Tuesday, May 26, 2015

When you’re backlighting cylindrical parts

I see backlighting used all the time in machine vision training classes and at trade shows, typically for gauging or locating shapes. Look closely though and you’ll see the targets are flat objects – boxes, stamped parts – those kinds of things. Never machined steel shafts.

There’s a good reason for that. Unless you’re using a collimated backlight you won’t get a true image. That’s because the backlight emits light over 180 degrees, and some of those rays strike the target shaft and reflect in to the camera, as shown in this rather crude sketch.

This means you will see bright pixels in what should be dark areas of the image, and those can play havoc with your vision tools.

Interestingly, I observed this in a recent application note from National Instruments. “Developing a High-Speed, High-Accuracy Measuring System for Automotive Screw Inspection” includes some screenshots from the system. If you look closely at image 3 in the gallery you’ll see what I mean.

Now there are ways around this. The best is to use collimated light (where all the rays travel in the same direction,) but if you can’t do that use the smallest backlight possible and position it as far behind the target as possible. That way you’ll cut down on those tangential rays coming off the part and into the camera.

There is no charge for this snippet of advice. All I ask is that you keep coming back. If you’d like to link to this page, even better.

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