Note: Spectroscopy with natural (unpolarized) light on anisotropic samples will show directional effects, since the electric vectors of natural light are not isotropic, but are restricted to a plane perpendicular to the beam. In two-photon processes, for example, luminescence measured with linear polarizers in both beams, an aligned sample is not required since it will be created by the exciting photon through photoselection. However, aligned samples allow a larger number of independent experiments to be recorded and thus contain more information.