Abstract: This invention relates to a backlight panel for a panel display device, comprising a light pipe, at least one source of illumination and a diffuse reflector. The light pipe has a first boundary surface comprising a first substantially planar surface interrupted by a first pattern of discrete surface relief features for extracting light from the light pipe and for directing said extracted light at the diffuse reflector, and a second boundary surface comprising a second substantially planar surface interrupted by a second pattern of laterally elongate surface relief features, each comprising an inclined surface angled towards the light source for directing light internally reflected from the second boundary surface towards the first boundary surface at a correspondingly lower angle of incidence. The areal density of the first pattern varies across the first side and generally increases along the transmission direction.