Liquid crystal display device

Abstract: A liquid crystal display device comprises a liquid crystal display panel (10), a light source (111) for illuminating a back surface of the liquid crystal panel, a light guide (13) for transmitting a light emitted by the light source to the back surface of the liquid crystal panel, a holding member (112,113) for accommodating and fixing the light source at one end portion thereof and the light guide so that a surface of the light guide faces the back surface of the liquid crystal display panel, and a frame member (12) for attaching the liquid crystal panel to the holding member so as not to disturb transmission of a heat emitting from the light source to the air through the holding member. Thus, transfer of the radiated heat to the frame member 12 through the holding member can be prevented. As a result, the temperature difference which occurs in the liquid crystal display panel 10 can be reduced, and occurence of irregular display density can be prevented and the display quality can be improved.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device for a laptop or note book type personal computer or word processor, and more particularly, to a liquid crystal display device having a uniform display surface and an improved display quality. 2. Description of the Related Art Figs. 1 and 2 show a conventional backlighted liquid crystal display device. In the figures, reference symbol 1 denotes a liquid crystal display plate; 2, an illuminating device (a back light); 21, a light source, such as a cold cathode tube (CCFT); 3, a light guide tube made of a transparent material; 4, a chassis made of a plastic; 22 and 23, plastic holding members for holding the light source 21; and 5, a metal frame (a metal bezel) for covering the four sides of the entire device. In the conventional liquid crystal display device, the liquid crystal display plate 1 and the light source 21 are covered by the single frame member 5, and transfer of heat occurs between the liquid crystal display plate 1 and the illumination device 2. Thus, when the light source 21 is turned on, the generated heat transfers through or stays inside the frame member 5, increasing the temperature of the frame member 5. Such a heat reaches the liquid crystal display plate 1. The liquid crystal display plate 1 is characterized in that the threshold voltage thereof varies due to a fine temperature difference. Thus, when a temperature difference occurs between the portion of the liquid crystal display plate 1 near the light source 21 and the portion thereof which is located at a distance from the light source 21, the liquid crystal display plate 1 has an irregular display density and hence the display quality thereof deteriorates. A liquid crystal display device of the above-described type is known from JP-A-3-59588. SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal display device which can prevent transmission of heat emitted from a light source to a liquid crystal display plate and which improves the display quality of the liquid crystal display plate. The present invention provides the liquid crystal display device defined by claim 1, whose preamble reflects the state of the art according to US-A-4 909 604. The sub-claims 2 and 3 are directed to preferred embodiments of the invention. According to the invention, the frame member is disposed separately from the light source so that it can expose the holding members. Consequently, the heat from the light source dissipates substantially into the air from the holding members, and the amount of heat transferred from the light source to the liquid crystal display panel can be greatly reduced. As a result, irregular display density of the liquid crystal display panel can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing the entirety of a conventional liquid crystal display device; Fig. 2 is a cross-sectional view of part of the conventional liquid crystal display device of Fig. 1; Fig. 3 is a perspective view showing the entirety of a liquid crystal display device of an embodiment of the present invention; Fig. 4 is a cross-sectional view of the essential parts of the liquid crystal display device shown in Fig. 3; and Fig. 5 is a plan view of the liquid crystal display device shown in Fig. 3. DESCRIPTION OF THE PREFERRED EMBODIMENT Description will now be made of one preferred embodiment of the present invention. Fig. 3 is a perspective view of the entire device, and Fig. 4 is a cross-sectional view of the essential parts of a liquid crystal display device according to the present invention. Fig. 5 is a plan view of Fig. 3. As shown in Figs. 3 through 5, a liquid crystal display device includes a liquid crystal display panel 10, an illuminating device 11 for illuminating the liquid crystal display panel 10 from behind, and a frame member 12 (a metal bezel) for fixing the liquid crystal display panel 10 at the front of the illuminating device 11. The illuminating device 11 includes a light guide plate 13 disposed at the rear of and parallel to the liquid crystal display panel 10, a light source 111 disposed at one end portion of the light guide plate 13, and a holding member 112,113 (a plastic chassis) for holding the light source 111 and the light guide plate 13. The light guide plate 13 is made of a flat plate-shaped acrylic resin. The thickness of the light guide plate 13 is about 2.0 mm, and the length thereof is greater than a display region 101 of the liquid crystal display panel 10. The light source 111 is a straight cold cathode tube (CCFT) consisting of a light emitter, such as a filament, and an outer tube which covers the light emitter. The diameter and length of the straight cold cathode tube are. for example, 4.1 mm and 164 mm, respectively. The power consumption of the straight cold cathode tube is 2.0 W. The holding member consists of a front holding member 112 for holding the light source 111 from the front side thereof, and a rear holding member 113 for holding the light source 111 from the rear side thereof. Both of the front and rear holding members 112 and 113 are made of a plastic which does not transmit light therethrough and which has a low coefficient of heat transfer, such as polycarbonate or ABS resin. The front and rear holding members 112 and 113 are closely fixed to each other at the end portions thereof located on the outer side of the light source 111 by means of screws, whereby leakage of the light from the light source 111 is prevented. The front holding member 112 is disposed only on the light source side of the light guide plate 13. The rear holding plate 113 is disposed over the entire rear surface of the light guide plate 13 and also serves as a reflection sheet which reflects the light directed in a rearward direction in the light guide plate 13 toward the liquid crystal display panel 10. The holding member 112 has a shoulder portion 112a for positioning the frame member 12 near the display region 101 of the liquid crystal display panel 10. The contact area between the shoulder portion 112a and the frame member 12 is minimized with the heat separation taken into consideration. As shown in Figs. 3 through 5, the frame member 12 has a picture frame-like shape when viewed from above so that the illuminated surface can be displayed therethrough. The outer edge portion of the frame member 12 located near the light source 111 has an L-shaped cross-section so that it can be engaged with the shoulder portion 112a. The outer edge portion thereof other than that located near the light source 111 covers the end surface of the light guide plate 13, whereby entry of disturbing light is prevented. Fixing claws (not shown) may be provided on the outer edge portion of the frame member 12 other than that located near the light source 111 so as to fix the frame member 12 to the liquid crystal display panel or illuminating device. In the liquid crystal display device arranged in the manner described above, the light from the light source 111 enters into the light guide plate 13 from the end surface thereof and is directed inwardly. The inwardly directed light is reflected in the forward direction by the rear holding plate 113 and thereby illuminates the rear surface of the liquid crystal display panel 10. At that time, the temperature of the holding member increases due to the heat generated by the light source 111. However, in this embodiment as shown in Fig. 4, since the frame member 12 is separated from the light source 111 so that it exposes the holding member 112,113, most of the heat from the light source 111 does not stay inside the frame member 12 but is radiated in the air from the holding member. The holding member 112,113 is made of a material having a low coefficient of heat transfer. Thus, transfer of the radiated heat to the frame member 12 through the holding member can be prevented. As a result, the temperature difference which occurs in the liquid crystal display panel 10 can be reduced, and occurrence of irregular display density can be prevented and the display quality can be improved. Specifically, the temperature of a corner portion A (see Fig. 5) of the liquid crystal display panel 10 which is located close to the light source 111 and which has the highest temperature and that of a central portion B located farthest from the light source 111 and having the lowest temperature were measured, and the measured temperatures were compared with those of the conventional liquid crystal display device shown in Fig. 1. The measurements were conducted on the surface of the liquid crystal display panel 10 one hour after the light source 111 had been turned on. Table 1 shows the results of the measurements. As shown in Table 1, a temperature difference is reduced in the present embodiment when compared with that in the conventional device. No display irregularity was visually observed. Tube current Surface temperature Portion A Surface temperature Portion B Temperature difference between A-B Comparative example 5 mA 29.8°C 26.5°C 3.3°C Present embodiment 27.8°C 26.5°C 1.3°C Comparative example 6.5 mA 31.5°C 27°C 4.5°C Present embodiment 28.5°C 26.8°C 1.7°C The invention may be embodied in other specific forms without departure from its scope as defined by the claims. For example, although the light source 111 is disposed only at one end portion of the light guide plate 13 in the above embodiment, it may be disposed at each of the two end portions thereof.

Claims: A liquid crystal display device, comprising: a liquid crystal display panel (10); a light source (111) disposed alongside a portion of the periphery of said display panel; a light guide plate (13) disposed parallel to said display panel and adapted to transmit light from said light source so as to backlight said display panel; holding means (112,113) arranged to support said light source and light guide plate; and a frame member (12) attaching said display panel at its periphery to said holding means, characterized in that said holding means comprises holding members (112,113) each made of a material having a low thermal conductivity; and said frame member (12) and said holding members are dimensioned and arranged such that said light source (111) and part of said holding members are located outside said frame member, so as to inhibit the trapping of heat produced by said light source. A liquid crystal display device according to claim 1, wherein one of said holding members (112) includes an engaging portion (112a) located adjacent to the surface of said light guide plate (13) which faces the back surface of said liquid crystal display panel (10), said engaging portion (112a) engaging said frame member (12). A liquid crystal display device according to claim 1 or claim 2, wherein said holding members (112,113) are made of a plastic having a light-shading property.

Inventors: OKAJIMA HIROYUKI, URATANI SHINICHI, OKAJIMA, HIROYUKI, URATANI, SHINICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G02F1
IPC codes: G02F1