imaging and machine vision europe february/march 2011 www.imveurope.com


18


spectral imaging


Spectroscopy meets imaging


Multispectral and hyperspectral imaging techniques, which combine spectroscopy and imaging, are opening up a range of applications from environmental monitoring and agricultural research to automation of plastic recycling, as Greg Blackman discovers


Imaging outside of the visible spectrum, whether in the ultraviolet or infrared, can tell you a lot about an object that might not otherwise be seen with visible light. Inspection of fruit and vegetables in infrared, for instance, can expose bruises and rotten areas under the skin that wouldn’t necessarily show up with standard colour imaging. Cameras like JAI’s LQ-200CL 4CCD colour line scan camera, which has four sensors providing an additional near-infrared (NIR) channel to the standard red, green, and blue, is designed for just such an application. Other systems are available offering greater numbers of channels – German company Chromasens has recently released a camera system with 12 spectral channels on a single sensor, providing highly accurate spectral analysis of different colours for tasks like print inspection. Spectral imaging combines spectroscopy


with imaging, whereby spectral information is captured at the pixels of a sensor. The systems typically have numerous channels covering different wavelength ranges, rather than merely the three bands of RGB colour. Related but distinct techniques include multispectral imaging, that is, the sensor images at a number of discrete wavelength bands, and hyperspectral imaging, in which the sensor still images at discrete bands – but over a continuous wavelength range. As an example, pixel lines on a hyperspectral sensor covering the short wave infrared (SWIR) range would analyse a band, say, between 1.4 to 1.5µm for


the first line, 1.5 to 1.6µm for the second, 1.6 to 1.7µm for the third, and so on. Multispectral imaging, on the other hand, might measure at specific wavelength bands, with the incoming light partitioned with filters, but it wouldn’t be over a continuous range. Both techniques are often used in remote


sensing projects, such as satellites monitoring Earth. Infrared detector manufacturer Sofradir, which has its headquarters near Paris, France, has provided both multispectral


and hyperspectral detector arrays for space projects. It has supplied its Saturn 1,000 x 256 pixel 30µm pitch SWIR detectors for the Italian-led PRISMA (Precursor Hyperspectral Mission Application) satellite programme, a system of Earth observation hyperspectral imagers currently under development. ‘The objective of the PRISMA project, as for a


lot of hyperspectral space missions operating in the SWIR, is monitoring the Earth’s water cycle,’ says Philippe Chorier, head of space projects at Sofradir. ‘In the short wave band there are a lot of wavelengths at which water absorbs radiation. The data provides a map of Earth’s humidity levels and this, in turn, provides data on vegetation as well as agriculture.’ Molecules such as water will have a spectral fingerprint that will signify its presence at specific wavelengths of light. Hyperspectral imagers capture a set of images each comprising


This unmanned aerial vehicle (UAV) was part of a research project conducted by Carinthian Tech Research that aimed to classify vegetation cover from the air using multispectral imaging. The UAV was equipped with multispectral cameras from Quest Innovations


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