The present invention relates to a liquid crystal display device, a light guide plate and a method for producing a light guide plate. Particularly, it relates to a technique concerning a light guide plate in a back-lighting type or front-lighting type liquid crystal display device.
A portable computer called lap-top personal computer has been popularized with the advance of miniaturization of a personal computer in recent years. A liquid crystal display device is generally used as a display device in the lap-top personal computer. Color expression of the liquid crystal device has progressed in recent years. A so-called backlighting type liquid crystal display device in which an illuminating means is disposed on the back of a liquid crystal display panel so that a display surface is lit from the back holds the main current of such a liquid crystal display device. A back-lighting means in such a back-lighting type liquid crystal display device needs to light the whole flat surface of the liquid crystal display panel evenly with high and uniform luminance. It may be conceived that luminance of a light source is increased to improve back-lighting luminance. The increase of luminance of the light source is, however, self-limited because it causes increase of electric power consumption and temperature rise in the liquid crystal display device.
There are known various configurations for the back-lighting type liquid crystal display device. For example, the background art includes techniques disclosed in JP-A-4-162002, JP-A-6-67004, etc.
FIG. 2 is a view showing the configuration of the back-lighting means in the conventional back-lighting type liquid crystal display device using an edge-lighting system. In FIG. 2, the reference numeral 1 designates a light source; 2, a light guide plate; 3, a diffusing sheet; 5, a first prism sheet; 5xe2x80x2, a second prism sheet; 6, a light scattering layer; and 7, a reflection sheet.
In the configuration shown in FIG. 2, the light source 1 constituted by a lamp such as a cold cathode tube, a hot cathode tube, or the like, is disposed on an edge face of the light guide plate 2 made of a light-transmissible material so that illuminating light emitted from the light source 1 is led into the light guide plate 2. The diffusing sheet 3 made of a translucent white synthetic resin and having a light scattering effect for making luminance of an illuminating surface uniform over the whole surface is provided on an upper surface (light emitting surface) of the light guide plate 2. The first and second prism sheets 5 and 5xe2x80x2 for converging scattered light to a certain degree to enhance the frontal luminance of the liquid crystal display device are further disposed on an upper surface of the diffusing sheet 3.
On the other hand, the light scattering layer 6 is provided on a surface (rear surface) of the light guide plate 2 opposite to the light emitting surface of the light guide plate 2 so that light led into the light guide plate 2 is scattered in the direction toward the diffusing sheet 3. The reflection sheet 7 is further disposed on a lower surface of the light scattering layer 6.
The light scattering layer 6 is configured as follows. FIG. 3 is a view showing the configuration of the light scattering Layer 6 depicted in FIG. 2. As shown in FIG. 3, the light scattering layer 6 is formed by printing a plurality of light diffusing substances using titanium oxide, glass beads, or the like, as a predetermined pattern on the rear surface of the light guide plate 2 by a technique of screen printing, or the like. Generally, the intensity of light emitted from the light source 1 decreases as the position of the light becomes farther from the light source 1. Therefore, the light scattering layer 6 is formed so that the pattern area of the light scattering layer 6 in the light guide plate 2 increases as the position becomes farther from the light source 1.
JP-A-7-294745 has proposed also a light guide plate in which grating grooves as an alternative to the aforementioned light scattering layer 6 are formed in a surface (rear surface) of the light guide plate opposite to the light emitting surface of the light guide plate so that light incident on the light guide plate is reflected at the grating grooves.
On the other hand, a reflection liquid crystal display device as described in xe2x80x9cApplied Physics; Vol. 67, No. 10, p.1159 (1998)xe2x80x9d is known as a technique for achieving a low power-driven liquid crystal display device without use of back-lighting. In such a reflection liquid crystal display device, room light or sunlight taken in is reflected at a layer formed on the back of a liquid crystal and having a reflecting function to thereby achieve elimination of back-lighting. The visibility of the reflection liquid crystal display device is, however, lowered in the dark place. To apply the refection liquid crystal display device to a wider working environment, it is necessary that a measure counter to the lowering of visibility is taken while the characteristic of the reflection liquid crystal display device is kept the best.
A front-lighting type liquid crystal display device as shown in FIG. 4 has been proposed to solve the aforementioned problem. FIG. 4 is a view showing the configuration of a (front-lighting type) reflection liquid crystal display device having a front-lighting means (hereinafter merely referred to as front-lighting type liquid crystal display device).
In FIG. 4, the reference numeral 2 designates a light guide plate; 6, a light scattering layer (formed by screen printing in the same manner as the light scattering layer in FIG. 3) formed on a surface (upper surface in FIG. 4) of the light guide plate 2 opposite to the light emitting surface of the light guide plate 2; and 1, a light source disposed on an edge face of the light guide plate 2. The light source 1 and the light guide plate 2 including the light scattering layer 6 constitute a front-lighting means. In the front-lighting type liquid crystal display device shown in FIG. 4, the light source 1 is not turned on in the bright place and display is watched through the light guide plate 2 of high transparency. In the dark place, the light source 1 is switched on so that the front-lighting means operates in place of external light.
Little scattering, high transparency and smallness in the quantity of light exiting from the upper surface in FIG. 4 are required as performance of the light guide plate in the aforementioned front-lighting type liquid crystal display device. Also in the light guide plate used in the front-lighting type liquid crystal display device, there is known a configuration in which grating grooves as an alternative to the light scattering layer 6 in FIG. 4 are provided in a surface of the light guide plate 2 opposite to the light emitting surface of the light guide plate 2.
In FIG. 4, the reference numeral 31 designates an absorption film; 32, a reflection polarizer; 33, a diffusing film (diffuser); 34, a glass substrate; 35, a thin-film transistor (TFT); 36, a liquid crystal cell array; 37, an LCD electrode; 38, a color filter; 39, a glass substrate; 40, a diffusing film (diffuser); 41, a phase-contrast film; and 42, a polarizer. The detailed description of the respective parts will be omitted because such a front-lighting type liquid crystal display device (the reflection liquid crystal display device having the front-lighting means) configured as described above is commonly known.
The conventional back-lighting type liquid crystal display device as shown in FIGS. 2 and 3 was configured so that light emitted from the light source 1 was led into the light guide plate 2 and scattered by the light scattering substances in the light scattering layer 6. A considerable part of the quantity of light incident on the light guide plate 2 was, however, reflected at the reflection sheet 7 to thereby cause an energy loss. Moreover, a considerable part of the quantity of light incident on the light guide plate 2 exited from the light guide plate 2 horizontally with respect to the liquid crystal display panel so as not to contribute to display. Hence, enhancement of luminance of the liquid crystal display device was limited. That is, there was a predetermined limit on the condition that light led into the light guide plate 2 could be efficiently utilized for display.
Moreover, the light scattering layer 6 was formed by printing light diffusing substances on the light guide plate 2. Hence, steps such as surface treatment (for improving printability of ink) using plasma treatment, screen printing, ultraviolet-curing treatment, etc. were required after injection molding of the light guide plate. A relatively great deal of labor was taken also for the production of the light guide plate 2.
On the other hand, in the conventional back-lighting type liquid crystal display device using the light guide plate provided with the grating grooves formed therein, enhancement of luminance was achieved but there was a problem that moirxc3xa9 was caused by interference between the regular pattern of light exiting from the light guide plate through the grating grooves and the regular pattern of a constituent member, such as a liquid crystal cell array, of the liquid crystal display unit. Hence, there was a disadvantage that a sheet for diffusing light strongly must be additionally used in order to solve the problem. As a result, sufficient enhancement of luminance could not be achieved. Moreover, the grating grooves parallel with the longitudinal direction of the light source were formed to have one and the same sectional shape so as to cross the light guide plate. Hence, in the case of the light guide plate provided with the grating grooves formed therein, it was difficult to obtain uniform luminance over the whole surface of the light guide plate. This was because it was difficult to adjust luminance in the direction parallel with the longitudinal direction of the light source though it was possible to adjust the number and depth of the grating grooves in the direction perpendicular to the longitudinal direction of the light source. Hence, luminance at edge portions of the light guide plate in the direction parallel with the longitudinal direction of the light source became lower than that at a center portion of the light guide plate. There was a problem that it was difficult to make luminance uniform over the whole panel. Moreover, in the case of the light guide plate provided with the grating grooves formed therein, the liquid crystal cell array itself was ununiform in in-plane transmittance. Hence, if the ununiformity of in-plane transmittance was required to be corrected on the light guide plate side, the distribution of back-lighting luminance in the light guide plate provided with the grating grooves formed therein could not be made ununiform intentionally to correct the ununiformity of in-plane transmittance of the liquid crystal cell array.
Moreover, the light guide plate provided with the grating grooves formed therein was produced by injection molding. The production of a mold for molding the light guide plate or the production of a master stamper for producing a stamper for molding the light guide plate was performed by forming the grating grooves one by one by a mechanical cutting operation. Hence, a great deal of time and labor was taken for the production of the mold or master stamper. As a result, this was a barrier to reduction of the cost of the light guide plate.
Further, in the front-lighting type liquid crystal display device, the required characteristics of the light guide plate for front-lighting are as follows.
(1) The haze (turbidity, cloudiness) of the light guide plate is low.
(2) The surface reflectance is low.
(3) The intensity of light exiting from the upper surface in FIG. 4 is small.
(4) The intensity of the vertical component of light exiting from the lower surface in FIG. 4 with respect to the light exiting direction is large.
In the light guide plate having the light scattering layer of the printed light-diffusing substances or in the light guide plate provided with the grating grooves formed therein, however, it was difficult to obtain the light guide plate satisfying the aforementioned characteristics simultaneously. Moreover, in the front-lighting type liquid crystal display device using the light guide plate having the light scattering layer of the printed light-diffusing substances or using the light guide plate provided with the grating grooves formed therein, there was the same problem as that in the aforementioned back-lighting type liquid crystal display device.
Therefore, a technical theme of the present invention is to solve the problems in the background art. An object of the present invention is to provide a light guide plate which can achieve enhancement of back-lighting or front-lighting luminance without increase of luminance of a light source, and a liquid crystal display device using such a light guide plate.
To achieve the foregoing object, the liquid crystal display device according to the present invention uses a light guide plate provided with a plurality of concave small dots so that the angle of light led into the light guide plate from a light source disposed on a side edge face of the light guide plate is changed efficiently to move the light to a light emitting surface of the light guide plate so that the light exits from the light emitting surface toward a liquid crystal cell array. The shape, shape arrangement, size, distribution, etc. of the concave small dots formed for changing the light travelling direction are made proper. For example, each of the concave small dots is shaped like a triangle with a sectional inclination angle of from 50 to 60xc2x0 in sectional view and shaped like an approximate rectangle or square in plan view in a direction perpendicular to a surface of the light guide plate and disposed so that a long side of the approximate rectangle or a side of the approximate square is approximately parallel with the longitudinal direction of the light source.
According to the aforementioned configuration, in the case of back-lighting, the quantity of light exiting from the lower surface of the light guide plate toward a reflection sheet is reduced while the quantity of light exiting from the upper surface of the light guide plate toward the liquid crystal cell array is increased. Moreover, the vertical component of the light exiting from the upper surface of the light guide plate toward the liquid crystal cell array is increased. Hence, the luminance of the liquid crystal display device can be enhanced without increase of luminance of the light source. Moreover, the luminance distribution of the liquid crystal display device can be made proper. Hence, visibility can be enhanced. On the other hand, in the case of front-lighting, the quantity of light exiting from the light guide plate directly toward an observer is reduced while light exiting from the light guide plate toward the liquid crystal cell array is increased. Moreover, the quantity of the vertical component of the light exiting from the light guide plate toward the liquid crystal cell array is increased. Hence, the luminance of the liquid crystal display device can be enhanced without increase of luminance of the light source. Moreover, the luminance distribution of the liquid crystal display device can be made proper. Hence, visibility can be enhanced