HYPERSPECTRAL IMAGING g
and cost competitiveness. Te light collection efficiency is twice that of previous generation cameras, Hyvärinen claimed, enabling high-speed applications with less illumination. And manufacturing the cameras is easier, because the company has minimised the assembly and adjustment phases. And now, Specim’s Fenix cameras have
both visible and near-infrared and short- wave infrared imaging spectrometers, spanning wavelengths from 400 to 1,000nm and 900 to 2,500nm, integrated behind common front optics. Tis single instrument collects this entire spectral data range for each image pixel simultaneously. Compared to a dual camera set-up, where each camera has its own front optics, this design eliminates the extra data processing step of overlapping two images from two different cameras, Hyvärinen explained.
Collecting snapshots Beyond line scanning, snapshot spectral imaging builds the 3D cube of images in a single moment, which makes it much easier to follow dynamic scenes. Commonly companies array wavelength filters into a grid on a detector and combine it with standard camera objective lenses. However, this often comes at the expense of spatial resolution, Edmund Optics’ Lange noted. He highlighted sensors based on this principle developed by Belgian firm Imec. Te sensors restrict groups of several neighbouring pixels on silicon-based detectors to a particular wavelength. Using Fabry-Perot filters it provides sensors that can exceed 100 colours, spanning 600 to 1,000nm or 450 to 960nm off-the-shelf. As Fabry-Perot filters allow other harmonics through, Imec adds another filter to avoid cross-talk with these wavelengths. Several smaller camera manufacturers using these Imec sensors exploit Edmund
To make continuous variable filters, Delta Optical Thin Film coats materials onto fused silica substrates with gradually changing thickness
Optics’ VIS-NIR series of objective lenses. ‘Tey have good transmission over the entire visible and infrared range with a lot of choices, in terms of focal length,’ Lange said. Snapshot hyperspectral images could be
used when surgeons are removing tumours, said Delta Optical Tin Film’s Pust. ‘You really need to have a snapshot camera to tell you that’s where the tumour is, and that’s where healthy tissue is instantaneously,’ he said. It’s also difficult to align image series from scanning hyperspectral cameras on drones, Pust said, but it’s much easier with snapshot cameras. Delta Optical Tin Film’s Bifrost
continuous variable bandpass filter technology is found today in snapshot hyperspectral cameras, explained Pust. To make continuous variable filters, the company coats materials onto fused silica substrates with gradually changing thickness. Te thickness determines the light wavelength that can pass through each area. Te approach enables very high transmission and allows more design flexibility than filters coated directly onto sensors have. ‘When you deposit directly on the chip, for patterning to enable the snapshot technology, you need to remove parts of the coating again,’ explained Pust. ‘Tat has a limit as to the thickness that you can remove again. We don’t have that limit without that photographic lift-off process.’ Pust said that the result surpasses
Specim’s FX17 camera is used in Picvisa’s machine vision solutions for waste treatment and recycling
rival filters, which don’t have a signal to background ratio better than 10:1. ‘With our filters, we can really block all the lines that you do not want, apart from that narrow
16 IMAGING AND MACHINE VISION EUROPE OCTOBER/NOVEMBER 2020
‘Tere are now cameras on the market that are easier to handle for many more applications’
bandpass you’re looking at,’ he said. ‘We can suppress all the other wavelengths by four orders of magnitude.’ Sensors exploiting such filters initially
needed scanning systems to build up hyperspectral data cubes. But in 2018 a further advance by Fraunhofer Institute for Applied Optics and Precision Engineering, in Jena, Germany, enabled snapshot hyperspectral imaging. ‘Tey combined our filter in a smart way with microlens arrays,’ said Pust. Tat enables high- quality snapshot hyperspectral cameras, commercialised by companies such as German firm Cubert.
Innovating past the hype For the HySpex division of Norwegian firm Norsk Elektro Optikk, the optical design goal ‘is almost always to achieve a very high performance at the pixel level’, according to Trond Løke, chief executive officer. Te quality of the hyperspectral systems
it builds, such as its Mjolnir product, always begins with a high-performance optical design. ‘Before that camera was introduced, you could either have good quality data or you could have a small hyperspectral system – and we were selling only good quality
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Specim
Delta Optical Thin Film
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