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  • EARTH RESOURCES AND REMOTE SENSING  (3)
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  • 1
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    Multiple vs single scattering - Assessment of magnitudes (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: All orders of scattering are analyzed for two artifical canopies. The SHL canopy consists of Small Horizontal Leaves that are much smaller than the leaf-to-leaf spacing. The IHL canopy consists of Infinite Horizontal Layers, where each leaf is of infinite extent (a horizontal plane). Hemispheric leaf reflectances and transmittances independent of the direction of illumination lead to exact solutions for these models. Sunlight that penetrates to a given leaf area index level is much stronger in an SHL canopy than that in IHL; but the difference becomes muted when leaf transmittance is large. Multiple scattering enhances the hemispheric canopy reflectance more strongly in SHL than it does in IHL. The enhancement depends linearly on leaf transmittance in SHL and on the transmittance squared in IHL. Comparison with measured reflectances indicates that IHL model grossly underestimates multiple scattering in soybean canopies.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Applied Optics (ISSN 0003-6935); 30; 1562-156
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    Penetration of sunlight into a canopy - Explicit models based on vertical and horizontal leaf projections (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The projections of leaf areas onto a horizontal plane and onto a vertical plane are examined for their utility in characterizing canopies for sunlight penetration (direct beam only) models. These projections exactly specify the penetration if the projections on the principal plane of the normals to the top surfaces of the leaves are in the same quadrant as the sun. Inferring the total leaf area from these projections (and therefore the penetration as a function of the total leaf area) is possible only with a large uncertainty (up to + or - 32 percent) because the projections are a specific measure of the total leaf area only if the leaf angle distribution is known. It is expected that this uncertainty could be reduced to more acceptable levels by making an approximate assessment of whether the zenith angle distribution is that of an extremophile canopy.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Boundary-Layer Meteorology (ISSN 0006-8314); 36; 335-349
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    NASA TECHNICAL REPORTS
  • 3
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    A ray tracing model for leaf bidirectional scattering studies (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A leaf is modeled as a deterministic two-dimensional structure consisting of a network of circular arcs designed to represent the internal morphology of major species. The path of an individual ray through the leaf is computed using geometric optics. At each intersection of the ray with an arc, the specular reflected and transmitted rays are calculated according to the Snell and Fresnel equations. Diffuse scattering is treated according to Lambert's law. Absorption is also permitted but requires a detailed knowledge of the spectral attenuation coefficients. An ensemble of initial rays are chosen for each incident direction with the initial intersection points on the leaf surface selected randomly. The final equilibrium state after all interactions then yields the leaf bidirectional reflectance and transmittance distributions. The model also yields the internal two dimensional light gradient profile of the leaf.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Format: text
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