Schematic (a) and rendering (b) of HSI optical design. (1) Entrance cosine-response optic located over a concave structure; (2) 45° folding mirror; (3) aspheric achromatic multiplets; (4) mechanical shutter; (5) beam splitter plate; (6) VNIR spectrometer; (7) NIR spectrometer; and (8) step motors.
HSI in zenith (a) and nadir (b) view measuring the down- and up-welling irradiance, respectively.
Dashed line: fraction of the signal coming from the cone defined by the angular interval [0,θ]. Full dots: fractional contribution of the signal arriving from 10° zenithal angular sectors (θ ± 5°). The upper x-axis reports the radius of the area contributing to the signal as a function of θ and for an instrument height of 3.5 m.
Example of a single spectral data acquisition over a vegetated surface. Down- and up-welling irradiances (E d and E u , upper panel) and resulting BHR (lower panel) for the VNIR (blue lines covering the whole range) and NIR (green lines covering the restricted range highlighted by the dashed vertical lines) spectrometers. The reflectance peak at 760 nm clearly visible in the NIR spectrometer data is originated by the fluorescence infilling of the O2-A band.
Measured and modeled down-welling irradiance at 747 nm for day 08/04/2009. The cosine of the solar zenith angle and the time of Sun culmination (dashed vertical line) are reported for reference.
(a): Reflectance factor measured by HSI (HSI BHR) and manual setup (field spectroscopy (F.S.) HCRF) at noon; (b) and (c): diurnal evolution of HSI BHR and FS HCRF for wavelengths 680 and 800 nm, respectively. Measurements were collected under clear sky conditions on 07/28/2009; the time of Sun culmination (dashed vertical line) is reported for reference
HSI installed at the Alpine grassland field-site. The masts used for eddy covariance, micrometeorology and webcam are visible in the background.
Temporal changes of average grassland reflectance spectra (n = 10) collected around solar noon during 2009 (data every ten days are plotted). Measurements acquired with the VNIR spectrometer.
Example of MTCI time series. (a) seasonal trend; (b) diurnal trend (DOY = 216).
Example of F 760 time series. (a) seasonal trend; (b) diurnal trend during a day of clear sky (DOY = 216).
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