid pattern. The goal should be to acquire at a minimum 10 rows and 7 columns of dots or grid indices, or at least 100 feature inputs.
- For images with a large field of view, use a smaller, less cumbersome calibration plate to allow for multiple poses throughout the field of view to calibrate the entire image.
- For highly distorted images, use multiple poses to acquire a larger number of calibrated references to increase the accuracy of the calibration.
- Maintain consistent, even illumination.
- Place the calibration grid on the same plane as the object being inspected. If, for example, you are examining a 3D object, and you're inspecting the top of that object, the calibration grid must be at the same height.
Choosing a type of calibration grid - dots versus checkerboard:
Grid of Dots
- Relatively high accuracy, up to 0.05 (1/20) pixels.
- Tolerates up to 30° perspective distortion.
- Excellent tolerance to noise and inconsistent lighting.
- Each dot must be between 10 and 50 pixels in diameter (dots of 15 pixels or greater will yield more accurate results).
Checkerboard
- The highest degree of accuracy, up to .025 (1/40) pixels.
- Tolerates up to 45° perspective distortion.
- Good tolerance to noise and inconsistent lighting.
- Squares should be at least 15 pixels in width.