Hyperspectral spectro-imager survey for state of art concept identification

Over the past twenty years, a very high number of different space hyperspectral spectrometers have been designed, developed and launched. Recently, several new compact designs based on freeform optics have been proposed in the literature. We thus considered that it was interesting to make a survey of the already existing hyperspectral imagers and innovative freeform designs to identify the most promising solutions to increase the compactness and the performance. Some surveys on hyperspectral imagers have already been done in the past. For example, the survey by Herring et al, in 1993 which proposed some hyperspectral spectrometer concepts. However, these studies are now outdated and cannot be used to identify state of art concepts. The review proposed by Kumar et al., in 2015 presented the advantages to browse the different hyperspectral imager concepts but provided only superficial technical details that cannot be used to identify the specific designs that improve both the compactness and the image and spectral qualities. We thus decided to make a bibliographic survey to analyze the most recent designs of different space hyperspectral imagers with the objective to select the concepts that will provide the best compromise between the volume and the performance. We have categorized the different systems into four groups: prism based, Offner, Three Mirror Anastigmat (TMA) and Dyson spectrometers. We analyzed these different concepts and we identified and the advantages and the drawbacks. We concluded that the most interesting state-of-art designs are the Freeform Offner, double-pass TMA and Dyson spectrometers. These designs present the most interesting performance and combine with a compact volume. We thus established a preliminary trade-off that summarizes the advantages and the drawback of these concepts. This trade-off analysis could be used as a starting point for any future study aiming at designing compact hyperspectral imager. Finally, we present the progress of some important and recent technological developments related to the manufacturing of freeform and convex gratings.