HYPERSPECTRAL IMAGING
Spectral inspection
Greg Blackman on how advances in lighting, optics, and sensor technology are turning hyperspectral imaging into an industrial inspection tool
T
he acquisition of Perception Park by Stemmer Imaging is a long-term investment in hyperspectral imaging,
Mark Williamson, Stemmer Imaging’s managing director, told Imaging and Machine Vision Europe at the Vision 2018 trade fair in Stuttgart in November. Stemmer Imaging bought a stake in the Austrian hyperspectral soſtware provider in October; for two years prior to the acquisition Stemmer had acted as distributor for Perception Park’s technology. Perception Park’s processing platform renders complex hyperspectral data usable for machine vision, which is where Stemmer Imaging’s interest in the technology lies. ‘Hyperspectral imaging has become accessible to the traditional system integrator,’ Williamson said during a VDMA-organised panel discussion at Vision 2018. ‘Five years ago that wasn’t possible.’ Te accessibility of hyperspectral imaging
is now apparent in not only soſtware but also hardware, with cameras from companies such as Finnish spectral imaging firm Specim now designed for industrial use, in the case of Specim through its FX series. Spectroscopy and spectral imaging used to be the domain of scientists, but today’s spectral imaging equipment, while still reasonably complicated and costly, can theoretically be set up on the factory floor by non-experts. And its benefits are being seen in industrial areas such as recycling, food and pharmaceutical production. Williamson told Imaging and Machine Vision Europe during Vision 2018 that a hyperspectral imaging system could cost around €30,000, but he added that this would be the cost of just the feasibility study not so long ago.
The cameras Materials can be identified by how light interacts with them; compounds will have unique spectral signatures that act like a
fingerprint. In this way, spectroscopy can distinguish between different plastics, identify a pharmaceutical product, or grade the purity of a whisky. A spectrometer gives a point reading, whereas a hyperspectral camera builds on this, so that each pixel in the sensor collects many spectra over tens or hundreds of wavelength bands. Te resulting information is known as a data cube made up of measurements at each wavelength over the range. Te hyperspectral cameras that have been
around the longest are those based on prisms or gratings, which use a dispersive element and image a narrow line of the object through a slit. Tese are line scan cameras or push broom scanners, scanning line by line to build a 2D image. Specim’s FX cameras use push broom scanning. Snapshot hyperspectral cameras are also now available, such as Ximea’s XiSpec, based on Imec’s mosaic hyperspectral image sensors. Tese cameras capture the entire data cube in one go, rather than scanning across an object line by line.
The optics New optical components are improving the accuracy and reliability of hyperspectral cameras. Delta Optical Tin Film has developed continuously variable bandpass filters (CVBF) specifically for hyperspectral imaging. Te filters have been used in a snapshot camera from Fraunhofer IOF and in cameras from Glana Sensors. Te coating thickness on these filters – and therefore the spectral characteristic – changes from one end to the other along one direction of the filter. Te result is a bandpass filter in which the centre wavelength changes continuously along its length. Delta’s filters for hyperspectral imaging will
only transmit a narrow band of light for any given position within the working wavelength range of the filter. All other wavelengths above
14 Imaging and Machine Vision Europe • December 2018/January 2019
Hyperspectral imaging can reveal defects in pharmaceuticals
or below this band are suppressed to at least OD4 in the wavelength range, where silicon detectors are sensitive, from 200nm to 1,200nm. ‘You get a very clean signal with a very high signal-to-background ratio,’ Dr Oliver Pust, Delta’s director of sales and marketing, said. ‘Te gradual change in coating thickness
gives you a real hyperspectral filter,’ he continued. ‘Other filter approaches have a number of discrete bands, making them inherently multispectral. But Delta’s filter is truly continuous, so each pixel will see a slightly different centre wavelength.’ Delta has filter versions covering 450nm to 950nm and 800nm to 1,100nm. Tere is also a version from 450nm to 850nm that can be used for slightly smaller sensors. Fraunhofer IOF’s snapshot camera uses a
micro-lens array. Mounting a continuously variable bandpass filter directly in front of the image sensor gives a hyperspectral detector, but
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