Thursday, April 14, 2011

Scheimpflug adjustment – solving a problem you didn’t know you had

I’m getting used to Opto-Engineering coming out with interesting lens arrangements intended to solve imaging problems, but their latest – 3D bi-telecentric lenses with Scheimpflug adjustment – had me scratching my head. Fortunately, Wikipedia came to my aid. The Scheimpflug Principle is all about taking pictures of surfaces that are not parallel to the image plane, and that’s becoming more of an issue as 3D imaging continues to grow.

Here’s the problem: you set up a laser to project a line perpendicular to a surface. Then you view this line from a 45o angle. That’s the basic principle of structured light or laser triangulation. The problem though is that you position the camera so the laser line runs horizontally across the image, but the top and bottom are out of focus because the lens doesn’t have enough depth-of-field (dof) to keep the whole image sharp.

That’s not a problem providing the laser line doesn’t deviate much from horizontal, except that it will because the whole object of the exercise is to measure changes in height. So you close down the aperture to maximize the dof and then find you need a more powerful laser, at which point your Laser Safety Officer starts to take an unhealthy interest in your project.

The answer is to angle the image sensor so as to keep the whole image in focus, which is where the Scheimpflug Principle comes in to play. Captain Theodor Scheimpflug noticed that the dof problem can be avoided if 3 planes – image (your camera sensor,) object, and lens – all intersect at the same point. Of course, this means rotating the lens relative to the senor, which you can’t do with regular lenses. (Ordinarily, the lens plane is parallel to the image plane.)

And that is where Opto-Engineering come to your rescue. Their 3D bi-telecentric lenses with Scheimpflug adjustment make it possible to create this angle. So if you’re doing 3D machine vision based on structured light, it might be a good idea to learn more about what this lens can do for you.

Google will have lots of places to look, but two of the most helpful sites were “The Scheimpflug Principle,” (it’s a pdf,) and “Focusing the Tilt-Shift Lens” by David Summerhayes. Scheimpflug adjustment – you didn’t know you needed it.

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