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Freeform based hYperspectral imager for MOisture Sensing (FYMOS)

Graham, Christopher and Girkin, John M. and Bourgenot, Cyril (2021) 'Freeform based hYperspectral imager for MOisture Sensing (FYMOS).', Optics express., 29 (11). pp. 16007-16018.


We present FYMOS, an all-aluminum, robust, light weight, freeform based, near infrared hYperspectral imager for MOisture Sensing. FYMOS was designed and built to remotely measure moisture content using spectral features from 0.7-1.7µm integrating an InGaAs sensor. The imaging system, operating at F/2.8, is based on the three-concentric-mirror (Offner) spectrograph configuration providing a spectral resolution of 8 nm optimized for broad spectral coverage with sufficient resolution to make assessments of water levels. To optimize the optical performance, whilst minimizing weight and size, the design incorporates a bespoke freeform blazed grating machined on a commercial 5 axis ultra precision diamond machine. We achieve a 30% improvement on the RMS wavefront error in the spatial and spectral fields compared to a conventional Offner-Chrisp design with similar aperture and the monolithic Primary/Tertiary mirror eases the manufacturing assembly whilst minimizing weight. We demonstrate the performance of FYMOS by measuring the evaporation rate of water on a soil sample and results are processed with a physical multilayer radiative transfer model (MARMIT) to estimate the mean water thickness.

Item Type:Article
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Available under License - Creative Commons Attribution 4.0.
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Date accepted:27 April 2021
Date deposited:25 May 2021
Date of first online publication:10 May 2021
Date first made open access:25 May 2021

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