Inter-comparison of hemispherical conical reflectance factors (HCRF) measured with four fibre-based spectrometers

K Anderson,A Mac Arthur,M Balzarolo,F Fava,L Vescovo,T Julitta,J Pacheco-Labrador,M Rossini

doi:10.1364/oe.21.000605

Abstract

We describe the results of an experiment designed to compare the radiometric performance of four different spectroradiometers in ideal field conditions. A carefully designed experiment where instruments were simultaneously triggered was used to measure the Hemispherical Conical Reflectance Factors (HCRF) of four targets of varying reflectance. The experiment was in two parts. Stage 1 covered a 2 hour period finishing at solar noon, where 50 measurements of the targets were collected in sequence. Stage 2 comprised 10 rapid sequential measurements over each target. We applied a method for normalising full width half maximum (FWHM) differences between the instruments, which was a source of variability in the raw data. The work allowed us to determine data reproducibility, and we found that lower-cost instruments (Ocean Optics and PP Systems) produced data of similar radiometric quality to those manufactured by Analytical Spectral Devices (ASD -here we used the ASD FieldSpec Pro) in the spectral range 400-850 nm, which is the most significant region for research communities interested in measuring vegetation dynamics. Over the longer time-series there were changes in HCRF caused by the structural and spectral characteristics of some targets.

Highlights

Measurements of reflectance quantities at eddy covariance sites are increasingly relying on hyperspectral instrumentation [1,2,3,4,5] because narrowband spectral indices are good proxies for plant physiological processes such as the xanthophyll cycle, or sun-induced chlorophyll fluorescence [7,8,9]
The work allowed us to determine data reproducibility, and we found that lower-cost instruments (Ocean Optics and PP Systems) produced data of similar radiometric quality to those manufactured by Analytical Spectral Devices (ASD –here we used the ASD FieldSpec Pro) in the spectral range 400-850 nm, which is the most significant region for research communities interested in measuring vegetation dynamics
This is true for the new generation of lightweight, miniaturised spectroradiometers which are compact enough to be deployed across eddy covariance sites in a cost-effective manner

Summary

Introduction

Measurements of reflectance quantities at eddy covariance sites are increasingly relying on hyperspectral instrumentation [1,2,3,4,5] because narrowband spectral indices are good proxies for plant physiological processes such as the xanthophyll cycle (e.g. photochemical reflectance index, PRI [6]), or sun-induced chlorophyll fluorescence [7,8,9]. Despite the fact that these instruments are increasingly being utilised in operational settings, and that there is a strong need for measurements to be cross-comparable across different sites, there is very little published work demonstrating whether data show good reproducibility [10] This is true for the new generation of lightweight, miniaturised spectroradiometers (e.g. those manufactured by companies such as Ocean Optics and PP Systems) which are compact enough to be deployed across eddy covariance sites in a cost-effective manner. The network of eddy covariance towers across Europe overseen by the Integrated Carbon Observation System (ICOS; http://www.icos-infrastructure.eu/) is calling for input from the hyperspectral measurement community about which systems are best suited to deployment on flux towers Before such advice can be provided, information is needed about the relative radiometric performance of the different systems available. This paper provides a first field-based quantitative assessment of the reproducibility of hemispherical conical reflectance factors (HCRF) measured by four individual fibre-optic based hyperspectral spectroradiometers, including three miniaturised systems (two from Ocean Optics (http://www.oceanoptics.com/) and one from PP Systems (http://www.ppsystems.com/)) that have already been shown to be ideally suited to deployment at eddy covariance sites [2, 4, 5]

Methods

Results

Conclusion