SPONSORED: HYPERSPECTRAL IMAGING
Gaining an edge when designing a hyperspectral imaging system
H
yperspectral and multispectral imaging techniques were originally
developed for military and space applications for aerial imaging. In recent years, other industries
have seen the advantage of these techniques and adopted them, such as quality control inspection in agriculture and semiconductors, DNA sequencing and PCR testing, anti-counterfeiting, emissions monitoring, and traditional machine vision applications in industrial settings – the list goes on.
Hyperspectral imaging is used to capture a wavelength intensity map of a field of view (FOV) with high spatial resolution. This combination of spectral data at each pixel, combined with the corresponding spatial data enables the analysis of a range of characteristics including colour, chemical content and other spectroscopic detail.
Important system components Most hyperspectral imagers are configured as a scanning ‘push-broom’ imager. This means that for each frame capture, the FOV observed by the system imaging lens is collected through a slit aperture onto or through a diffractive element (grating) and dispersed onto a two-dimensional (2D) sensor array. The axis of the sensor which is congruent with the slit aperture captures the
spatial information of the scene, while the spectral information of that scene is collected by the sensor pixel array orthogonal to the slit aperture. In practice, this push-broom configuration is used to collect data on a moving scene, whether the scene is moving relative to the camera (fixed inspection on parts on a conveyer) or the camera is moving relative to the scene (camera attached to a drone flying above the scene). All of these frames can then be combined to create a hyperspectral ‘data cube’ which contains X and Y (spatial) information on the scene, with a range of spectral data for each X and Y location. An alternative to the
traditional push-broom imaging configuration is to image the scene directly using a 2D sensor array and a linear variable filter (LVF). An LVF is a unique type of interference filter where spectral performance varies along one axis and is constant along the orthogonal axis. It can be a narrow bandpass design, providing spectral discrimination, or a long pass design and used as an order sorting filter in a grating-based spectrometer. In both cases, spectral performance shifts in a consistent manner as a function of spatial position on the filter. The use of a filter to
replace the grating or prism in an imaging system results in a significant reduction in weight, size and cost of the imager. High spatial resolution allows for discrimination of fine detail, texture and resolution of a scene. The design of an imaging
system using a linear variable filter requires attention to a number of unique details.
“An LVF is a unique type of interference filter where spectral performance varies along one axis and is constant along the orthogonal axis”
The spectral resolution of an LVF camera is limited by the bandwidth of the filter, the operating F-number of the imaging system and the distance between the filter and the detector. Alignment of the filter to the detector array is important in extracting spectral information.
Important Design Considerations Choosing the right light source is a significant consideration when using an LVF camera. Fluorescent or LED lighting consists of strong spectral bands and therefore solar or incandescent lighting is preferred. An important tradeoff of the LVF camera is that it does not generate a true hyperspectral
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data cube. The wavelengths in the spectrum correlate to the position of the camera’s LVF/pixel wavelength. A wavelength versus frame
number is created for each pixel, but the spectral scan for each point will potentially contain a different number of points at different wavelength intervals from other points in the image. Nevertheless, the LVF camera images can be stitched together and each point can produce a unique spectrum allowing for identification or characterisation of the object at that point. Omega Optical Holdings companies Optometrics and Omega Optical have aided countless partners to select the right components to maximise their system performance. Optometrics has the rare combination of manufacturing Ruled, Holographic (reflection and transmission), and replicated gratings which enable the team to recommend a grating from its extensive library which will minimise scatter and maximise efficiency. Omega Optical is a leader in the production of LVFs, and will help partner companies to navigate the trades when selecting LVF bandwidth, gradient, and physical size. EO
Further information
www.omegafilters.com
www.optometrics.com
May 2022 Electro Optics 27
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