Sunday, July 7, 2013

Solve your machine vision lighting problems


In machine vision contrast is king. Creating contrast between what you want and do not want to see makes it possible to apply edge tools that are the basis of just about every machine vision application. (Yes I know about blob analysis, but that too relies on creating contrast. And pattern-matching is little more than fancy edge detection.)

But there is another way. By measuring distance to the target, laser triangulation 3D eliminates the variation in contrast resulting from changes in surface reflectivity. And in the real world the things we need to inspect often vary slightly. Ink dries faster or slower, there’s natural grain in the finish, or the condition of molding dies changes through a production run. All these can upset a conventional front lighting approach.

Note though that low angle and back lighting techniques, by virtue of using geometry rather than surface appearance, are largely immune to changes in appearance.

Take corks as an example, the kind that keep wine in bottles. Being a natural product (I hope you’re not using those awful ‘plastics’,) they have lots of lines on them, so how you do distinguish between lines and edges? With 3D vision of course.

Now I didn’t just happen to pick on corks; they are the subject of an application study in Inspect-online. “No Leakage” (June 27th, 2013,) describes a system developed by Spanish integrator baxicat visiĆ³n S.L. This uses two camera-laser pairs to inspect the ends of corks as they pass by in a kind of Ferris wheel arrangement.

The article gives lots of good details on how the task was accomplished, which included enlisting the assistance of point cloud specialists Aqsense. If you’re struggling with a surface that varies in appearance, I suggest you read it.

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