There are several factors to take into account when choosing or creating a calibration grid of dots for vision applications while utilizing NI Vision Software:
- Minimum Dot Size. The actual size of the dot will vary based on the number of pixels of the camera and the field of view selected. An optimal size is at least 9 pixels (3 x 3), which will ensure the camera will detect the dot accurately and IMAQ Vision will correctly process the dots.
- Minimum Dot Spacing. The individual dots should be at least 4 pixels apart. This will prevent two or more dots from being processed as one dot.
- Quantity of Dots Consideration. As the number of dots in the field of view increases, the calibration of the image (in software) becomes better. For irregular or curved surfaces, a greater number of dots will insure the contour of the field of view is more accurately represented.
- Grid Layout. The dots need to be in vertical and horizontal lines that are perpendicular to each other to make up the grid:
NI Vision Software ships with 3 calibration examples for LabVIEW that install by default in <LabVIEW>\examples\Vision\2. Functions\Calibration. You may use the calibration grid installed with the software by navigating to Start»All Programs»National Instruments»Vision»Documentation»Calibration Grid. The dots have radii of 2 mm and center-to-center distances of 1 cm. If your field of view is too small to view a large grid of dots at this scale, you can print the grid at a smaller scale. Since a calibration grid is included with the installation of the Vision Software, NI does not currently sell any calibration grids individually.