Lens quality is key in machine vision
Two lenses could have nearly identical specifications yet produce very different image quality.
By Ann Thryft, Contributing Technical Editor -- Test & Measurement World, 8/1/2010 12:00:00 AM
Machine-vision camera lenses are usually treated as accessories: Their characteristics may not be specified, or even considered, until the vision system has already been designed. Yet, the lens is responsible for much of an image's quality, said Greg Hollows, director of machine-vision solutions for Edmund Optics. "The optics provided by a camera lens can be thought of as a signal conditioner," he said. "The lens conditions the signal that goes from the object viewed, via the lens, to the camera's image sensor."
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When specifying optics for a machine-vision system, system designers and integrators must consider resolution, field of view, depth of field, and working distance. Two lenses could have nearly identical specifications yet produce very different image quality, said Hollows. That's because the composition of a lens design—the number of elements and their overall tolerancing and quality—determines much of the image quality. For example, two lenses can have the same focal length, but their designs may be targeted at different working distances. If one is designed for surveillance and the other for close-up semiconductor inspection, their individual elements will differ, such as lens curvature or the spacing between internal lenses.
Integrators usually choose a sensor first and then match a lens to that sensor, said Hollows. "But it's best to start considering lenses when you're at most 60% to 70% of the way toward making your sensor choice, because it may be difficult to find the right lens that meets all of your other, non-sensor-related needs."
The inspection of semiconductors and some solar wafers requires high-powered lenses that can help operators evaluate small defects or make fine measurements. For these applications, integrators should look for a lens that provides the smallest detail needed at the highest levels of contrast possible, said Hollows. In semiconductor inspection, high resolution and a small field of view are often needed to examine very small detail in a small area, but integrators often want to combine these qualities with longer working distances, large depth of field, and compact package sizes.
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"Unfortunately, integrators may need to trade off some lens qualities, since physics dictates that these qualities often vary in opposite directions," said Hollows. "For example, to achieve higher resolution, a lens must have a low f number, but to achieve a larger depth of field, a lens requires a higher f number. In addition, a lower f number will make lens diameter grow, affecting package size."
Integrators might also want to consider using telecentric lenses to ensure measurement accuracy and repeatability in a metrology or inspection system and eliminate variation, said Hollows. Telecentric lenses are especially suited for metrology because they remove parallax, or perspective effects, thus eliminating measurement errors related to the apparent change in size of an object if its position moves closer to or farther away from the lens. Unlike conventional lenses, telecentric lenses yield the same size field of view regardless of the distance an object is from the lens. Since the object does not appear to change in size, the measurements remain consistent. Compared to standard lenses, however, telecentric lenses are limited in the size of their field of view, the lenses are larger, and they cost more. Their use has been increasing in metrology because software that can analyze the images they produce has improved and is easier to use.
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