Abstract: A device comprising the following parts; a light guiding plate in thermoplastic material; a source of light placed on at least one edge of the light guiding plate; a reflecting surface placed along the surface of the light guiding plate, a series of grooves being present on the lower surface, parallel to the light guiding plate edge on which there is the source of light; the grooves having: an hollow shape; the total groove surface, measured on the incision projections on the plate surface, comprised between 10% and 60% with respect to the plate surface; the width of each groove between 0.2 and 5 mm.