If I wanted to measure the size of a through hole I’d start out by using a backlighting. Providing I was careful not to saturate the “hole” area of the image that would work pretty well. If I needed greater accuracy I might go with a telecentic lens and telecentric or collimated lighting as that would help me avoid any edge effects. But what if I wanted to know if the hole was clear?
I’ve actually encountered this problem. Sometimes in machining chips will get stuck in a drilled hole. Backlighting will show me the hole but may not reveal a little curl of metal that could cause a problem when the part goes in to service.
“Beam profiling finds use in industrial parts inspection”, (Laser Focus World, August 13th, 2013,) discusses an interesting example of such a challenge. In this case the hole was very small and backlighting wasn’t enough to check that the hole was clear.
The solution was to look at how much light came through the hole. How? Well the title of the article gives you a clue, but for more details you’ll need to click the link.
It’s an interesting solution, but I can’t help wondering if there’s an easier way to do this. If you’ve ever dealt with such a problem, perhaps you could share your solution.