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A Compact Laboratory Spectro-Goniometer (CLabSpeG) to Assess the BRDF of Materials. Presentation, Calibration and Implementation on Fagus sylvatica L. Leaves
Katholieke Universiteit Leuven, Dept. of Biosystems, M3-BIORES, Celestijnenlaan 200E, BE-3001 Leuven, Belgium
Katholieke Universiteit Leuven, Dept. of Computer Science, Celestijnenlaan 200A, BE-3001 Leuven, Belgium
ouncil for scientific and Industrial Research (CSIR) Natural Resources and the Environment – Ecosystems Earth Observation P.O.Box 395, 0001 Pretoria, South Africa
Katholieke Universiteit Leuven, Dept. of Land Management and Economics, Celestijnenlaan 200E, BE-3001 Leuven, Belgium
* Author to whom correspondence should be addressed.
Received: 2 August 2007; Accepted: 6 September 2007 / Published: 7 September 2007
Abstract: The design and calibration of a new hyperspectral Compact Laboratory Spectro-Goniometer (CLabSpeG) is presented. CLabSpeG effectively measures the bidirectionalreflectance Factor (BRF) of a sample, using a halogen light source and an AnalyticalSpectral Devices (ASD) spectroradiometer. The apparatus collects 4356 reflectance datareadings covering the spectrum from 350 nm to 2500 nm by independent positioning of thesensor, sample holder, and light source. It has an azimuth and zenith resolution of 30 and15 degrees, respectively. CLabSpeG is used to collect BRF data and extract BidirectionalReflectance Distribution Function (BRDF) data of non-isotropic vegetation elements suchas bark, soil, and leaves. Accurate calibration has ensured robust geometric accuracy of theapparatus, correction for the conicality of the light source, while sufficient radiometricstability and repeatability between measurements are obtained. The bidirectionalreflectance data collection is automated and remotely controlled and takes approximatelytwo and half hours for a BRF measurement cycle over a full hemisphere with 125 cmradius and 2.4 minutes for a single BRF acquisition. A specific protocol for vegetative leafcollection and measurement was established in order to investigate the possibility to extractBRDF values from Fagus sylvatica L. leaves under laboratory conditions. Drying leafeffects induce a reflectance change during the BRF measurements due to the laboratorySensors 2007, 7 1847 illumination source. Therefore, the full hemisphere could not be covered with one leaf. Instead 12 BRF measurements per leaf were acquired covering all azimuth positions for a single light source zenith position. Data are collected in radiance format and reflectance is calculated by dividing the leaf cycle measurement with a radiance cycle of a Spectralon reference panel, multiplied by a Spectralon reflectance correction factor and a factor to correct for the conical effect of the light source. BRF results of measured leaves are presented.
Keywords: BRDF-retrieval; BRF-measurement; multi-angular; hyperspectral; spectroradiometer; vegetation; heat-stress.
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Biliouris, D.; Verstraeten, W.W.; Dutré, P.; Van Aardt, J.A.; Muys, B.; Coppin, P. A Compact Laboratory Spectro-Goniometer (CLabSpeG) to Assess the BRDF of Materials. Presentation, Calibration and Implementation on Fagus sylvatica L. Leaves. Sensors 2007, 7, 1846-1870.
Biliouris D, Verstraeten WW, Dutré P, Van Aardt JA, Muys B, Coppin P. A Compact Laboratory Spectro-Goniometer (CLabSpeG) to Assess the BRDF of Materials. Presentation, Calibration and Implementation on Fagus sylvatica L. Leaves. Sensors. 2007; 7(9):1846-1870.
Biliouris, Dimitrios; Verstraeten, Willem W.; Dutré, Phillip; Van Aardt, Jan A.; Muys, Bart; Coppin, Pol. 2007. "A Compact Laboratory Spectro-Goniometer (CLabSpeG) to Assess the BRDF of Materials. Presentation, Calibration and Implementation on Fagus sylvatica L. Leaves." Sensors 7, no. 9: 1846-1870.