A thin planar illumination device with a high light utilization efficiency is provided. The receiving tray includes a bottom plate, a pushing plate, and a shielding plate. The top side of the tongue has a protrusion that cooperates with a meshing recess located in the lower side of the upper lip of the groove of the coupling devices, and a portion extending generally parallel with the plane of the panel to form a contact surface cooperating with the lower side of the upper lip of the groove. Angular misalignment, which may exist between the anisotropic film and the orientation layer, is minimized. Each pair of docking points comprises a pair of flanged wheels with one flexible annular intermediate section.