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We present uncertainties associated with the measurement of coniferous needle-leaf optical properties (OPs) with an integrating sphere using an optimized gap-fraction (GF) correction method, where GF refers to the air gaps appearing between the needles of a measured sample. We used an optically stable artificial material simulating needle leaves to investigate the potential effects of: 1) the sample holder carrying the needles during measurements and 2) multiple scattering in between the measured needles. Our optimization of integrating sphere port configurations using the sample holder showed an underestimation of the needle transmittance signal of at least 2% in flat needles and 4% in nonflat needles. If the needles have a nonflat cross section, multiple scattering of the photons during the GF measurement led to a GF overestimation. In addition, the multiple scattering of photons during the optical measurements caused less accurate performance of the GF-correction algorithms, which are based on the assumption of linear relationship between the nonGF-corrected signal and increasing GF, resulting in transmittance overestimation of nonflat needle samples. Overall, the final deviation achieved after optimizing the method is about 1% in reflectance and 6% in transmittance if the needles are flat, and if they are nonflat, the error increases to 4%-6% in reflectance and 10%-12% in transmittance. These results suggest that formulae for measurements and computation of coniferous needle OPs require modification that includes also the phenomenon of multiple scattering between the measured needles. 14 15

Optical properties (OPs) of non-flat narrow plant leaves, i.e., coniferous needles, are extensively used by the remote sensing community, in particular for calibration and validation of radiative transfer models at leaf and canopy level. Optical measurements of such small living elements are, however, a technical challenge and only few studies attempted so far to investigate and quantify related measurement errors. In this paper we review current methods and developments measuring optical properties of narrow leaves. We discuss measurement shortcomings and knowledge gaps related to a particular case of non-flat nonbifacial coniferous needle leaves, e.g., needles of Norway spruce (Picea abies (L.) Karst.). KeywordsNeedles, optical properties, reflectance, transmittance, integrating sphere, leaf, conifers, gap fraction

Needle-leaf chlorophyll content (Cab) of a Norway spruce stand was estimated from CHRIS-PROBA images using the canopy reflectance simulated by the PROSPECT model coupled with two canopy reflectance models: 1) discrete anisotropic radia-tive transfer model (DART); and 2) PARAS. The DART model uses a detailed description of the forest scene, whereas PARAS is based on the photon recollision probability theory and uses a simplified forest structural description. Subsequently, statisti-cally significant empirical functions between the optical indices ANCB 670 − 720 and ANMB 670 − 720 and the needle-leaf Cab content were established and then applied to CHRIS-PROBA data. The Cab estimating regressions using ANMB 670 − 720 were more robust than using ANCB 670 − 720 since the latter was more sensitive to LAI, especially in case of PARAS. Comparison between Cab esti-mates showed strong linear correlations between PARAS and DART retrievals, with a nearly perfect one-to-one fit when using ANMB 670 − 720 (slope = 1.1, offset = 11 μ g · cm − 2 ). Further com-parison with Cab estimated from an AISA Eagle image of the same stand showed better results for PARAS (RMSE = 2.7 μ g · cm − 2 for ANCB 670 − 720 ;RMSE = 9.5 μ g · cm − 2 for ANMB 670 − 720 )than for DART (RMSE = 7.5 μ g · cm − 2 for ANCB 670 − 720 ;RMSE = 23 μ g · cm − 2 for ANMB 670 − 720 ). Although these results show the potential for simpler models like PARAS in estimating needle-leaf L. Yáñez-Rausell is with the Laboratory of Geo-Information Science and Remote Sensing, Wageningen University, PO BOX 47, 6700 AA Wageningen, The Netherlands (l.yanezrausell@gmail.com) and the Department of Geography, Univ. of Zurich, Winterthurerstrasse 190, Switzerland (lucia.yanezrausell@geo.uzh.ch these results show the potential for simpler models like PARAS in estimating needle-leaf Cab from satellite imaging spectroscopy data, further analyses regarding parameterization of radiative transfer models are recommended.