A flat illumination device such as may be used in a liquid crystal display device, includes a light guiding plate having a front and a rear surface and at least one illumination window. The device further includes a light source that is coupled to an edge of the light guiding plate by means of an energy conserving coupling section, and which provides light that is conditioned and transmitted through the light guiding plate. Light extractors are disposed on at least one side of the light guiding plate to assist in the transmission and conditioning of the light that is provided to the light guiding plate by the light sources. Extractors form a non-uniform pattern that is a two-dimensional, non-monotonic irregular function of the distance of their elementary area a point on the perimeter of the light guiding plate.