Telecentric lenses measure up
By Jon Titus, Contributing Technical Editor -- Test & Measurement World, 9/1/2007
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My July column (“Calibrate camera coordinates and colors”) described techniques you can use to calibrate vision systems. In some cases, though, basic calibrations do not suffice. General-purpose machine-vision lenses produce perspective in images so that distant and nearby components of the same type appear to have different dimensions. In the same way, cars far down a highway appear to our eyes as smaller than cars just ahead of us. We know the cars didn’t shrink, but vision software analyzing an image of them might think they did.
If components exist within a thin plane, perhaps on a printed-circuit board (PCB), minor distortions due to perspective may cause no measurement problems. But if manufacturing fixtures do not always place PCBs at the same distance from a camera or if components exist at different levels, say on a daughter card or subassembly, the accuracy of dimensional measurements can suffer.
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| Figure 1 a) This image shows different dimensions for pins on a PCB. b) This image, which was taken through a telecentric lens, provides accurate information. Courtesy of Edmund Optics. |
Because a telecentric lens accepts only parallel light rays, if the area you want to inspect increases, you will need a larger lens. A 16-mm-diameter telecentric lens used with a 2/3-in. CCD sensor, for example, offers an 11-cm horizontal field of view. As precision telecentric lenses get bigger, their cost goes up markedly. To reduce costs, you could use a small lens to inspect a product one section at a time rather than trying to inspect the entire product at once.
Although telecentric lenses overcome perspective errors, engineers also use them because of their overall accuracy. Greg Hollows, vision integration partners coordinator at Edmund Optics, explained that telecentric lenses have fewer lens-to-lens variations. Thus, if you need several lenses in a vision system, telecentric lenses help ensure you get comparable high-quality images from each lens. Lenses come with iris adjustments and may include a variable focus at slightly higher cost. And unlike general-purpose vision lenses that work with many camera-sensor sizes, telecentric lenses mate with only a few sensor sizes.
If you need dimensional accuracy over a given distance range, you should determine the dimensions of the area you want to inspect—the field of view—and then determine which lens and camera will work together. Lens suppliers can help match a lens to your requirements and equipment. Telecentric lenses come at premium prices because of their tight manufacturing tolerances and the small market for them. But better measurements can justify their cost.
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