ANALYSIS AND OPINION: HYPERSPECTRAL IMAGING IN MACHINE VISION
How to use multi-view 360-degree lenses for efficient defect inspection
Beatrice Danese from Opto Engineering gives a wide view of the 360-degree lens options used by machine vision systems to provide fast, accurate and reliable defect detection in the food and beverage and pharmaceutical industries
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any applications of machine vision, like OCR, barcode reading, defect detection or
integrity checks, may require multiple checks to be performed,” begins Beatrice Danese in her seminar detailing the advantages of multi-view cameras and special lenses for all-round, single- shot inspection at the Machine Vision Conference this year, “for example the inspection of the top or the bottom of the inner walls or the outer surface.” The common complications that arise from these kinds of inspection requirements include randomly located and randomly orientated samples, features and defects, while space constraints present an additional challenge in certain cases.
Drawbacks of a multi-camera system “Common solutions involve multi-camera systems and line scan approaches,” explained Danese. “As the name suggests, multi-camera systems get multiple views, and this for sure allows you to get high- resolution images of the sample within a flexible system – enabling the inspection of various forms and parts of an image, but this approach also suffers from some major drawbacks. Typically, such an approach requires
a large footprint because the minimum working distance in each direction is usually around 100mm.”
Multi-camera systems often require special high-speed hardware and software “Moreover, multi-camera systems often require special high-speed hardware and software to be used, [meaning] you need multiple BUS ports or GPUS, complicating the system even further. Using multiple cameras also means you acquire multiple images.
“So more setup time is needed to
calibrate and synchronise the entire system. And lastly, acquiring multiple images with multiple hardware components [increases] the risk of component failure.”
“Multi-view lenses give four different outer views in one shot”
Line scan offers little improvement “The other typical approach is line scan – using line scan cameras to scan the outer surface of objects – to get all the details of interest. In this case, you get very high-resolution images of the object with the outer surface perfectly captured, but [either] the object or the camera must be mechanically rotated to perform the acquisition, making the system incompatible with high-speed line applications. Meanwhile, additional line scan cameras may be required to inspect the top and the bottom of the object at the same time.”
360-degrees for one-shot inspection “360-degree lenses are special lenses that allow you to get a [more] complete image of the inspected sample with just one camera and no need for part rotation,” said Danese. “You basically have a 2D image of a 3D surface using one single camera in one single shot, and you can inspect either both the top and outer surfaces, or the [entire] inner surface of the part, depending on the design of the 360-degree lens. But it’s not a complex solution, claims Danese, because “you don’t need many other components. You just [need] one parallel lens to inspect everything and you don’t need to mechanically rotate the parts.” Because the inspection is achieved from just one camera shot, image processing is much faster and simpler, as Danese says, “because you simply need to generalise one image that can be unwrapped by software to get all the defects you want to highlight.”
Applications for 360-degree optics “There are three main categories of 360-degree lenses,” outlined Danese.
32 IMAGING AND MACHINE VISION EUROPE AUGUST/SEPTEMBER 2024
There are “outer inspection lenses for the inspection of the top and the outer sides of samples, inner inspection lenses for the inspection of the inner surfaces of cavities, and then there are multi-mirror lenses,” which make use of mirrors to provide multiple viewing angles of an object for inspection.
One type of outer inspection option
is the pericentric lens. “These feature a converging design,” says Danese. “Arrays converge on the optical axis, [giving] the possibility to inspect the top and the outer sides of a sample, like a vial [for example,] in one shot.” Using software, it’s possible to then
unwrap the image to analyse the sample for defects, says Danese: “Object diameters that can be covered using [pericentric lenses] go from seven to 60mm, so a wide range of samples can be inspected.”
Unwrapping the image for analysis Pericentric catadioptric lenses are another type of outer inspection lens outlined by Danese. “These allow [the user] to inspect both the top and outer surface of objects thanks to a catadioptic design that integrates mirrors and so allows you to have a more compact solution with high resolution, chromatic correction and the possibility to work on non-visible wavelengths.” These attributes mean “pericentric
lenses can be used to perform OCR or matrix code reading on vials for track and trace processes or the identification of defects or codes. But it can also be used to check flip-off caps for correct sealing, stopper absence or dents and scratches. So it’s extremely useful in the pharmaceutical industry.
Inner inspection lenses to look deeper Featuring a “cylindrical field of view that’s able to adapt to the cavity of cylindrical samples,” explains Danese, inner inspection lenses can “inspect both the bottom inner surfaces and inner sidewalls of the cavity at the same time. You have
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