Backlight panel for providing area backlighting of a panel display device

A backlight panel for providing area backlighting of a panel display device for displaying one or more backlit graphic information features, wherein the backlight panel comprises a light pipe, at least one source of illumination and a diffuse reflector;

• the light pipe comprises a substantially plate-like body of an optically transmissive material, said body comprising opposite first and second sides providing, respectively, opposite first and second boundary surfaces of the light pipe for guiding light along the light pipe by means of internal reflections off said boundary surfaces, said sides being bounded by a peripheral edge of said body;
• the peripheral edge comprises an input edge portion at which said source of illumination is provided to inject light into the light pipe for transmission by internal reflections off said boundary surfaces along a transmission direction within the light pipe, the transmission direction defining a lateral direction extending transversely to the transmission direction;
• the diffuse reflector is positioned opposite the first side so that light escaping the light pipe through the first boundary surface is diffusely reflected by the diffuse reflector back into the light pipe and generally towards the second side, said area backlighting being provided by light escaping from the light pipe through the second boundary surface;
• the first boundary surface of the light pipe comprises a first substantially planar surface interrupted by a first pattern of surface relief features, said surface relief features being discrete and the discrete surface relief features being spaced apart across the first side both in the transmission direction and in the lateral direction to form said first pattern for extracting light from the light pipe and for directing said extracted light at the diffuse reflector, the proportion of the first boundary surface covered by the discrete surface relief features as compared with said first planar surface defines an areal density of said first pattern, and the areal density of said first pattern varies across the first side and generally increases along the transmission direction;
• and the second boundary surface comprises a second substantially planar surface interrupted by a second pattern of surface relief features, each of said surface relief features being elongate in the lateral direction and said elongate surface relief features being spaced apart from one another across the second side in the transmission direction to form said second pattern, each elongate surface 5 relief feature comprising an inclined surface, each of said inclined surfaces being angled towards said light source relative to said second planar surface of the second boundary surface for directing light internally reflected from the second boundary surface towards the first boundary surface at a correspondingly lower angle of incidence.