Hello inko,
In Vision Builder, there are a few edge detection filters that would help you with the caliper measurement. In the Inspection Steps palette, under 'Enhance Images', there are a couple of options that could be useful to you.
The 'Filter Image' step has edge detection filters to choose from, that would make it easier to define transitions at the edges. The 'Threshold Image' step would enable you to distinguish between bright and dark portions of the image according to a threshold value that you can set. It might be a bit tricky to use the threshold step, as it cannot detect both the dark and bright spots at the same time.
I hope this helps! If this does not answer your question, could you post a sample image in your next post to help me understand your issue further?
Vivek Nath
National Instruments
Applications Engineer
Machine Vision