Bidirectional reflectance spectroscopy: 1. Theory

Hapke, Bruce

doi:10.1029/jb086ib04p03039

Cited by 1,653 publications

(1,300 citation statements)

References 33 publications

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“…The woody parts are treated 227 as opaque foliage elements thus they only scatter or absorb incident radiation. The canopy is (lower) 228 bounded by a soil medium with anisotropic scattering functioning according to Hapke (1981). The 229 horizontal exchange of rays with neighboring areas is arranged by cyclic boundary conditions, 230 meaning that laterally exiting rays of the bounding box are re-cast from the opposite plane at the same 231 trajectory angle to extend scattering to an infinitely extended forest.…”

Section: Background 46mentioning

confidence: 99%

Effects of woody elements on simulated canopy reflectance: Implications for forest chlorophyll content retrieval

Verrelst

Schaepman

Malenovský

et al. 2010

Remote Sensing of Environment

103

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Effects of woody elements on simulated canopy reflectance:implications for forest chlorophyll content retrieval AbstractAn important bio-indicator of actual plant health status, the foliar content of chlorophyll a and b (Cab), can be estimated using imaging spectroscopy. For forest canopies, however, the relationship between the spectral response and leaf chemistry is confounded by factors such as background (e.g. understory), canopy structure, and the presence of non-photosynthetic vegetation (NPV, e.g. woody elements)--particularly the appreciable amounts of standing and fallen dead wood found in older forests.We present a sensitivity analysis for the estimation of chlorophyll content in woody coniferous canopies using radiative transfer modeling, and use the modeled top-of-canopy reflectance data to analyze the contribution of woody elements, leaf area index (LAI), and crown cover (CC) to the retrieval of foliar Cab content. The radiative transfer model used comprises two linked submodels: one at leaf level (PROSPECT) and one at canopy level (FLIGHT). This generated bidirectional reflectance data according to the band settings of the Compact High Resolution Imaging Spectrometer (CHRIS) from which chlorophyll indices were calculated. Most of the chlorophyll indices outperformed single wavelengths in predicting Cab content at canopy level, with best results obtained by the Maccioni indexWe demonstrate the performance of this index with respect to structural information on three distinct coniferous forest types (young, early mature and old-growth stands). The modeling results suggest that the spectral variation due to variation in canopy chlorophyll content is best captured for stands with medium dense canopies. However, the strength of the up-scaled Cab signal weakens with increasing crown NPV scattering elements, especially when crown cover exceeds 30%. LAI exerts the least perturbations. We conclude that the spectral influence of woody elements is an important variable that should be considered in radiative transfer approaches when retrieving foliar pigment estimates in heterogeneous stands, particularly if the stands are partly defoliated or long-lived. Abstract 11A sensitivity analysis for the detectabilility of chlorophyll content in woody coniferous canopies 12 using radiative transfer modeling is presented. Foliar content of the photosynthetic pigments 13 chlorophyll a and b (Cab) is widely regarded as a bioindicator of the actual plant health status and can 14 be quantified using imaging spectroscopy. At stand level, however, the relationship between spectral 15 response and leaf chemistry tends to be weakened due to confounding factors such as canopy 16 structure, presence of non-photosynthetic vegetation (NPV, e.g. woody elements) and forest 17 background (e.g. understory). Especially in old-growth forests significant fractions of standing and 18 fallen dead woody material are present. We analyzed the contribution of crown wood cover, crown 19 LAI and crown coverage on the retrieval of Cab content on...

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Section: Background 46mentioning

confidence: 99%

Effects of woody elements on simulated canopy reflectance: Implications for forest chlorophyll content retrieval

Verrelst

Schaepman

Malenovský

et al. 2010

Remote Sensing of Environment

103

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“…The Negentropy (e.g., see [55] and [58]), an entity closely related with the mutual information, has also been used as an objective function to obtain . It is defined as (15) where is a Gaussian random vector with the same mean and covariance as [59]. Negentropy is nonnegative and is equal to zero if and only if has Gaussian distribution.…”

Section: Ica and Ifamentioning

confidence: 99%

“…The nonlinear model holds when the mixing scale is microscopic (or intimate mixtures) [13], [14]. The linear model assumes negligible interaction among distinct endmembers [15], [16]. The nonlinear model assumes that incident solar radiation is scattered by the scene through multiple bounces involving several endmember [17].…”

Section: Introductionmentioning

confidence: 99%

Does Independent Component Analysis Play a~Role in Unmixing Hyperspectral Data?

Nascimento

Dias

2003

Pattern Recognition and Image Analysis

114

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Abstract-Independent component analysis (ICA) has recently been proposed as a tool to unmix hyperspectral data. ICA is founded on two assumptions: 1) the observed spectrum vector is a linear mixture of the constituent spectra (endmember spectra) weighted by the correspondent abundance fractions (sources); 2) sources are statistically independent. Independent factor analysis (IFA) extends ICA to linear mixtures of independent sources immersed in noise. Concerning hyperspectral data, the first assumption is valid whenever the multiple scattering among the distinct constituent substances (endmembers) is negligible, and the surface is partitioned according to the fractional abundances. The second assumption, however, is violated, since the sum of abundance fractions associated to each pixel is constant due to physical constraints in the data acquisition process. Thus, sources cannot be statistically independent, this compromising the performance of ICA/IFA algorithms in hyperspectral unmixing. This paper studies the impact of hyperspectral source statistical dependence on ICA and IFA performances. We conclude that the accuracy of these methods tends to improve with the increase of the signature variability, of the number of endmembers, and of the signal-to-noise ratio. In any case, there are always endmembers incorrectly unmixed. We arrive to this conclusion by minimizing the mutual information of simulated and real hyperspectral mixtures. The computation of mutual information is based on fitting mixtures of Gaussians to the observed data. A method to sort ICA and IFA estimates in terms of the likelihood of being correctly unmixed is proposed.Index Terms-Independent component analysis (ICA), independent factor analysis (IFA), mixture of Gaussians, unmixing hyperspectral data.

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“…We next corrected each image for limb-darkening and phase-angle effects, thereby predicting each pixel's normal reflectance r,z, the value of I/F when incidence, emission, and phase angles are all zero. This correction was performed using the Hapke photometric function [Hapke, 1981[Hapke, , 1984[Hapke, , 1986 The various photometrically corrected images were then converted into simple cylindrical map projections; the map scale chosen, 4 pixels deg-•, preserves the intrinsic resolution of all but one of the original images. Finally, the map-projected images that share the same filter and same phase angle were mosaicked together, thereby producing global maps of lowphase green data, high-phase violet data, etc.…”

Section: Galileo Low-phase and High-phase Maps Of Iomentioning

confidence: 99%

Regolith variations on Io: Implications for bolometric albedos

Simonelli¹,

Dodd²,

Veverka³

2001

J. Geophys. Res.

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Abstract. Global maps of Io produced from two sets of Galileo images (solar phase angles 4ø-14 ø and 71ø-86 ø, respectively) reveal that this satellite's color and albedo patterns change dramatically with phase. At low phase the equatorial band is the brightest, whitest part of Io; that is, it is brighter than the polar regions at all wavelengths. At high phase, however, the equatorial band becomes a dark gray, exhibiting little contrast with the polar regions at violet and green wavelengths and appearing darker than the polar regions at red wavelengths. To quantify these phase-related changes, we derive global maps of the Henyey-Greenstein asymmetry factor # that show how the strength of backscattering by Io's regolith varies from region to region. In the green and red, where albedo patterns change most radically with phase, the equatorial band forms a welldefined unit of strongly backscattering material; material that is more weakly backscattering at these wavelengths, mainly in the polar regions, shows a broad continuum of different # values. In the violet the phase-related changes in albedo patterns are more subdued, and the scattering units are poorly defined. Using this information on #, we generate a global map showing variations in the bolometric Bond albedo A B, the true "energy balance albedo" that governs insolation-based surface temperatures. The mean albedo of this map, AB •--0.52, is similar to A• values computed previously for Io, but the distribution of albedos is markedly different. In previous Bond albedo maps the highest A• coincided with the bright equatorial band seen at low phase; the new map, however, more closely resembles Io's high-phase albedo patterns (i.e., the band of high Bond albedos at the equator is absent). This change in A• patterns has a significant effect on how insolation-based temperatures vary with latitude on Io; in particular, it increases the satellite's equator-to-pole temperature contrast. IntroductionBy comparing images taken during Galileo's first and second orbits of Jupiter, SimoneIll et al. [1997] discovered that the color and albedo patterns on Io change with solar phase angle a in ways too radical and widespread to be explained by the appearance of new volcanic plume deposits or lava flows between orbits. For example, Io's equatorial band is much brighter than its poles at low phase, but according to these authors the equator-to-pole albedo contrast weakens dramatically as a increases. Simonelli et al. proposed that Io's surface can be divided up into two photometric units (an equatorial unit of strongly backscattering material and more weakly backscattering material located mainly in the polar regions) and that the different scattering behaviors of these two units are evidence for major region-to-region variations in Io's regolith properties. In particular, they proposed that there may be significant variations between equator and pole in the average size and/or internal structure of regolith particles.The early Galileo observations discussed by Simonelli et ...

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Bidirectional reflectance spectroscopy: 1. Theory

Cited by 1,653 publications

References 33 publications

Effects of woody elements on simulated canopy reflectance: Implications for forest chlorophyll content retrieval

Effects of woody elements on simulated canopy reflectance: Implications for forest chlorophyll content retrieval

Does Independent Component Analysis Play a~Role in Unmixing Hyperspectral Data?

Regolith variations on Io: Implications for bolometric albedos

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