Friday, April 17, 2009

HySpex, new small Hyperspectral scanner

The HySpex(TM) line of hyperspectral cameras are a result of the knowledge and experience accumulated through more than a decade (starting in 1995) of research activities in the field of imaging spectroscopy at NEO. The spectrometer design is flexible, and the specifications and performance can be tailored to the user's requirements and needs.

The HySpex camera is a line imager, meaning that all spectra across a spatial line in the image are recorded simultaneously. Some form of scanning, depending on the application, is used to record sequential lines, building up a hyperspectral image with two spatial dimensions. The scanning can be done by either camera movement (e.g. aircraft) or movement of the scene (e.g. conveyer belt).

All the HySpex cameras perform very well in terms of the characteristics commonly used to describe the quality of hyperspectral line imagers:

* Spatial resolution
* Spectral resolution
* Spectral keystone
* Smile effect
* Alignment of sensor, slit and dispersive element
* Responsivity
* Stray light
* Second order suppression
* Sensor characteristics
* Radiometric accuracy

All the spectrometer modules are based on the same general architecture, with lenses and gratings separately optimized for the different modules.

The fore-optics and collimating optics is based on a special configuration of two aspherical mirrors. This unique design avoids introduction of spherical and chromatic aberrations before the grating, and at the same time minimizes stray light. The transmission grating is polarization independent and has been chosen among a large selection tested gratings. The lens system for final focus is optimized for minimization and equalization of the point spread function across the FOV and spectral range. Additionally, spectral keystone and smile effects are only a small fraction of a pixel. The optical performance is nearly diffraction limited, as can be seen on the figure. A detailed tolerance analysis has been performed, ensuring that the tolerances of the optical and mechanical parts are specified sufficiently tight in order to make sure that the real performance is close to the simulated performance. The spectrometer modules are extremely rugged with no moving parts.

A detailed experimental characterization is performed for each camera to verify that the performance is according to the specifications. The results of this characterization are documented in a detailed test report which follows each instrument. The end user thus has full access to all relevant performance characteristics.

All the cameras are also spectrally and radiometrically calibrated. The basic output of the systems is thus a radiance spectrum in W/(m^2 nm sr) for each pixel in the image.

Dedicated image processing algorithms can be implemented to provide an output tailored for a specific application (anomaly detection images, thematic maps, etc), possibly in real time for e.g. on-line applications.


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