Useful backlighting tip

If you’ve ever tried backlighting a polished metal cylinder – like a roller bearing, for example – you’ll know it’s not as easy as much of the machine vision literature would imply. The problem is that if your backlight is larger than the diameter of the cylinder you will inevitably see reflections from the sides of the pin. This makes it difficult to ascertain the actual edge and as a result, diameter measurements will be unreliable.

One solution is to use a telecentric or collimated backlight. Edmunds make a range, However, they are rather expensive and are only available in smaller diameters. But help is at hand!

Spencer Luster of Light Works recently published an email tip suggesting an alternative technique, one that I’d never heard of. This involves using two circular polarizers either side of the cylinder you’re trying to gauge. I don’t really understand all the physics of polarization, but judging by the images Spencer offers, it seems to do the trick.

I couldn’t find the paper on the Light Works web site, so if you want to learn more I suggest you use the “Contact” button and email a request. I’m sure Spencer will be very pleased to hear from you.