Reflectance and transmittance measurements of anisotropically scattering samples in focusing Coblentz spheres

A procedure for obtaining the correct hemispherical reflectance and transmittance of anisotropically scattering samples in focusing Coblentz spheres are presented. Consideration is taken not only to the angle-dependent detector efficiency and to multiple reflections between sample and detector, but a separation of the scattered light into a low and a high angle fraction is also introduced. To validate the formalism, the correction procedure is applied to six samples with different scattering characteristics in the visible and near-infrared region. The agreement with results from a double beam spectrophotometer, equipped with an integrating sphere, was found to be good. Without making the separation into high and low angle scattering, it was not possible to reach agreement between the results from the Coblentz and integrating spheres, and errors of the order of 10% could be obtained.