Backlighting device

Abstract: A light reflector covers a linear light source that is disposed adjacent to a light incident end face of a light conducting plate. The interval between the end portions of the light reflector adjacent to the light incident end face of the light conducting plate is made smaller than the thickness of the light incident end face. Alternatively, the light-exit-side portion of the light reflector located between the linear light source and the light incident end face of the light conducting plate is disposed within the thickness of the light incident end face.

Description: BACKGROUND OF THE INVENTION The present invention relates to a backlighting device for illuminating a transmission-type or semi-transmission-type panel from its back side. In recent years, thin and highly legible liquid crystal display devices having a backlighting device are used as display devices for lap-top-type or book-type word processors, computers, or the like. Edge-light-type backlighting devices are commonly used for that purpose in which devices a linear light source such as a fluorescent tube is disposed adjacent to an end face of a transparent light conducting plate. In many of the edge-light-type backlighting devices, a light diffusive substance is partially coated on one major surface of the light conducting plate and this major surface is entirely covered with a reflecting plate. In particular, in recent years, with improved performance of word processors and personal computers, display devices are required to be more compact and to be further improved in legibility. In the backlighting device, it is attempted to reduce its overall size by reducing the maximum external dimensions of the light conducting plate with respect to the light-emitting area of the backlighting device which area corresponds to the display area of a liquid crystal display panel. However, in the backlighting device in which the area (maximum dimensions) of the light conducting plate is reduced with respect to the light-emitting area of the backlighting device, a high-brightness portion (bright line) appears near the end face of the light conducting plate adjacent to the linear light source so as to be parallel with the end face, and deteriorates the luminance uniformity in the light-emitting area, i.e., deteriorates the legibility. To solve the above problem, it has been proposed to dispose a light absorptive dark or black tape or the like at the position where a bright line appears. However, other problems then occur. For example, the power-luminance conversion efficiency is lowered due to the light absorption characteristic of the dark or black tape or the like. EP-A-442529 discloses a light plate (1), and a linear light source (4) surrounded by a reflector (5). However there is no statement of the relationship between the slot in the reflector (5) and the dimensions of the light plate. SUMMARY OF THE INVENTION An object of the present invention is to provide a backlighting device in which the external dimensions can be made as small as possible with respect to the effective light-emitting area, and which can provide superior legibility. The inventors have made various investigations into the relationship between the structure around the end face of the light conducting plate adjacent to the linear light source and the luminance distribution of the light exit surface of the light conducting plate, and have obtained the knowledge that the bright line occurs in the following manner in the edge-light-type backlighting device. Light rays, which should undergo repeated total reflection in the light conducting plate, are diffuse-reflected by adhesive layers, such as double-sided adhesive tapes, which are used to bond, to the light conducting plate, the end portions of a light reflector that covers the linear light source. Even in the case where adhesive layers such as double-sided adhesive tapes are not used, if gaps are formed between the light conducting plate and the end portions of the light reflector, light rays reflected by the end portions of the light reflector enter the light conducting plate from its surfaces (top and bottom surfaces) other than the end face and do not satisfy the condition for total reflection. The inventors have found the above-described object of the invention can be attained by properly adapting the contact state (contact portions) of the end portions of the light reflector and the end portions of the light conducting plate. According to one form of the invention, a backlighting device for a panel comprises: a light conducting plate having a central longitudinal plane and made of transparent material, and affording a major exit surface from which, in use, light is output from the device; a linear light source disposed adjacent to an end face of the light conducting plate; and a light reflector extending from the exit surface of the plate at one end thereof around behind the light source to the other face of the plate at the same end thereof and arranged to reflect light from the light source into the plate, characterised in that the separation (a) of the reflective surfaces of the light reflector at the end of the plate is less than to the thickness (b) of the plate at the said end, and in that end portions of the light reflector adjacent to the end face of the light conducting plate are disposed within the thickness of the end face. According to another form of the invention a backlighting device for a panel comprises: a light conducting plate having a central longitudinal plane and made of transparent material, and affording a major exit surface from which, in use, light is output from the device; a linear light source disposed adjacent to an end face of the light conducting plate; and a light reflector extending from the exit surface of the plate at one end thereof around behind the light source to the other face of the plate at the same end thereof and arranged to reflect light from the light source into the plate, characterised in that the separation (a) of the reflective surfaces of the light reflector at the end of the plate is less than or equal to the thickness (b) of the plate at the said end, and in that at least one of the end portions of the light reflector has a step portion that engages the light conducting plate. Preferably, a light-exit-side portion of the light reflector located between the linear light source and the end face of the light conducting plate is disposed within the thickness of the end face. Further, the light reflector may be formed of a light reflecting sheet, in which case at least part of a light-exit-side end portion of the light reflecting sheet is bonded to a portion of the light exit major surface of the light conducting plate close to the end face. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be hereinafter described in detail with reference to the accompanying drawings. Fig. 1 is a perspective view of a conventional backlighting device; Fig. 2 is a sectional view a conventional backlighting device; Figs. 3 and 4 are partial sectional views showing a light source and a portion close thereto of two conventional backlighting devices; Fig. 5 is a sectional view of a backlighting device according to an embodiment of the present invention; and Figs. 6-8 are sectional views of backlighting devices according to different embodiments of the invention each of which views shows a portion of the backlighting device including a light source. DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Fig. 1, edge-light-type backlighting devices commonly used for display devices for computers have a linear light source 4 such as a fluorescent tube disposed adjacent to an end face of a transparent light conducting plate 1. In many of the edge-light-type backlighting devices, as shown in Fig. 2, a light diffusive substance 6 is partially coated on one major surface of the light conducting plate 1, and this major surface is entirely covered with a reflecting plate 3. However, in the backlighting device in which the area (maximum dimensions) of the light conducting plate 1 is reduced with respect to the light-emitting area of the backlighting device, a high-brightness portion (bright line) appears near the end face of the light conducting plate 1 adjacent to the linear light source 4 so as to be parallel with the end face, and deteriorates the luminance uniformity in the light-emitting area, i.e., deteriorates the legibility, as illustrated in Figure 3. As shown in Fig. 3, light rays, which should undergo repeated total reflection in the light conducting plate 1, are diffuse-reflected by adhesive layers 7, such as double-sided adhesive tapes, which are used to bond, to the light conducting plate 1, the end portions of a light reflector 5 that covers the linear light source 4. Figure 4 shows what can happen even in the case where adhesive layers such as double-sided adhesive tapes are not used. Thus if gaps are formed between the light conducting plate 1 and the end portions of the light reflector 5 light rays reflected by the end portions of the light reflector 5 can enter the light conducting plate 1 from its surfaces (top and bottom surfaces) other than the end face and then do not satisfy the condition for total reflection. Fig. 5 is a sectional view of a backlighting device according to a first embodiment of the invention. In Fig. 5, a light conducting plate 1 may be made of any material which transmits light efficiently, for instance, quartz, glass, or a transparent natural or synthetic resin such as an acrylic resin. A light diffusing sheet 2 transmits the light output from the surface of the light conducting plate 1 while scattering it. In the invention, one or a plurality of light diffusing sheets are used. The back major surface of the light conducting plate 1 is given a function of diffusing the light that is input to the light conducting plate 1. The light diffusing function is provided, for instance, by forming light diffusing elements 6 on the back major surface. The light diffusing elements 6 may be made of a light scattering substance such as TiO 2 , BaSO 4 , SiO 2 . Preferably, a paint containing a pigment having a large diffuse reflectance, printing ink, or the like is formed in dots on the surface of the light conducting plate 1 by screen printing, for instance. Alternatively, the surface itself of the light conducting plate 1 may be provided with the light diffusing function by roughening or coarsening the surface, forming small holes or small protrusions, or by some other means. The light diffusing elements 6 are preferably formed in dots, lines, or some other pattern or form so that the light diffusiveness per unit area of the light conducting plate 1 gradually increases as one moves away from the light source 4. Alternatively, the light diffusing function may provided in the inside of the light conducting plate 1 by dispersing therein, for instance, a large number of minute particles whose refractive index is larger than that of the light conducting plate 1. A light reflecting plate or sheet 3 is so disposed as to cover almost the entire back major surface 26 of the light conducting plate 1 opposite to the light exit surface 25. The light reflecting plate or sheet 3 may be made of any material which reflects light. Where it is required to be specular, it is preferably made of metal, e.g. silver, aluminum, platinum, nickel, chromium, or the like. Preferably, it is composed of a metal plate or a plastic film base of, for instance, polyester and a coat of metal, e.g. silver, aluminum, or the like formed thereon by evaporation or sputtering. Where the light reflecting plate or sheet 3 is required to be diffusive, it may be made of any material which diffuse-reflects the incident light. Examples of such a material are resins such as polyester mixed with a light diffusive substance (for instance, TiO 2 , BaSO 4 and SiO 2 ), a resin such as polyester that is foamed to provide light diffusiveness, and a plate made of metal, for instance, aluminum coated with the above light diffusive substance. A linear light source (rod-like light source) 4 is disposed adjacent to at least one end face of the light conducting plate 1. Preferably, the linear light source 1 is so disposed that its central axis is substantially parallel with the end face of the light conducting plate 1 to allow the light to enter the light conducting plate 1 through the end face, and the portion of the surface of the linear light source 4 other than the portion opposed to the end face of the light conducting plate 1 is surrounded or covered by a light reflector 5. In the invention, the light reflector 5, which covers the linear light source 4, may be of any material as long as the portion of its surface opposed to the linear light source 4 is capable of light reflection. Where the light reflector 5 is a specular reflecting plate or sheet, it may be made of a metal such as silver, aluminum, platinum, nickel, chromium, or the like. Preferably, it is composed of a metal plate or a plastic film base of, for instance, polyester and a coat of metal e.g. silver, aluminum, or the like formed thereon by evaporation or sputtering. Where the light reflector 5 is a diffuse-reflecting plate or sheet, it may be made of any material which diffuse-reflects the incident light. Examples of such materials are resins such as polyester mixed with a light diffusive substance (for instance, TiO 2 , BaSO 4 and SiO 2 ), a resin such as polyester that is foamed to provide light diffusiveness, and a plate made of metal, for instance, aluminum coated with the above light diffusive substance. Examples of the linear light source 4 are a fluorescent tube, a tungsten incandescent tube, an optical rod, and an LED array. Among those examples, the fluorescent tube is preferable. From the viewpoints of power consumption and uniformity of the luminance distribution in the effective light-emitting area, it is preferred that the length of the uniform light-emitting portion excluding the electrode portions be approximately equal to the length of the adjacent end face of the light conducting plate 1. Fig. 6 shows a second embodiment of the present invention and an example of an arrangement of the end portions of the light reflector 5 and the end face of the light conducting plate 1 adjacent to the linear light source 4, which arrangement is a feature of the invention. As shown in Fig. 6, an internal dimension (a) of at least parts of portions of the light reflector 5 namely the reflective surfaces 11 which portions extend between the light source 4 and the end face of the light conducting plate 1 is smaller than a thickness (b) of the end face of the light conducting plate 1. Even where the end portions of the light reflector 5 is laid on the surfaces of major surfaces of the light conducting plate 1, the advantages of the invention can be obtained if the internal dimension (a) of the light reflector 5 satisfies the above condition. If the internal dimension (a) of the light reflector 5 is smaller than the thickness (b) of the light conducting plate 1, the phenomenon shown in Fig. 4 can be prevented that light- rays reflected by the light reflector 5 enter the light conducting plate 1 through its surfaces (top and bottom surfaces) other than the end face, and take paths that do not satisfy the condition for total reflection in the light conducting plate 1. Thus, the generation of a bright line can be prevented. This second embodiment shown in Fig. 6 is the preferred embodiment of the invention and will be described below in more detail. In Fig. 6, the end faces of the light reflector 5 on the side of the light conducting plate 1 are substantially in contact with the end face of the light conducting plate 1. Backlighting devices of the invention as described above find particular use in the liquid crystal display panels. Further embodiments will now be described. It is preferred that, as shown in Fig. 7, the light reflector 5 covering the linear light source 4 have a step portion 8 that engages the light conducting plate 1. With this configuration, the light reflector 5 and the light conducting plate 1 can stably be connected to each other, improving the ease of operation in mass-production. It is preferred that, as shown in Fig. 8, at least part 35 of the step portion 8 e.g. its surface opposed to the light conducting plate 1 be darkened or blackened so as to absorb light. This configuration is effective in preventing a bright line, because even if there is a gap between the end portion of the light reflector 5 and the light conducting plate 1, the dark or black part 9 of the step portion 8 can absorb light rays traveling through the gap. Where the light reflector 5 is composed of a plurality of plates or sheets, the end portions of the innermost plate or sheet may be so disposed as to satisfy the above-described condition, i.e. provide a step configuration to provide the same advantages as in the case where the light reflector 5 has the step portion 8. In the invention, it is preferred that the ratio a/b of the internal dimension a (see Fig. 8) namely the distance between the reflective surfaces 11 of the light reflector 5 on the side of the light conducting plate 1 to the thickness b of the end portion of the light conducting plate 1 be 0.8 to 1 exclusive. To improve the efficiency of light utilization of the backlighting device, it is preferred that the ratio a/b be 0.9 to 1 exclusive or 0.9 to 0.975. To efficiently introduce light into the light conducting plate 1, it is preferred that both end portions of the light reflector 5 be located within the thickness of the end portion of the light conducting plate 1. According to another preferable configuration of the invention, the portion of the light reflector 5 adjacent to the light exit surface 25 of the light conducting plate 1 is located within the thickness of the light incident end face of the light conducting plate 1. No particular limitation is imposed on the other end portion of the light reflector 5 i.e. that juxtaposed to the end face adjacent the surface 26 of the plate 1. More specifically, where the light reflector 5 has a step portion 12 as described above, the step portion 12 of the end portion of the light reflector 5 adjacent to the light exit surface of the light conducting plate 1 is opposed to an edge 13 that is formed by the light incident end face and the light exit surface of the light conducting plate 1. The edge 13 may be located at the junction of the end face 20 of the plate 1 and the exit surface 25 of the plate. With this configuration, the phenomenon shown in Fig. 4 can be prevented that light rays reflected by the light reflector 5 enter the light conducting plate 1 through a surface other than the end face, and take paths that do not satisfy the condition for total reflection in the light conducting plate 1. Thus, a bright line can effectively be prevented from appearing on the top surface of the light conducting plate 1 near the light source 4. According to a further preferable configuration of the invention, the step portion 12 of the light reflector 1 is substantially in contact with the edge 13 of the light conducting plate 1. With this configuration, the light emitted from the linear light source 1 can efficiently be introduced into the light conducting plate 1 through the end face. In the following, the invention will be described in further detail by way of Examples and a Comparative Example. Example 1 As shown in Fig. 5, a cold cathode fluorescent tube 4 of 3 mm in diameter and 180 mm in length (Harrison Electric Co., Ltd.) was disposed adjacent to the shorter-side end face of a 4-mm-thick rectangular acrylic plate 1 (205 mm x 160 mm; Delaglass A produced by Asahi Chemical Industry Co., Ltd.). As shown in Fig. 6, the fluorescent tube 4 was surrounded by a light reflector 5 in which a silver film (produced by Reiko Co., Ltd.) was provided on a polycarbonate member so that the light output from a slit (width: 3.9 mm) (i.e. a = 3.9 mm, b = 4.0 mm; a:b = 0.975:1) of the silver film which slit is opposed to the end face of the light conducting plate 1 is that light enters the light conducting plate 1 through the end face. On the other hand, a pattern 6 of circular dots having a pitch of 1 mm was screen-printed on the back major surface 26 of the light conducting plate 1 with ink containing a light diffusive substance (titania). A screen block copy was produced by CAD under the following conditions. That is, a pattern was drawn so that the coating ratio of the light diffusive substance had the minimum value of 20% at a position close to the linear light source 4 and the maximum value of 95% at a position farthest from the light source 4, and gradually increased from the minimum position to the maximum position. A 0.13-mm-thick white diffuse-reflecting plate 3 made of polyester (Merinex 329 produced by ICI) was so disposed as to cover the entire surface 26 of the light conducting plate 1 which is coated with the light diffusive substance. A 0.18-mm-thick light diffusing plate 2 (8B36 produced by GE) made of polycarbonate was so disposed as to cover the almost entire light exit surface 25 of the light conducting plate 1 with its coarser surface located on the side opposite to the light conducting plate 1. The cold cathode fluorescent tube 4 was driven by an inverter with an AC voltage of 30 kHz and a constant current (tube current: 6 mA). The surface luminance was measured under these conditions with a luminance meter (BM-8 produced by Topcon Corp.). An average luminance of 99 points (uniformly distributed) in the effective light-emitting area was 1,200 cd/m 2 . Almost no abnormal light-emitting portion (bright line) was observed near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4. Example 2 An average luminance as measured using the same devices under the same conditions as in Example 1 except that the end portions of the light reflector 5 had a step portion 8 (see Fig. 7) was 1,200 cd/m 2 . Almost no abnormal light-emitting portion (bright line) was observed near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4. It became easier, in terms of mechanism, to keep the positional relationship between the silver film and the light conducting plate 1. Example 3 An average luminance as measured using the same devices under the same conditions as in Example 2 except that the parts 9 of the surfaces (opposed to the light conducting plate 1) of the step portions 8 of the light reflector 5 which engage the light conducting plate 1 are darkened or blackened (see Fig. 8) was 1,200 cd/m 2 . Almost no abnormal light-emitting portion (bright line) was observed near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4. It became easier, in terms of mechanism, to keep the positional relationship between the silver film and the light conducting plate 1. Further, even if the dimensions of the light reflector 5 had small variations, no bright line was observed. Comparative Example 1 An average luminance as measured using the same devices under the same conditions as in Example 1 except that the end portions of the light reflector 5 were so disposed that the light output from a slit (width: 4.1 mm) (i.e. a = 4.1, b = 4.0; a:b = 1.025:1) of the silver film entered the light conducting plate 1 through the end face (see Fig. 4) was 1,100 cd/m 2 . An abnormal light-emitting portion (bright line) was observed near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4. The maximum luminance at that portion was 2,100 cd/m 2 . Example 4 An average luminance as measured using the same device under the same conditions as in Example 4 except that one end portion of a light reflecting sheet 16 (0.05-mm-thick silver film) was bonded to the surface of the light conducting plate 1 adjacent to the light incident end face through a 0.06-mm-thick transparent adhesive tape 14 was 1,200 cd/m 2 . Almost no abnormal light-emitting portion (bright line) was observed near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4. It became easier, in terms of mechanism, to keep the positional relationship between the silver film and the light conducting plate 1. Example 5 An average luminance as measured using the same device under the same conditions as in Example 4 except that the adhesive tape 14 was darkened or blackened was 1,100 cd/m 2 . Almost no abnormal light-emitting portion (bright line) was observed near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4. Example 6 An average luminance as measured using the same device under the same conditions as in Example 4 except that Ag was evaporated on the end face of the adhesive tape 14 to make it reflective was 1,220 cd/m 2 . The amount of abnormal light-emitting portion (bright line) near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4 was less than in the case of Example 4. Example 7 An average luminance as measured using the same devices under the same conditions as in Example 4 except that a black paint was applied to the end face of the adhesive tape 14 to make it absorptive was 1,200 cd/m 2 . The amount of abnormal light-emitting portion (bright line) near the end face of the light conducting plate 1 adjacent to the fluorescent tube 4 was less than in the case of Example 4. According to the invention, as described above, a compact backlighting device can be provided in which the external dimensions can be made as small as possible with respect to the effective light-emitting area, and which can provide superior legibility.

Claims: A backlighting device for a panel, comprising: a light conducting plate (1) having a central longitudinal plane (32) and made of transparent material, and affording a major exit surface (25) from which, in use, light is output from the device; a linear light source (4) disposed adjacent to an end face of the light conducting plate; and a light reflector (5) extending from the exit surface (25) of the plate at one end thereof around behind the light source to the other face (26) of the plate at the same end thereof and arranged to reflect light from the light source into the plate (1), characterised in that the separation (a) of the reflective surfaces (11) of the light reflector (5) at the end of the plate is less than the thickness (b) of the plate at the said end, and in that end portions of the light reflector adjacent to the end face of the light conducting plate are disposed within the thickness of the end face. A backlighting device for a panel, comprising: a light conducting plate (1) having a central longitudinal plane (32) and made of transparent material, and affording a major exit surface (25) from which, in use, light is output from the device; a linear light source (4) disposed adjacent to an end face of the light conducting plate; and a light reflector (5) extending from the exit surface (25) of the plate at one end thereof around behind the light source to the other face (26) of the plate at the same end thereof and arranged to reflect light from the light source into the plate (1), characterised in that the separation (a) of the reflective surfaces (11) of the light reflector (5) at the end of the plate is less than the thickness (b) of the plate at the said end, and in that at least one of the end portions of the light reflector has a step portion (12, 22, 28) that engages the light conducting plate. A backlighting device as claimed in claim 2 characterised in that a light absorbing portion (35) is provided at least at part of a surface of the step portion of the light reflector which surface is opposed to the light conducting plate. A backlighting device as claimed in any one of claims 1 to 3 characterised in that the light reflector comprises a light reflecting sheet, and in that at least part (16) of the end portion of the light reflecting sheet (5) which is juxtaposed to the exit surface (25) of the plate (1) is bonded (14) to a portion of the said exit surface of the light conducting plate close to the end face thereof. A backlighting device as claimed in any one of claims 1 to 4 characterised in that a diffuser sheet (2) is disposed over the exit surface (25) of the light conducting plate (1) and is positioned externally of the light reflector (5).

Inventors: KASHIMA KEIJI, YOSHIDA NAOKI, KASHIMA, KEIJI, YOSHIDA, NAOKI
Applicants: TOSOH CORP, TOSOH CORPORATION
Application year: 1995
Main classification: F21V33
IPC codes: F21V8, G02F1, G02B6