(Regarding Japanese Unexamined Patent Publication No. 5-88174) An additional sheet such as a light diffusion sheet needs to be omitted and the thickness of the entire light guide plate needs to be thinned in order to thin a backlight used in the liquid crystal display device, and the like.
However, if the thickness of the light guide plate is thinned, the light guide plate becomes thin like a sheet, and thus the light guide plate tends to easily warp. If the light guide plate easily warps, the assembly of the backlight becomes difficult and the light may leak from the area where the light guide plate warped. Thus, a method of laminating the light guide plate to the back surface of a liquid crystal display panel with an adhesive etc. without sandwiching an air layer between the light guide plate and the liquid crystal display panel is known for the method of preventing the warp of the thinned light guide plate.
A liquid crystal display device in which the light guide plate is adhered to the back surface of the liquid crystal display panel includes that described in a first comparative example of Japanese Unexamined Patent Publication No. 5-88174 (see FIG. 3 of Japanese Unexamined Patent Publication No. 5-88174). As shown in FIG. 1, in such liquid crystal display device 11, an acrylic plate having smooth surfaces (index of refraction of 1.49) is used for the light guide plate 12, and the light guide plate 12 is laminated to the back surface of a scattering type liquid crystal display panel 14 so as not to sandwich the air layer using a connection layer 13 having an index of refraction higher than the light guide plate 12 (two-pack type curable silicone having index of refraction of 1.51). A light source 15 including a cold cathode tube is installed at positions facing both end faces of the light guide plate 12.
As shown in FIG. 2A, in the liquid crystal display device 11, the light L exit from the light source 15 and entered into the light guide plate 12 transmits through the light guide plate 12 to the connection layer 13 and further enters the liquid crystal display panel 14, and then scattered by pixels in a scattered state (clouded state) to be exit to the front side, thereby causing the pixels to emit light.
However, in such liquid crystal display device, the light is not totally reflected at the interface of the connection layer 13 and the light guide plate 12 as with the light L shown with a broken line in FIG. 2A since the index of refraction of the connection layer 13 is higher than the index of refraction of the light guide plate 12. Thus, the light L entered to the light guide plate 12 cannot be guided through the light guide plate 12, and is exit from the liquid crystal display panel 14 in the vicinity of the light source 15. As a result, the light emission luminance is high and bright at locations close to the light source 15, but the light emission luminance is low and dark at locations distant from the light source 15 (i.e., central portion of the light sources 15), as shown in the luminance distribution in FIG. 2B.
In a first example described in Japanese Unexamined Patent Publication No. 5-88174 (see FIG. 1 of Japanese Unexamined Patent Publication No. 5-88174), a thin film 16 having an index of refraction lower than the light guide plate 12 is partially formed on the surface of the light guide plate 12, where the area ratio of the thin film 16 is set large at locations close to the light source 15 and the area ratio of the thin film 16 is set small at locations distant from the light source 15, as shown in FIG. 3, to resolve the unevenness of the light emission luminance. The light guide plate 12 formed with the thin film 16 is laminated to the back surface of the liquid crystal display panel 14 through the connection layer 13 having an index of refraction higher than the light guide plate 12. Here, the light guide plate 12 is formed by an acrylic plate having an index of refraction of 1.49, a two-pack type curable silicone having an index of refraction of 1.41 is used for the thin film 16, and a two-pack type curable silicone having an index of refraction of 1.51 is used for the connection layer 13.
In the first example of Japanese Unexamined Patent Publication No. 5-88174, the light in the light guide plate 12 is guided through the light guide plate 12 by being totally reflected at the interface of the light guide plate 12 and the thin film 16 since the thin film 16 is formed on the surface of the light guide plate 12. Furthermore, the ratio of light transmitted between the thin films 16 and exit from the liquid crystal display panel 14 is small since the area ratio of the thin film 16 is large at locations close to the light source 15, and the ratio of light passed between the thin films 16 and exit from the liquid crystal display panel 14 is large since the area ratio of the thin film 16 is small at locations distant from the light source 15 where the amount of light that reaches is small, and consequently, the evenness of the light emission luminance is achieved in the entire display surface of the liquid crystal display device.
In a second example described in Japanese Unexamined Patent Publication No. 5-88174 (see FIG. 2 of Japanese Unexamined Patent Publication No. 5-88174), as shown in FIG. 4, a concave-convex portion 17 having a prism shape is partially formed on the surface of the light guide plate 12, where the degree of roughness of the concave-convex portion 17 is set low at locations close to the light source 15 and the degree of roughness of the concave-convex portion 17 is set high at locations distant from the light source 15. The light guide plate 12 formed with the concave-convex portion 17 is laminated to the back surface of the liquid crystal display panel 14 through the connection layer 13 having a lower index of refraction than the light guide plate 12. Here, the light guide plate 12 is formed by an acrylic plate having an index of refraction of 1.49, and a two-pack type curable silicone having an index of refraction of 1.41 is used for the connection layer 13.
In a second example of Japanese Unexamined Patent Publication No. 5-88174, the light in the light guide plate 12 is enclosed in the light guide plate 12 by being totally reflected at a smooth region of the surface of the light guide plate 12 and then guided through the light guide plate 12 since the index of refraction of the connection layer 13 is lower than the index of refraction of the light guide plate 12. The light entering the concave-convex portion 17 is scattered by the concave-convex portion 17 so as to be transmitted to the connection layer 13, and scattered by the pixels in the scattered state of the liquid crystal display panel 14 thereby emitting light. Furthermore, the ratio of light scattered at the concave-convex portion 17 and exit from the liquid crystal display panel 14 is small since the degree of roughness of the concave-convex portion 17 is low at locations close to the light source 15, and the ratio of light scattered at the concave-convex portion 17 and exit from the liquid crystal display panel 14 is large since the degree of roughness of the concave-convex portion 17 is high at locations distant from the light source 15 where the amount of light that reaches is small, and consequently, the evenness of the light emission luminance is achieved in the entire display surface of the liquid crystal display device.
In the first example of Japanese Unexamined Patent Publication No. 5-88174, the directional characteristics of the light in a plane perpendicular to the light guide plate 12 are shown in FIG. 5A. The spread (directional characteristics) of the light immediately before entering the light guide plate 12 is ±90°, but the spread of light immediately after entering the light guide plate 12 is±arcsin(1/1.49)=±42.2°since the index of refraction of the light guide plate 12 is ng=1.49. The critical angle of total reflection at the interface of the light guide plate 12 and the thin film 16 is,arcsin(1.41/1.49)=71.1°.The critical angle 71.1° is 18.9° when measured from the horizontal direction.
Therefore, the light (light in the shaded range in FIG. 5B) in the range of 18.9° to 42.2° and the light in the range of −18.9° to −42.2° measured from the horizontal direction out of the light having the spread of ±42.2° that entered the light guide plate 12 transmit through the thin film 16 without being reflected at the interface with the thin film 16 when entered to the interface with the thin film 16. The light of the shaded range in FIG. 5B is not guided through the light guide plate 12 as it is transmitted through the thin film 16 in the vicinity of the light source 15, and hence sufficient amount of light cannot be guided from the light source 15 to a distant location, and the light emission luminance cannot be sufficiently evened.
In the second example of Japanese Unexamined Patent Publication No. 5-88174 as well, the light in the range of 18.9° to 42.2° and the light in the range of −18.9° to −42.2° measured from the horizontal direction out of the light having the spread of ±42.4° that entered the light guide plate 12 transmits through the connection layer 13 without being reflected at the smooth region of the light guide plate 12 since the index of refraction of the connection layer 13 is 1.41, same as the thin film 16 of the first example. Therefore, in the second example as well, the light of the shaded range in FIG. 5B is not guided through the light guide plate 12 as it is transmitted through the connection layer 13 in the vicinity of the light source 15, and hence sufficient amount of light cannot be guided from the light source 15 to a distant location, and the light emission luminance cannot be sufficiently evened.
In the liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 5-88174, the vicinity of the light source lights up brightly since the light easily leaks in the vicinity of the light source thereby causing luminance unevenness at the display surface, and the display surface becomes dark due to lowering of the usage efficiency of the light.
In the present specification, an idiomatically used method is sometimes used to represent the directional characteristics and the directivity spread of the light. For instance, the spread of light is sometimes simply expressed as ±42.2° instead of being expressed as −42.2° to +42.2° (i.e., −42.2≦X≦42.4, where X is the spread of light).
(Regarding Japanese Unexamined Patent Publication No. 2001-110218)
A liquid crystal display device in which the thickness of the light guide plate is thinned and the attenuation of light by the additional sheet is eliminated by omitting the additional sheet such as a prism sheet, and the luminance of the display surface is enhanced by narrowing the directional characteristics of the exit light includes that disclosed in Japanese Unexamined Patent Publication No. 2001-110218.
FIG. 6 is a schematic view showing a structure of a liquid crystal display device disclosed in Japanese Unexamined Patent Publication No. 2001-110218. The liquid crystal display device 21 has a liquid crystal display panel 23 overlapped over the entire surface of an area light source device 22. The area light source device 22 includes a wedge-shaped light guide plate 24, where a first light transmissive layer 25 having a lower index of refraction than the light guide plate 24 is overlapped on the lower surface of the light guide plate 24, and a light deflection layer 26 having substantially the same index of refraction as the light guide plate 24 is overlapped on the lower surface of the first light transmissive layer 25. The light deflection layer 26 is obtained by integrally forming a micro-mirror 28 made from metal material on the back surface of a second light transmissive layer 27 having substantially the same index of refraction as the light guide plate 24. A light source 29 such as a cold cathode fluorescent lamp is arranged facing the end face on the large thickness side of the light guide plate 24.
The optical path is shown with an arrow in FIG. 6, where the light radiated from the light source 29 passes the side surface of the light guide plate 24 and enters the light guide plate 24 in the area light source device 22. The light that entered the light guide plate 24 is guided through the light guide plate 24 while repeating total reflection between the upper surface and the lower surface of the light guide plate 24. Regarding the light in the light guide plate 24, the incident angle (angle formed by the light beam and a normal line at the lower surface of the light guide plate 24) of when entering the lower surface of the light guide plate 24 becomes smaller every time the light is totally reflected at the upper surface and the lower surface of the wedge-shaped light guide plate 24. When the incident angle of light entering the lower surface of the light guide plate 24 becomes smaller than the critical angle of total reflection at the interface of the first light transmissive layer 25 and the light guide plate 24, such light is transmitted through the lower surface of the light guide plate 24 and entered to the first light transmissive layer 25, and then entered into the second light transmissive layer 27 and reflected at the micro-mirror 28, where the light reflected at the micro-mirror 28 is transmitted through the second light transmissive layer 27, the first light transmissive layer 25, and the light guide plate 24, and exit from the upper surface (light exit surface) of the light guide plate 24.
In the area light source device 22 having such structure, only the light in a narrow range, in which the incident angle of when entering the lower surface of the light guide plate 24 is smaller than the critical angle of total reflection, out of the light guided through the light guide plate 24 is transmitted through the first and second light transmissive layers 25, 27 and reflected at the micro-mirror 28, and hence the light of narrow directional characteristics can be exit from the area light source device 22.
However, since the first light transmissive layer 25 having a lower index of refraction than the light guide plate 24 is closely attached to the lower surface of the light guide plate 24 without interposing an air layer even in the relevant area light source device 22, the light corresponding to the shaded range in FIG. 5B is transmitted through the first and second light transmissive layers 25, 27 and reflected at the micro-mirror 28, and then exit all at once from the front surface of the area light source device 22 when entered to the lower surface of the light guide plate 24 the first time as shown in FIG. 7. Thus, the vicinity of the light source 29 lights up brightly thereby causing luminance unevenness, and the entire display surface cannot evenly emit light even when such area light source device 22 is used. Moreover, the display surface becomes dark since the light leaks in the vicinity of the light source 29 and the usage efficiency of the light lowers.