BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the structure of a planar light source unit according to a first embodiment of the present invention.
FIG. 2 is a diagram showing the structure of a light guide plate for use in the planar light source unit shown in FIG. 1.
FIG. 3 is a diagram showing the luminance distribution on the light guide plate shown in FIG. 2.
FIG. 4 is a diagram showing the structure of a light guide plate for use in a planar light source unit according to a second embodiment of the present invention.
FIG. 5 is a diagram showing the operation of the light guide plate shown in FIG. 4.
FIG. 6 is a diagram showing the luminance distribution on the light guide plate shown in FIG. 4.
FIG. 7
a is an exploded perspective view showing the structure of a conventional planar light source unit.
FIG. 7
b is a side explanatory view showing the path of light in the planar light source unit shown in FIG. 7a.
FIG. 7
c is a side explanatory view illustrating light emitted from the light guide plate toward a diffuser shown in FIG. 7b.
FIG. 8
a is an exploded perspective view showing the structure of a conventional light guide plate having an anisotropic diffusion surface.
FIG. 8
b is a side explanatory view illustrating exiting light from the light guide plate shown in FIG. 8a.
FIG. 8
c is a sectional view taken along the line 8c-8c in FIG. 8b.
FIG. 9 is a diagram showing in the left half (a) a method of producing a hologram in an XZ plane and showing in the right half (b) the operation of the hologram thus produced.
FIG. 10 is a diagram showing in the left half (a) a method of producing a hologram in a YZ plane and showing in the right half (b) the operation of the hologram thus produced.
FIG. 11 is a diagram for explaining the reason why curved emission lines are generated in anisotropic diffusion by a hologram.
FIG. 12 is a side view showing the action of an anisotropic diffusion surface on which a laser beam perpendicularly impinges, which also shows a sectional view taken along the line A-A in the side view.
FIG. 13 is a side view showing the action of the anisotropic diffusion surface on which a laser beam obliquely impinges, which also shows a sectional view taken along the line B-B in the side view.
FIG. 14 is a diagram showing the luminance distribution on the light guide plate shown in FIGS. 8a and 8b.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 shows the general structure of a planar light source unit 20 according to a first embodiment of the present invention. In FIG. 1, the planar light source unit 20 includes LEDs (light-emitting diodes) 2, a light guide plate 1, a diffuser 3, a Px prism sheet 4, a Py prism sheet 5, and a reflector 6. The LEDs 2 are mounted on an LED substrate 2b and disposed at a position facing a light-receiving surface 1c of the light guide plate 1. The diffuser 3, the Px prism sheet 4 and the Py prism sheet 5 are stacked successively over an upper surface 1a of the light guide plate 1. The reflector 6 is disposed close to and facing a lower surface 1b of the light guide plate 1. In the illustrated coordinate system consisting of mutually orthogonal x, y and z axes, the y axis is the axis in the longitudinal direction of the light guide plate 1, which is perpendicular to the light-receiving surface 1c. The x axis is the axis in the width direction of the light guide plate 1, which is perpendicular to the y axis. The z axis is the axis in the thickness direction of the light guide plate 1.
FIG. 2 is a perspective view showing the light guide plate 1 and the LEDs 2 of the planar light source unit 20 shown in FIG. 1. As shown in FIG. 2, an anisotropic diffusion surface 1h and a smooth flat surface 1k are formed on the upper surface 1a of the light guide plate 1. The anisotropic diffusion surface 1h may, for example, be a hologram diffusion surface, or a hairline diffusion surface having a hologram-like groove pattern. The smooth flat surface 1k is formed only in a region of the upper surface 1a of the light guide plate 1 that extends over a predetermined distance from the light-receiving surface 1c, i.e. a region of the upper surface 1a close to the LEDs 2. The anisotropic diffusion surface 1h, which is a hologram diffusion surface or a hairline diffusion surface having a hologram-like groove pattern, is formed on the rest of the upper surface 1a of the light guide plate 1. The lower surface 1b of the light guide plate 1 is provided with a plurality of prism surfaces.
Light rays entering the light guide plate 1 through the light-receiving surface 1c travel in the light guide plate 1, and while traveling, the light rays exit upward from the upper surface 1a including the smooth flat surface 1k and the anisotropic diffusion surface 1h. At this time, light from the anisotropic diffusion surface 1h exits as anisotropically diffused light. Light exiting the smooth flat surface 1k is not emitted as diffused light but as ordinary refracted light. Therefore, no curving due to anisotropic diffusion occurs in the cross-section of the light exiting the smooth flat surface 1k.
FIG. 3 shows the luminance distribution of light exiting the light guide plate 1 as seen from above (as seen from the vicinity of the upper surface 1a of the light guide plate 1). In the figure, regions L1 shown by diagonal hatching are where rays are emitted even a little, and thickly shaded regions L2 are curved emission lines that are conspicuously bright.
In the first embodiment, as shown in FIG. 3, curved emission lines due to anisotropic diffusion are not present at all in a surface portion corresponding to the smooth flat surface 1k in FIG. 2 because ordinary refracted rays exit therefrom. In the other surface portion (corresponding to the anisotropic diffusion surface 1h in FIG. 2), curved emission lines L2 due to anisotropic diffusion are present, but the degree of curving of the emission lines L2 is relatively small, and the area of the gaps between the curved emission lines, which provide a low luminance, is relatively small. In this state, light exits upward from the light guide plate 1 and passes through the diffuser 3, the Px prism sheet 4 and the Py prism sheet 5 as illuminating light. Therefore, the direction of exiting light from the light guide plate 1 is corrected, and the luminance nonuniformity is improved. The problem that the curved emission lines due to anisotropic diffusion is also improved to a considerable extent. Thus, it is possible to emit illuminating light of substantially uniform luminance as a whole.
It should be noted that the reflector 6 reflects rays exiting downward from the lower surface 1b of the light guide plate 1 back into the light guide plate 1, thereby increasing the light utilization efficiency in the light guide plate 1.
A second embodiment of the planar light source unit according to the present invention will be explained below with reference to FIGS. 4 to 6. FIG. 4 shows a light guide plate 11 for use in the planar light source unit according to the second embodiment. A smooth flat surface 11k is provided near a light-receiving surface 11c of the light guide plate 11 in the same way as the first embodiment shown in FIG. 2. The light guide plate 11 further has band-shaped anisotropic diffusion surfaces 11h and band-shaped smooth flat surfaces 11k that are alternately provided thereon. In the illustrated example, the width of each smooth flat surface 11k and that of each anisotropic diffusion surface 11h are substantially equal to each other.
FIG. 5 shows the operation of the light guide plate 11 shown in FIG. 4. When internal light s in the light guide plate 11 is incident on one anisotropic diffusion surface 11h, a part of the incident light becomes diffused light φ, but the remaining light (i.e. light not passing through the slits of the hologram) is diffusely reflected by the anisotropic diffusion surface 11h and may be directed to the neighboring smooth flat surface 11k via the reflector 6 and so forth. As denoted by the dashed lines sr, the light passes through the smooth flat surfaces 11k while being subject refraction. The angles of refraction of the exiting light are distributed over a relatively wide angle range according to the angle of incidence on the smooth flat surface 11k. Accordingly, the exiting light sr can complementarily fill the gaps between the diffused lights φ exiting from the anisotropic diffusion surface 11h. It should be noted that internal light s is also reflected on the lower surface 11b and passes through the smooth flat surface 11k as exiting light s1. Therefore, the exiting light s1 complementarily enter the gaps between the diffused lights φ.
The complementarily filling effect of the smooth flat surface 11k can be increased by increasing the ratio of the width of the smooth flat surface 11k to that of the anisotropic diffusion surface 11h. Excessively increasing the width ratio, however, will nullify the characteristic advantage of the anisotropic diffusion surface that can enhance the overall brightness of the planar light source unit. Therefore, it is preferable to set the ratio of the width of the smooth flat surface 11k to that of the anisotropic diffusion surface 11h appropriately in a range within which the brightness enhancing purpose can be attained, thereby compatibly achieving the luminance uniformity of illuminating light and the enhancement of overall brightness of illuminating light.
Although FIG. 4 shows an example in which a plurality of smooth flat surfaces 11k are provided in a belt-like shape, each between a pair of adjacent anisotropic diffusion surfaces 11h, the present invention is not necessarily limited thereto. Smooth flat surfaces 11k of a triangular, quadrangular, circular or other shape may be dispersedly arranged in a zigzag or other pattern between each pair of adjacent anisotropic diffusion surfaces 11h. With this arrangement, the same advantageous effects as the above can be obtained.
FIG. 6 shows the luminance distribution of exiting light from the light guide plate 11 in FIG. 4 as seen from above (as seen from the vicinity of the upper surface 11a of the light guide plate 11). In the figure, a region L1 shown by coarse hatching corresponds to the smooth flat surface 11k near the light-receiving surface 11c in FIG. 4, where the luminance is relatively low. A region L3 shown by fine hatching corresponds to the area in which the smooth flat surfaces 11k and the anisotropic diffusion surfaces 11h are mixedly present in FIG. 4, where the luminance is relatively high and the luminance distribution is substantially uniform. In FIG. 6, there are practically no curved emission lines L2 as shown in FIG. 3. Thus, the light guide plate 11 according to the second embodiment shown in FIG. 4 can satisfactorily eliminate the disadvantage of the anisotropic diffusion surface and can ensure the required intensity of lighting.
It should be noted that the present invention is not necessarily limited to the foregoing embodiments but can be modified in a variety of ways without departing from the gist of the present invention.
Patent Application
20080018968
