Uniformly thin, high efficiency large area lighting panel with two facet grooves that are spaced apart and have light source facing facets with smaller slopes than the facets facing away from the light source

#This invention relates generally to a flat, thin, large area lighting panel, and more particularly to a large area lighting panel employing a uniformly thin light guide having a planar major surface, a parallel major surface which includes faceted grooves opposite said planar major surface, and a reflecting surface opposite said faceted grooved surface.

#Large area lighting panels employing thin light guides with edge lit end surfaces are known. Many of these large area lighting panels have in common the fact that they do not have a uniform thickness, which is desirable in many applications, such as back-lighting of liquid crystal displays (LCDs) for pagers, hand-held and portable computers, organizers, and the like. In addition, they are not as easily and inexpensively manufactured as are uniformly thin panels. Many large area lighting panels do not provide uniform light. This drawback is overcome to some degree by using a diffuser between the lighting panel and the display; however, this increases the thickness and the cost of the lighting panel. In some large area lighting panels, several individual sources project light into the guide. In some instances, this results in striations of the observed light emitted by the panel.

#It is an object of this invention to provide an efficient, uniformly thin large area lighting panel.

#The foregoing and other objects of the invention will be clearly understood from the following description when read in conjunction with the drawings of which:

#FIG. 1 is a perspective view of a light guide and display assembly in accordance with one embodiment of the invention;

#FIG. 2 is an enlarged cross-sectional view of the light guide of FIG. 1;

#The figures show display assembly 11 which includes a uniformly thin light guide 12 with parallel first and second end surfaces 13 and 14, joined by parallel side surfaces 16 and 17, a planar major upper surface 21, and a shaped major lower surface 22 parallel to the upper layer surface. The shaped lower surface includes optically flat facets 23 and 24. A thin, flat member 26 is placed adjacent to the shaped lower surface and includes a highly reflective light scattering face 27. The light guide typically has a thickness of 1 mm or less. A suitable light source not shown in FIG. 1, projects light illustrated by rays 25 into the light guide through the end surface 13. The facets 23 and 24 of the shaped lower surface 22 extend across the light guide between side surfaces 16 and 17. The facets are generally parallel to the front surface 13. The optically flat facets 23 are inclined upwardly at an angle between 1.degree. and 15.degree., with respect to the plane of the lower surface 22 and reflecting face 27. Preferably, the facets 23 are inclined between 1.degree. and 10.degree. with respect to the lower surface 22. The depth of the grooves formed by the facets 23, 24 is between 5 and 10 microns. The spacing between the grooves defines lands; the width of the lands is between 1 and 200 microns.

#Referring to FIG. 2, inclined facets 23 specularly reflect light entering the guide from the face 13 along the guide towards the end 14. A reflector 28 is placed adjacent the end 14 to reflect the light back into the light guide. The second facets 24 are inclined downwardly at a steep angle of between 35.degree. and 55.degree. with respect to the plane of the lower surface, preferably between 40.degree. and 50.degree.. The light reflected back into the light guide strikes the facets 24 and is either specularly reflected upwardly through the upper major surface 21 or it strikes the surface at less than the critical angle and is transmitted through the surface to strike the reflecting surface 27 where it is reflected back into the guide and through the upper surface. The light rays 30 illustrate the internal reflection and illumination of the upper surface. The advantage of the inclination of the facets is that they allow the light to travel along the guide and scatter within the guide and thereby provide a more uniform projection of light through the upper surface. In addition, light may be reflected off the surfaces 23 and through the upper surface 2 1. This is illustrated in by ray 30a. The specular reflection of the light provides a highly efficient lighting panel. In the embodiment of FIGS. 1 and 4, the emitted light strikes a diffuser 29 which scatters the light and illuminates an LCD 31. It is, of course, to be understood that the large area lighting panel may be employed to provide large area light for any lighting purpose.

#FIGS. 2 and 3 are enlarged views of the large area lighting panel of FIG. 1. A light source 32 projects light into the light guide from the end 13. The light source illustrated includes an elongated incandescent lamp 33 which has a linear, elongated filament 34 which, when heated, emits light in all directions along its length as shown by the arrows 36. A reflector 37 reflects the light back toward the end surface 13. The entering light is reflected along the light guide from the upper surface 21 as well as from the side surfaces 16 and 17, and from the facets 23. Light reflected from reflector 28 which is not emitted from the major upper surface 21 may escape through the end 13 where it is reflected back into the guide by the reflector 37. The flat portions 35 between the faceted grooves transmits light along the panel as shown by ray 30b. This acts together with the facets 23 to direct light along the panel. Additionally, by varying the width of the flat surfaces, longitudinal uniformity of illumination can be further enhanced. Preferably, the side surfaces 16 and 17 are provided with a reflector (not shown) which prevents any light from escaping through these surfaces. Thus, the only way light can leave the light guide is through the upper major surface 21. The relatively uniform light flux provided by the internally reflecting facets 23 and the multiple internal reflections provides a uniform large area lighting panel. In the presence of the multiple reflections caused by the specially designed inclination angle of facets 23 in this invention, the emitted light is uniform over the surface of the panel. In the absence of such multiple reflections, the emitted light is much more intense along the panel opposite the light source when the source does not extend entirely across the width of the panel.