FEATURE HYPERSPECTRAL IMAGING g
translation stage behind the lens. The sensor has 100 spectral bands in the SWIR range, or 150 bands in the VNIR range. The high- resolution spatial image makes it a good choice for lab setups and application research. Imec also offers its Snapshot technology, which is based on a mosaic pattern of 3 x 3 or 4 x 4 filters covering the imager. The camera achieves video-rate spectral imaging at the cost of some of the spatial and spectral resolution. Snapshot VIS, RedNIR and NIR cameras are available with 16 or 25 spectral filters with real-time acquisition; the SWIR version is available with nine filters, with 16 bands coming soon. Snapshot cameras would typically be used for industrial implementation. ‘Plastic sorting might use either a push-
broom scanning camera, imaging line by line and using the motion of the conveyor to do the scan,’ explained Charle. ‘The other would be an area scan camera like our Snapshot camera, which behaves similar to a regular machine vision camera.’ He added: ‘We have feedback from industrial partners that, because this is a snapshot camera, it can give a complete field of view quickly, rather than waiting to build up a scan.’ A robot bin picking application, for example, can classify objects in the full field of view and identify a new location for the robot arm to move to. Waste sorting is often done on a conveyor belt with many people around
‘If we can make [SWIR] available in a system then these applications will come’
the conveyor picking and removing material. There will be occlusions from material lying on top of items, so you need to reacquire images regularly to automate this process, Charle explained. Imec has a few feasibility studies with industrial partners where it has shown that the snapshot SWIR camera with nine bands gives sufficient information to classify around 20 different types of fabric – polyester, cotton, wool, linen and others. ‘That was enough to have a high success rate and reliability in classification and sorting,’ Charle said. ‘A human would only have a 66 per cent accuracy. With this study, we have shown that with just nine spectral channels in the shortwave infrared – carefully positioned to measure the required spectral features – you can realise this sorting application.’ Sorting all plastics is a bit tougher,
according to Charle, because the dye colour of the plastic will have an impact on the success of the classification. Black plastic, from which electronic items are often made, is more difficult to classify than white plastic because it does not reflect a lot of light. Hyperspectral
28 Electro Optics March 2021
Demo using Imec’s Snapscan camera, based on a line scan sensor that is scanned on an internal translation stage
imaging depends on the light reflected from the sample, so dark plastic won’t give a good signal. ‘Hyperspectral imaging needs a lot of light, so that’s still a challenge,’ he said. ‘To be completely independent of dye
colour you would need to move to the mid- wave infrared, but this is a different imaging technology and is not price effective at the moment,’ Charle added. ‘There are still challenges, but for instance
in fabric sorting we have clear success,’ he said. ‘For bright plastics, then hyperspectral imaging is a no-brainer – we have this as a demo; you can easily recognise the different plastics with a nine-band sensor.’ In terms of lighting for SWIR hyperspectral imaging, Charle said that the best source is still a halogen lamp. There are SWIR LEDs available, but they tend to be only narrow- band versions that don’t cover the SWIR range fully. In the visible spectrum there are wide-band LEDs available, which is good for machine vision and hyperspectral in the visual range. Charle said that there’s still a way to go, however, to provide wide-band SWIR LEDs. There are other opportunities in recycling, and Imec has worked on paper sorting. Recycling plants pay per kilo for paper or cardboard, but the quality of it depends on its moisture content – too moist and it can’t be recycled effectively, but it will weigh a lot more. Moisture content is usually determined by manual sampling, but suppliers might put moist paper on the bottom of the batch and good paper on the top. ‘There is a clear absorption valley in the SWIR range caused by water, so a SWIR camera can be used to do a quality inspection on the incoming paper and measure the water content,’ explained Charle.
Each of Imec’s SWIR hyperspectral sensors undergo a characterisation stage, where the
sensor is tested to see how it responds to light for each nanometre in the SWIR range, 1,000 to 1,700nm. In this way, a spectral sensitivity model is built up for the camera. The camera is also validated once the bandpass filter has been integrated. The true spectral information is captured by the system and given as a digital reading. The characterisation data is used to interpret the digital reading, to convert it back to true irradiance and reflectance spectral information. ‘Our goal is to go as close as possible
to the end-user; we don’t want the user to worry about how to go from raw data to good hyperspectral data,’ Charle said. ‘But when the application starts, the customer will need to tune the system.’ Imec has a support team of application
experts and can help with feasibility studies. ‘There needs to be a further industrialisation step now to improve uptake of hyperspectral imaging,’ Charle noted. ‘It’s a new technology. We expect that customers will firstly explore the potential of hyperspectral imaging by doing application research, after which they will bring this to OEMs and start to make systems in volume. ‘SWIR imagers are still niche,’ he continued. ‘When there can be a volume application attached to it, we will push to convince sensor manufacturers to go to higher volumes, which will drive prices down and make this technology more available.’ Charle said that eventually Imec wants
to integrate SWIR sensors on its CMOS processes, similar to what it has done with its chips on CMOS in the visible and NIR range. ‘This is a long way ahead,’ he said. ‘People have to start using the technology. If we can make it available in a system then these applications will come, and then this market pull will follow.’ EO
@electrooptics |
www.electrooptics.com
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