HYPERSPECTRAL IMAGING


a prism or grating,’ Pust said. ‘Tey can be more mechanically stable and they’re also more compact. Tey’re also typically cheaper than a classical hyperspectral imager.’ A nice side benefit from a filter-based camera,


Pust noted, which is especially useful for drone applications in forestry or agriculture, is that 3D height information can easily be reconstructed from the image sequence when viewing the object at different angles. ‘Tat comes basically for free; it’s just a matter of soſtware,’ he said. An additional lidar system has to be attached to the drone to capture 3D information when imaging with a classical hyperspectral camera. Glana Sensors has also incorporated Delta


Optical Tin Film’s hyperspectral filters in its cameras. Glana worked with a modified DSLR camera at first, and has now mounted the filter inside a machine vision camera from Lumenera. Tis camera doesn’t use a micro- lens array because Glana wanted to have 3D capability, which is not possible with a snapshot camera. Glana also focuses more on high spatial resolution. ‘With every snapshot camera, there is a compromise to be made between spatial and spectral resolution,’ Pust observed.


The light needs to be high power and also emit a broad spectral range, which isn’t necessarily natural for LEDs


it still needs some kind of scanning between camera and object. Fraunhofer’s camera works in such a way that the micro-lens array axis is tilted with respect to the filter axis, Pust explained. Te setup can be described as a multi-aperture camera, where each of the micro-lenses looks through a different region of the filter, with a different centre wavelength onto the same object in space. Push broom scanners require careful


synchronisation to get an accurate data cube, according to Pust. Attaching a hyperspectral


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camera to a drone, for example, is much easier with a filter-based approach, compared to a push broom sensor, because every image acquired has the complete 2D object. Pattern recognition can therefore be used to align the image spatially on an unstable platform. Filter-based cameras can also reduce the data


load, Pust observed. He gave the example of obtaining a data cube of 1,000 x 1,000 pixels, which would require 1,000 images using a classical push broom scanner. With a filter- based approach, the user would only need to acquire as many images as the number of spectral channels required in post-processing, which could be 50 or 100. Tis therefore reduces the amount of data by at least an order of magnitude. ‘Te filter-based cameras are more compact


and more robust, because they don’t need a certain distance between the sensor, the slit, and the dispersive element, as is required with


The lights Illumination is another component where dedicated hyperspectral solutions are springing up. Typically, hyperspectral systems will use halogen bulbs because they give broadband illumination, as well as being relatively inexpensive. Broadband light is important for hyperspectral systems, since they image over a large spectral range. Now, however, broadband LED lights are being designed with industrial hyperspectral systems in mind. ‘Halogen has quite a lot of drawbacks,


especially for any company trying to integrate hyperspectral technology into an automated system,’ commented James Gardiner at US lighting firm Metaphase Technologies. Te company offers dedicated broadband hyperspectral LED illumination, in both the visible to near-infrared, 400nm to 1,000nm, and the shortwave infared (SWIR), 1,000nm to 1,700nm. Te first disadvantage with halogen bulbs is


that they run at more than 100°C and, unlike LEDs, they radiate a lot of their heat in the same direction as their light output. Radiating heat onto food or pharmaceuticals, for example, isn’t ideal. One of Metaphase’s biggest application areas


at the moment is inspecting gel caps commonly used for ibuprofen and fish oil capsules. Te hyperspectral system uses SWIR illumination


December 2018/January 2019 • Imaging and Machine Vision Europe 15


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