Wednesday, February 18, 2009

Line scan in action

Now that you’ve got a few successful vision applications under your belt how do you feel about tackling a line scan project?

Line scan cameras make even seasoned machine vision pros nervous. The whole basis of image acquisition is different to conventional area or matrix cameras. Rather than grabbing a frame, line scan assembles an image one row of pixels at a time. Typical applications are inspecting a continuous strip of material, (usually termed “web inspection,”) or unwrapping a cylindrical surface, such as a can of beans.

Either way, there are clearly some challenges in terms of synchronizing the acquisition of each line of the image with the movement or rotation of the part. Capture lines too fast and you oversample. Capture too slow and you miss regions. Too long an exposure and the image blurs. See what I mean? It’s complicated.

To make matters worse, line scan cameras come in several flavors. Obviously there’s color and monochrome, but the color subdivide into Bayer filter types and those with prisms and three sensors. Then there are the Time Delay Integration, or TDI, cameras, which are matrix cameras that think they’re line scan (or is it the other way around?)

Basler, purveyors of an impressive range of line scan cameras have attempted to clarify this murky world by placing some neat animations on their web site. (Color here, and monochrome here.) These show in clear, graphical terms, the basic principles of the various types of line scan cameras.

For anyone needing a better grasp of the principles and options, a short visit will be time well spent.

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