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Abstract: To understand the connection between
single-particle optics and the optics of a closely packed surface,
controlled laboratory measurements of bidirectional reflectance
distribution functions on layers of
polymer and glass spheres are carried out. The measurements
are compared with predictions from five
radiative-transfer models; the Hapke’s models, the Lumme-Bowell
model, the BRF algorithm of
Mishchenko et al., and the discrete ordinate radiative
transfer. It is found that models of strict
numerical radiative-transfer equations (RTEs) predict
measurements well in some regions but have
errors in both forward- and backward-scattering directions.
The improved Hapke’s model, although it
has an anisotropic multiple-scattering term, still produces
considerable errors compared with the
strict RTE. The difference can be attributed to the exclusion
of a diffraction contribution in the Hapke
model.