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.

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.

Another source for machine vision cables

Cables are the unsung heroes of machine vision: if it weren’t for their sterling efforts all that camera work in converting photons to bits would be for naught. Yet so often the cable is an afterthought. And when we do come to hook camera to PC, how often do we find that the cable on hand wont reach, or the connector doesn’t fit? Add to that the need for screw-in connectors and suddenly you’re scratching your head wondering where to go for parts.

Wonder no more, because today I would like to share with you a new cable vendor. Well they’re new to me but I believe they’ve been in business for some time. I’m talking about Newnex Technology Corp.

California-based Newnex offer an extraordinarily wide range of Firewire, USB and GigE cables, and if you still can’t find what you need they’ll even make up a custom cable for you.

Something to add to your “Favorites” if nothing else.

Telecentric illumination – you didn’t know you needed it

When I visited the Robots & Vision Show in Chicago back in the summer I was impressed by the telecentric backlight that Edmund Optics had on their stand. (Well OK, I was impressed by the design ingenuity – take a telecentric lens and put an LED at the focal point, then charge nearly $900 to solve a problem most people don’t recognize.)

I guess it isn’t selling too well because I notice that there’s now a video on the EO web site that explains the benefits. It’s worth watching because in certain backlighting applications the light does offer some real advantages (although the presenter, Greg Hollows of Edmund Optics also points some of the disadvantages, which is very honest of him.)

Bottom line? In a precision gauging application this light can be a real life (and career) saver. Watch the video and see if you need it.

Better backlighting

I haven’t checked but I think it’s a safe bet to say that every piece of machine vision marketing material features a backlit image. (For the newcomers, backlighting is when you put a light behind the object you’re looking at so as to create a silhouette. With this black and white image it’s then straightforward to apply gauging tools to check key dimensions.)

Unfortunately, even backlighting is not as simple as it might appear. The problem is that light is emitted in all directions from the surface of the backlight, and not just towards the camera. This means that rather than appearing crisp, edges can actually be rather fuzzy, which is a problem if you need to make precise and repeatable measurements.

The answer, as discussed in this excellent article on MachineDesign.com (June 2nd, 2009) is to use telecentric backlighting, which means that that only parallel rays of light are emitted from the surface towards the camera, thus avoiding stray reflections.

As always though, there is a price to be paid: telecentric backlights typically need more space than a regular backlight, and of course, cost more. But if you need to make precise measurements you’ve got to put one on your shopping list.