Freeform based hYperspectral imager for MOisture Sensing (FYMOS)

Graham, Christopher; Girkin, John M; Bourgenot, Cyril

Abstract

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.

Citation

Graham, C., Girkin, J. M., & Bourgenot, C. (2021). Freeform based hYperspectral imager for MOisture Sensing (FYMOS). Optics Express, 29(11), 16007-16018. https://doi.org/10.1364/oe.425660

• Journal Article Type: Article
• Acceptance Date: Apr 27, 2021
• Online Publication Date: May 10, 2021
• Publication Date: 2021
• Deposit Date: May 24, 2021
• Publicly Available Date: May 25, 2021
• Journal: Optics Express
• Electronic ISSN: 1094-4087
• Publisher: Optica
• Peer Reviewed: Peer Reviewed
• Volume: 29
• Issue: 11
• Pages: 16007-16018
• DOI: https://doi.org/10.1364/oe.425660
• Public URL: https://durham-repository.worktribe.com/output/1274719