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Hyperspectral Imaging - a Technology Update

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Hyperspectral Imaging - a Technology Update Hyperspectral Imaging - a Technology Update A White Paper by Dr Nick Barnett, Pro-Lite Technology Ltd May 2021 classification, object segmentation and improved colour characterisation. Imaging spectroscopy is often divided into two categories: multispectral and hyperspectral. Multispectral cameras measure light in a small number (typically 3 to 15) of spectral bands whereas hyperspectral cameras collect a much larger number (up to several hundred) of distinct, yet contiguous bands across a wide spectral range. Traditionally, hyperspectral imaging has found Introduction applications in remote sensing and agriculture New developments in imager technology and in using scanning or push-broom cameras installed on computer processing power are rapidly enhancing satellites, aircraft, or UAVs. In recent times, consumer digital camera performance, leading to a developments in spectral imaging technology have wider adoption of imaging in everyday use. been evolving at a pace. Advances in high Additionally, image processing techniques based resolution sensors, electronics and optics are on artificial intelligence and machine learning are providing enhanced push-broom technologies as increasing the capabilities of cameras and smart well as enabling numerous other forms of spectral devices for tasks such as object detection based on imaging. Alternatives to push-broom systems colour and two-dimensional geometric data. include methods using tunable spectral filters as However, these conventional RGB-type cameras well as designs based on Fourier Transform only make use of a limited range of wavelengths in spectroscopy. There are also snapshot spectral the visible part of the spectrum, so are missing out imaging systems employing mosaic arrays of filters on a lot of the available spectral information. and light field camera designs offering video-rate imaging spectroscopy. These technologies offer Spectral imaging captures the same geometric specific strengths that can sometimes be more image but in multiple narrow spectral bands that appropriate than the traditional push-broom can cover the broader visible, near infrared and cameras and have the potential to facilitate many shortwave infrared spectral ranges. This allows for new and exciting applications. identification of optical features of objects that are invisible to conventional cameras or the human The following paper provides a glimpse of some eye. Spectral features are directly attributable to new developments within hyperspectral imaging the chemical properties of an object, such that technology. It is not intended to be a spectral imaging enables tasks such as object comprehensive review but highlights what is a detection and identification, substance rapidly changing imaging landscape. www.pro-lite.co.uk White Paper: Hyperspectral Imaging – a Technology Update Page 1 Simply the Best: Traditional Push- Broom Cameras for the Highest Quality Data High quality scientific hyperspectral imaging data is required when looking for subtle differences in spectra. Cameras based on push-broom architectures typically provide the best quality data but even within this category there are varying levels of performance. Cameras made by HySpex (Norsk Elektro Optikk, NEO) are a good example of high quality devices that provide optimal spectral fidelity and sharp imaging optics. The care and attention taken to minimise optical distortions such as spatial and spectral misregistration and stray light are fundamental to final image quality Figure 1: The HySpex Baldur S-384 N SWIR hyperspectral and the ability to identify spectral features. camera (930-2500nm) is used to detect asbestos and separate it from other materials in mixed building waste. Five different The latest HySpex SWIR cameras use state-of-the- sample sets containing five different classes of materials are art MCT sensors with cooling down to 150K using a shown: asbestos (green); concrete (blue); terracotta (light blue); ceramics (purple) and unknown mix (red) Sterling cooler providing low background noise, high dynamic range, and exceptional signal to Internal Push-Broom Systems: noise. HySpex’s most recent SWIR cameras also provide exceptional spatial and spectral resolution. from the Laboratory to the Field These systems tend to cost more than some Typically, for laboratory or field measurements, alternatives, but the new Baldur range of industrial push-broom imaging applications require that the cameras from NEO (Figure 1) are more affordable sample object moves within the view of the while guaranteeing a similar level of high-quality camera, either on a conveyor belt or by using an data. Additionally, the combination of the Baldur external translation stage or rotating tripod head. cameras with Breeze run-time software (from However, new cameras have been developed with Prediktera AB) enables classification and the scanning mechanism built into the camera quantification of objects in real-time which is ideal body (the mechanical system moves the for process sorting and segregation applications. spectrometer entrance slit internally). With these cameras there is no need for an external translation stage or moving conveyor belt, instead the camera remains static. This can be useful when space is limited, and it also allows the camera to be easily attached to a microscope for hyperspectral microscopy applications. Examples of cameras using this approach include the VNIR and SWIR SnapScan cameras from Imec (Figure 2) and the VNIR and SWIR SOC-710 cameras from Surface Optics Corporation. www.pro-lite.co.uk White Paper: Hyperspectral Imaging – a Technology Update Page 2 to achieve with some of the older tunable filter technologies. Figure 2: The Imec SnapScan SWIR camera used to detect and classify different plastic materials. Tunable Filter Cameras for High Spatial Resolution and Microscopy Applications Hyperspectral cameras using liquid crystal or acousto-optical tunable filters (AOTFs) have been available for many years. In this approach the spectral bands are transitioned sequentially to create a resulting spectral hypercube. More Figure 3: Schematic of Fabry-Perot Filter and the Hinalea recently, tunable filter cameras based on piezo- model 4250 camera. actuated Fabry-Perot interferometry have been Fabry-Perot interferometer cameras also provide developed (for example Hinalea Imaging). great spectral flexibility as they can be used in a A Fabry-Perot interferometer consists of two hyperspectral mode, acquiring hundreds of parallel reflective mirror surfaces with a gap wavebands, or in a multi-spectral mode where the between the mirrors. The passband wavelength of data acquisition is limited to just a few wavebands. the filter is tuned by adjusting the distance In this multi-spectral mode of operation, it has between the mirrors (Figure 3). The filter is placed recently been demonstrated that these cameras in front of an imaging sensor and spectral bands can work at video rates with the application of real- are selected sequentially to generate a hyper-cube. time image classification. The acquisition time depends on the number of Tunable filter cameras are also ideal for spectral bands being acquired and the time hyperspectral microscopy as the cameras can be required to collect light at each band. These easily mounted with no additional scanning imagers are especially attractive for applications mechanism required. Canadian firm, Photon etc, requiring high spatial resolution as they utilise all has developed a high-performance hyperspectral available pixels on the camera sensor. The imaging platform based on tunable volume Bragg technology can be easily integrated into small gratings and global imaging (Figure 4). These camera designs which has previously been difficult essentially provide tunable bandpass optical www.pro-lite.co.uk White Paper: Hyperspectral Imaging – a Technology Update Page 3 filtering with high efficiency, high out-of-band replicas that share a common optical path. The rejection, and high spectral resolution. The entire resulting interference pattern is measured by a optical field of view is uniformly illuminated, and detector as a function of their relative delay and monochromatic images acquired sequentially. the Fourier Transform of the interferogram yields the intensity spectrum of the light (Figure 5). Figure 4: The IMA hyperspectral microscope from Photon etc used for photoluminescence imaging. a) PL image extracted at 770 nm, b) false-colour map of the PL central wavelength and c) two PL spectra extracted from the hyperspectral data – see corresponding colours. Images from Prof. David Cooke (McGill University) and Prof Mercouri Kanatzidis (Northwestern Figure 5: The Hera camera from NIREOS used for imaging University). chlorophyll fluorescence in leaves. These cameras offer high spectral resolution This newly developed camera works in a staring (<2nm in the visible) and high spatial resolution (in mode without the need for a translation stage or the order of 1μm) and have been used for scanning. It has potential advantages for photoluminescence, electroluminescence, hyperspectral imaging in low level light conditions fluorescence and reflectance in material science so is well suited to fluorescence imaging although and life science applications. it has also been shown to produce high quality spectral data for remote sensing and in art and Fourier Transform Hyperspectral cultural heritage applications. Imaging: Great for Low Light It is relatively straightforward to extend this Conditions technology
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