The optical response of rubrene crystals is described by providing the full UV-VIS dielectric tensor. Consistently with the molecular and crystal symmetries, the lowest crystal transition originates from an $A_u$ molecular transition, it is polarized along the $c$ axis (normal to the larger crystal face), and it is the origin of the emission, which is self-guided towards the edge where the corresponding polarization and intensity angular distribution are detected. By contrast, the $B_u$ molecular transitions give rise to Davydov states described by the other two components of the diagonal dielectric tensor.

The full UV-visible dielectric tensor and the corresponding directions of the principal axes of triclinic tetracene crystals are reported as deduced either by polarized absorption and ellipsometry measurements or by calculations based on the molecular and crystallographic data. The results allow the attribution of the polarized bands observed in both absorption and photoluminescence emission spectra. In particular, the spectral line shape and polarization of the emission are found to depend on the sample thickness, and the effect is attributed to the modification of the state of polarization of the emitted light during its propagation inside the crystal. Indeed, the directions of polarization of the lowest optical transitions and the directions of the principal axes of the dielectric tensor are demonstrated not to coincide, in contrast to the assumptions typically made in the literature, thus causing the mixed transverse/longitudinal character of light propagation.