Lens choices get more complicated
Higher-resolution components don't necessarily produce better images.
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 12/9/2010 12:28:59 PM
"The lens is the gating item in a machine-vision system: You can't make the image any better if the lens isn't good enough," said Stuart Singer, VP of Schneider Optics. "Probably 95% of the calls I get where people need help with a lens is when they've boxed themselves in: They've chosen the camera, the sensor, the lighting, and now they need a lens. But these choices should be made concurrently. When all those other choices have been made first, we have to negotiate what they can or can't give up, since they didn't consider the lens."
Higher-resolution components don't necessarily produce better images. Instead, lens and sensor resolutions must be matched, said Jason Baechler, manager of sales and marketing for Moritex USA's machine-vision division.
But before picking a lens or a sensor, you should refine your specifications, said Nicholas James, product line manager for Edmund Optics. "You need to choose both at the same time to make sure there's a lens for your application," he said.
Choosing a lens is more complicated than choosing a sensor, since multiple lens characteristics must be considered together and traded off against one another. Not all lens makers rank these characteristics in the same order.
"The most important lens features are field of view and resolution, followed by distortion, spectrum, and f number," said Jonathan Kane, president of Computer Optics. "These will also determine the space constraints, which are a major issue that OEMs and integrators haven't always considered. We try to get information on the physical parameters of the lens, such as its space and weight requirements, at the beginning of our discussions."
Craig Fitzgerald, VP of product and business development at Navitar, said his company helps customers find the right lens by asking them four questions to determine the field of view, the working distance, the CCD sensor size or type of camera they've chosen, and the size of the object to be imaged in the application. "We also try to find out how the object will be illuminated," he said.
Coordinating lens and sensor choices is highly dependent on the application, said Moritex's Baechler. "Although resolution is important, knowing that without also knowing the field of view is not very useful."
MTF (modulation transfer function) data tells you whether lens performance is strong across the entire field of view, and not only in the center, which is especially important in large-format and linescan lenses, said Baechler. Working distance is related to the entire envelope of the optics. Often, little space is available in semiconductor, solar, and other electronics manufacturing systems. "But in flat-panel inspection, panels are big and, generally, so is the manufacturing equipment, so you often want a long working distance, especially when various system components are moving," he said. "This is also often true for PCBs [printed-circuit boards]."
The first lens characteristic Schneider Optics' Singer asks customers about is magnification, determined by the size of the smallest defect that will need to be analyzed. Once you know the necessary magnification and the sensor's physical size, you can choose the lens architecture and type that are suitable for the application, such as a micro or macro lens or a telecentric lens. "For example, if you need the lens to produce a one-to-one image at the exact same size as the object, like in flat-panel display inspection, you go to the unity lenses," he said. "But if you need to enlarge something by a factor of one or two, you go to the macro lenses. Perhaps you need a giant reduction in flat panels, where you're reducing the object by 50 times. Here, you could use a telephoto or inverse telephoto lens."
After magnification, the next step is choosing a lens with a resolution commensurate with sensor pixel size, Singer continued. Other considerations are field of view, depth of field, and depth of focus. The working distance determines focal length but not lens type.
In order to get what the integrator or OEM wants, lens features must sometimes be traded off against each other, and against the cost of those features, since customers often want a lot of performance for the lowest possible cost, said Kane of Computer Optics. But good performance in a precision lens costs more.
"Often, integrators don't understand the relationship between field of view and precision," Kane said. "For a given number of pixels on a lens' focal plane array, in a larger area there's less resolution, and in a smaller area there's more resolution. For example, if you have a 10-pixel camera, you can only break up an image into 10 pieces. The smaller the field of view, the smaller the area each piece sees, so the greater the resolution. But if you increase field of view, each piece, or pixel, will see a larger area, and resolution will be lower."
Although sensors with smaller pixels are increasing camera resolution, those smaller pixels mean slower frame rates, so cameras don't gather data as quickly, said Fitzgerald. "For example, you might only be able to achieve 20 to 60 fps at 2.2-micron pixel resolution," he said. But he noted that most Navitar customers designing machine-vision systems would trade resolution for larger pixels and faster data gathering.
Another tradeoff customers must make is between depth of field and magnification. "Customers often ask for high depth of field plus high magnification, but those are directly opposed: the higher the magnification, the lower the depth of field," said James. "They also often want long working distance, but the farther away the optics are, the larger they must be to hold resolution constant. Resolution and contrast are the most important factors: Each inspection system requires a minimum, below which the application won't work."
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