The new calibration targets introduced with HALCON 12 offer added convenience over the previous style. You can easily perform an accurate calibration with just a single shot or use much fewer images than before, if a full 3D calibration is required.
MVTec provides targets for a field-of-view ranging from about 10 to 300 mm, which covers most applications. Should there be a need for larger or smaller targets, e.g. for bin-picking or microscopy applications, you can make your own using the operator create_caltab (it doesn’t actually make the target, but it creates a postscript and a description file so you can print the target and use it).
Here are some tips to choose a proper calibration target:
The size of the target should generally be between 100% and 200% of the field-of-view. Smaller or larger targets are possible, but special care has be taken during the calibration process in this case.
. If the target is smaller than the FOV, you can compensate for this by acquiring several calibration images in order to cover the FOV entirely.
. When working with low resolution images, make sure the diameter of the individual dots is no less than 20 pixels, Otherwise the mark extraction may fail. For an image with VGA resolution (640×480), for example, the target size must be twice the FOV to meet this requirement.
. If the target is significantly larger than the FOV (e.g. more much than 200%), you can still use it. Since you are able to fit fewer points into the FOV, you can compensate for it by moving the plate around and taking more images. Should you need to perform a 3D calibration, you might be limited by the depth-of-field, since the plate needs to be tilted. For 2D applications, however, this is not a problem.
. If you do decide to print your own plate, note that your office printer may not provide the accuracy you might need. As a rule-of-thumb, printing your own plate is OK for sizes 160 mm and larger. You can print smaller plates for proof-of-concept purposes, of course, but keep in mind that the accuracy will suffer or, in the worst case, the calibration may fail altogether. You can have your target professionally made by 3rd parties, or purchase one from your MVTec distributor once this becomes an issue.
With HALCON, it is possible to render an image of your target as seen by your imaging system. Email us and we will send you a program that shows how your target will appear based on your focal length, working distance, and other camera parameters. It will also calculate the dot diameter so you can check whether it meets the 20 pixel requirement.