Light-Emitting Display Device Including Light Diffusing Film

This invention relates to a light-emitting display device having an improved light diffusing film.

Light-emitting display devices are utilized in indicators of characters, marks and numerals. The light-emitting display device generally comprises a light-emitting element, a substrate on which the light-emitting element is disposed, a reflector which surrounds the light-emitting element, and a light diffusing film. The light diffusing film plays an important role in the point of uniformly diffusing the light emitted from the light-emitting element. In the past, a light-emitting display device as described in the following has been used. One light diffusing film consists of a light diffusing material which is a powdered material such as Al.sub.2 O.sub.3 SiO.sub.2 or CaCO.sub.3, and transparent material. Another light diffusing film consists of a transparent material having a rough face.

The brightness of the light-emitting display device using the above-mentioned light diffusing film, is decreased by the absorptive efficiency of the light diffusing film, in the case where the light diffusing material is such as Al.sub.2 O.sub.3, SiO.sub.2 or CaCO.sub.3. The brightness distribution of the light diffusing film is chiefly affected by the grain distribution and by the grain size of the light diffusing material. Therefore, the dispersion of the brightness and the brightness distribution widely vary in the prior art. As a result, it is difficult to produce a desirable characteristic for the light-emitting display device in the prior art.

The object of this invention is to provide a light-emitting display device which is satisfactory as regards the average brightness characteristics.

Another object of this invention is to provide a light-emitting display device which is satisfactory as regards the brightness distribution characteristics.

The objects of the present invention are achieved by a light-emitting display device including a light emitting element, a substrate on which the light emitting element is disposed, a reflector for effective reflecting the light from the light emitting element and a light diffusing film which includes a light diffusing part of a fibriform light diffusing material.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by references to the following detailed description when considered in connection with the accompanying drawing, wherein;

FIG. 1 is a sectional view of a light-emitting display device according to one embodiment of the present invention,

FIG. 2 is an oblique view of the light diffusing film in FIG. 1,

FIG. 3 is a sectional view of the glass fiber shown in the light diffusing film in FIG. 2,

FIG. 4 is a graph showing the relationship between the brightness and the position of the light diffusing film in the present invention,

FIG. 5 is an oblique view of a light diffusing film in a light-emitting display device according to another embodiment of the present invention.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the light-emitting display device of the present invention is described hereinunder.

In FIG. 1, a light emitting element 1 electrically connected to leads 4 and 5 which are formed on a substrate 2 in a desirable pattern, is disposed on the substrate 2 with the lead 4 interposed therebetween. A reflector 3 which surrounds the light-emitting element 1 is mounted on the substrate 2. The light from the light-emitting element 1 is reflected by the reflective wall 3a of the reflector 3 to a light diffusing film 6. A large number of rods 7 for dispersing the light are arranged to be parallel to one another at regular intervals in a transparent film 8. Also, a large number of fibriform materials for dispersing the light may be arranged to be parallel, to be in a mesh, or at random in the transparent film 8. Furthermore the fibriform material may be in the form of a solid rod or a flexible string.

The transparent film 8 is acrylic plastic with 0.2 mm thickness. The radius of rod 7 is about 50 .mu., and the rod 7 is subjected to treatment for making its surface uneven. Referring to FIG. 2, a plurality of rods 7 are arranged at regular intervals in the transparent film 8, for example with calendering treatment. The light diffusing film 6 is composed of two parts. One is a transparent part which passes the light emitted from the light-emitting element 1. Another is a light diffusing part which disperses the light from the light-emitting element 1.

The transparent part is made from a transparent material which is able to transmit the light from the light-emitting element 1. Glass, a high molecular compound, air and the like are suitable for the transparent material. In this embodiment thermo-plastic material and thermo-indurative material made from a high molecular compound are used as the transparent part. For the thermo-plastic material, acrylic-plastic, cellulose acetate, polyimide, polycarbonate, polyethylene, polystyrene and the like are suitable and for the thermo-indurative matter epoxy resins, phenolic resin, polyurethane and the like are suitable. The thinner the transparent part, the brighter is the light-emitting display device, because the transparent matter absorbs little light. Therefore, the light diffusing film, that is to say the diffuser including the transparent material, should be as thin as possible. But, the thinner the transparent part, the frailer is the transparent material. Consequently, the desirable thickness of the transparent part is 10.mu. to 1000.mu.; the optimum thickness is 50.mu. to 70.mu..

The light diffusing part includes a light diffusing material, for instance a fiber glass, a fibriform high molecular compound or a vegetable fiber. For a light diffusing material of fibriform high molecular compound, teflon and the like are suitable the same as for the above-mentioned transparent matter. In case the refractive efficiencies of the light diffusing material and the transparent material are equal, it is necessary to roughen the entire surface of the light diffusing material. In case the light diffusing material is different from the transparent material with regards to refractive efficiency, the roughening is not necessary, because the light is dispersed at the boundary between the light diffusing part and the transparent part on account of the different refractive efficiency. When the ratio of the different refractive efficiencies is above 1.5, the optimum brightness for the light-emitting display device is obtained.

FIG. 4 shows the relationship between the brightness and the position of the light diffusing film. The luminous intensity of the light-emitting display device is shown by a solid line, and the average luminous intensity characterized by the orientation, namely the average brightness, is shown in a dashed line. The average brightness of the light-emitting display device in the present invention is higher by 50% to 80% than the prior art. Therefore the average brightness of the light-emitting display device in the present invention is greatly improved. Furthermore, it is found to be better that the area ratio of the transparent part to the light diffusing part be equal to or more than 4 to 1 because otherwise, flickering of the light-emitting display device is observed.

Next, the light diffusing film of another embodiment of the present invention is formed in the following way. After many glass fibers are arranged in an intertwined mesh configuration, a press treatment is carried out. A plate of many pressed glass fibers shown in FIG. 5 is formed with a thickness of 0.15 mm. The plate of many glass fibers 8 intertwined at random can be applied to the light-emitting display device too. In this case, the light diffusing part of the plate is made of fiber glass, and the transparent part is made of air. The average brightness mainly depends on the thickness of fiber and the thickness of the plate. The relationship of the average brightness and the intensity ratio between the light-emitting display device in the present invention and in the prior art is shown in the table. The intensity ratio shows the ratio of the brightness distribution between maximum intensity and minimum intensity in the light-emitting display area in the light-emitting display device.

The prior art described in the table shows a light-emitting display device, wherein the light diffusing film consists of acrylic plastic and CaCO.sub.3 which is added into the light diffusing film at 15 weight percent, and the thickness of the light diffusing film is 0.15 mm.

The light diffusing film consists of acrylic plastic and fiber glass having a radius of 0.05 mm, the distance between the glass fibers at regular intervals being about 0.2 mm, and the thickness of the light diffusing film being 0.15 mm. As described in the table, the light-emitting display device has satisfactory brightness characteristics, particularly the characteristic regarding the average brightness and the brightness distribution.

The light diffusing film consists of air and fiber glass which has a radius of 0.05 mm, the distance between the glass fibers in a mesh configuration is about 0.3 mm, and the thickness of the light diffusing film is 0.15 mm. As described in the table, the light-emitting display device has a satisfactory average brightness. The light-emitting display device of the present invention is extremely useful for providing a light-emitting display device for which the average brightness is high.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.