Abstract
Technologies to determine spectral sky radiance distributions have evolved in recent years and have enabled new applications in remote sensing, for sky radiance measurements, in biological/diagnostic applications and luminance measurements. Most classical spectral imaging radiance technologies are based on mechanical and/or spectral scans. However, these methods require scanning time in which the spectral radiance distribution might change. To overcome this limitation, different so-called snapshot spectral imaging technologies have been developed that enable spectral and spatial non-scanning measurements. We present a new setup based on a facet mirror that is already used in imaging slicing spectrometers. By duplicating the input image instead of slicing it and using a specially designed entrance slit, we are able to select nearly 200 (14 × 14) channels within the field of view (FOV) for detecting spectral radiance in different directions. In addition, a megapixel image of the FOV is captured by an additional RGB camera. This image can be mapped onto the snapshot spectral image. In this paper, the mechanical setup, technical design considerations and first measurement results of a prototype are presented. For a proof of concept, the device is radiometrically calibrated and a 10 mm × 10 mm test pattern measured within a spectral range of 380 nm–800 nm with an optical bandwidth of 10 nm (full width at half maximum or FWHM). To show its potential in the UV spectral region, zenith sky radiance measurements in the UV of a clear sky were performed. Hence, the prototype was equipped with an entrance optic with a FOV of 0.5° and modified to obtain a radiometrically calibrated spectral range of 280 nm–470 nm with a FWHM of 3 nm. The measurement results have been compared to modeled data processed by UVSPEC, which showed deviations of less than 30%. This is far from being ideal, but an acceptable result with respect to available state-of-the-art intercomparisons.
Highlights
Technologies for assessing spectral imaging radiance data are based on scanning in either the spectral (Hardeberg et al 2002, Gupta and Voloshinov 2004, Mathews 2008, Sigernes et al 2012) or spatial domain (Hu et al 2005)
We present a new setup based on a facet mirror that is already used in imaging slicing spectrometers
Spectral snapshot imaging measurements have been performed with the 4D Imager, which is a new type of integral field spectroscopy (IFS) technology
Summary
Technologies for assessing spectral imaging radiance data ( called hyperspectral imaging) are based on scanning in either the spectral (Hardeberg et al 2002, Gupta and Voloshinov 2004, Mathews 2008, Sigernes et al 2012) or spatial domain (Hu et al 2005). One of many existent approaches is based on imaging a coded spatial and spectral information mixture on the detector and mathematically reconstructing the spectral snapshot image (Gehm et al 2007, Wagadarikar et al 2008) Another approach is the so-called integral field spectroscopy (IFS) snapshot technology, such as fiber bundlebased technologies (IFS-F), where single fibers capture the signal of one measurement channel (Hagen and Kudenov 2013). A couple of image slicing spectrometer (ISS) technologies (one example being IFS-M) are known for applications in astronomy or for microscopy purposes (Content 1997, Henault et al 2004, Laurent et al 2006, Gao et al 2009) These approaches are based on a facet mirror optical design, which slices the image into single parts that are spectrally resolved. The new system is called 4D Imager since measurements of an image (2D), spectral resolution (1D) and snapshot in time (1D) with well-balanced performance in all four dimensions could be achieved
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