A light-emitting device, in particular a backlight device, comprises a transparent substrate (2) having a front surface and a rear surface, in which associated to the rear surface are means for generating an electromagnetic radiation that is able to pass through the substrate and come out of the front surface. According to the invention, the device comprises a layer of porous alumina which operates so as to inhibit propagation of said electromagnetic radiation in the directions parallel to the plane of the substrate, thus improving the efficiency of extraction of light from the substrate and increasing the directionality of the emitted light.