LGE_000580
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`LG Electronics Ex. 1012
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`EP 0 878 720 A1
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`Description
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`BACKGROUND OF THE INVENTION
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`1. Field of the Invention
`
`The present invention relates to an illumination device and display device using this wherein a light-guide plate is
`arranged at the front face of an illuminated object and this illuminated object is two-dimensionally surface-illuminated,
`and more particularly relates to an illumination device and display device using this wherein properties such as illumi-
`nation function, recognisability, contrast and energy-saving that are based on the optical diffusion characteristic of this
`light-guide plate are greatly improved.
`
`2. Description of the Related Art
`
`Conventionally, various types of illumination device are employed that exhibit the function of surface illumination in
`respect of a display device that requires planar illumination, such as a liquid crystal display device.
`For example there is known a display device that is arranged at the back face of the object to be illuminated, such
`as a liquid crystal display panel; normally such an illumination device is arranged to be constantly lit. An illumination
`device is also known mounted on a liquid crystal display device having a reflective function. In the case of such an illu-
`mination device, a reflective plate is arranged at the back face of the liquid crystal display panel and the device is
`employed by illumination provided by external light. Furthermore, an illumination device is also known that is arranged
`together with a semi—transparent reflective plate at the back face of a liquid crystal display panel; this illumination device
`is used for reflection when the environment is brighter and to provide back lighting illumination when the environment
`is dark (for such devices, for example Early Japanese Patent Publication No. SHO.57-049271, Early Japanese Patent
`Publication No. SHO.57-054926 and Early Japanese Patent Publication No. SHO.58-095780 may be referred to).
`However, conventional illumination devices having solely an illumination function suffered from the problem that
`power consumption in order to keep the light source constantly lit was large; for example, they could not be used over
`a long time to provide illumination for portable equipment. Also, in the case where a conventional display device having
`solely a reflective function was mounted on a liquid crystal display device or the like, there was the problem that contrast
`of the display screen was low, making it impossible to employ them in a dark environment. Furthermore, illumination
`devices that are employed with a semi—transparent reflective plate inevitably have the problem that the display is dark
`both when used with reflection and when used with back lit illumination; this technique represents an unsatisfactory
`compromise and has not become popular at all.
`In these circumstances there has recently been proposed for example in Early Japanese Patent Publication No.
`H.6-324331 an illumination device that is arranged at the front face of a display device such as a liquid crystal display
`device. The illumination device of this proposal is incorporated in a thin liquid crystal display device and has the object
`of ensuring high contrast of illumination both when lit and when not lit. Specifically, a thin illumination device is arranged
`at the top face (front face) of a liquid crystal display and a reflective plate is arranged at the back face of the liquid crystal
`display. The illumination device comprises a light-guide plate and a light source that is arranged at the end face of this
`light-guide plate or in its vicinity. At the optical output face of the light-guide plate, there is formed an indented shape
`comprising faces practically parallel with this face and faces approximately perpendicular thereto. The indented shape
`may be formed for example of a plurality of ribs or projections of cylindrical or prismatic shape.
`However, the illumination devices arranged at the front face of these publications are adapted to light sources of
`rod or linear shape. For such light sources, fluorescent tubes, which are of high light-emitting efficiency are generally
`employed; however, fluorescent tubes need power of at least a certain level and suffer from the problem that their power
`consumption cannot be reduced below this. Also,
`if point light sources such as LEDs or electric light bulbs were
`employed, there was the problem that, since the lines of intersection at the root sections forming the ribs or prismatic
`projections and optical output faces are straight lines, the quality of illumination tends to be adversely affected by regu-
`lar reflection. Furthermore, in the case of point light sources, there was the problem that unevenness of brightness
`could not be eliminated by one-dimensional distribution control of the pattern of the projections.
`Also, illumination devices arranged at the front face in this way were subject to the problem of being easily affected
`by external damage to the light-guide plate, causing light to be emitted by dispersive reflection of optical flux from such
`damaged portions, lowering the contrast of the illuminated object such as the liquid crystal display when lit.
`Also, with such illumination devices of the type that were arranged at the front face, since the light source is
`arranged at the end face of the light-guide plate, a space needs to be provided at the end of the light-guide plate suffi-
`cient to screen the light source from the observer; if they are employed as illumination for a liquid crystal display or the
`like, a border is therefore necessary around the periphery of the display area. This resulted in waste of space and
`imposed considerable design limitations.
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`EP 0 878 720 A1
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`In one aspect, the present invention was made in order to solve the various problems of a conventional illumination
`device as described above.
`
`An object of the present invention is to provide an illumination device using a point light source of low power con-
`sumption and high quality, and a display device such as a liquid crystal display device using this.
`Also, a further object of the present invention is to provide an illumination device of low power consumption and
`high quality by employing as light source a light emitting diode (LED), and a display device such as a liquid crystal dis-
`play device using this. Yet a further object of the present invention is to provide an illumination device whereby illumina-
`tion can be achieved without loss of reflective function and a display device such as a notice board device or liquid
`crystal display device using this, and a device such as an electronic device or mobile telephone using this liquid crystal
`display device.
`Yet a further object of the present invention is to provide an illumination device with little deterioration of illumination
`function by low-cost, convenient means, and a display device such as a liquid crystal display device with little deterio-
`ration of display quality.
`Yet a further object of the present invention is to provide an illumination device whereby rays of light can be effi-
`ciently directed into the interior of a light-guide plate from a light source positioned remote from the light-guide plate
`end, which is space-saving, and has excellent design characteristics, a display device such as a liquid crystal display
`device, and a device such as an electronic device or mobile telephone using this liquid crystal display device.
`Furthermore, from the point of view of display devices in which an illumination device is mounted, in view of con-
`ventional reflective type liquid crystal display devices for the aforesaid display devices, it is an object to provide various
`types of electronic device such as liquid crystal display devices, portable telephone devices, timepieces, cameras or
`data terminal devices wherein the production of a bright line which is annoying to the observer can be prevented,
`wherein unevenness of brightness can be eliminated, which are of a type in which power consumption can be reduced
`and furthermore which afford an illumination function of high quality.
`Furthermore, conventionally, notice board devices having an illumination function were of a construction in which a
`casing was provided whose front face was covered by transparent glass and wherein a notice was illuminated by
`arranging a light source at the front edge of the notice. Also, they were of a construction in which the person viewing
`them could not directly see the light source due to an optical screening section, also serving as a casing, in front of the
`light source.
`However, conventional notice board devices suffered from the problem that they had to be of sufficient thickness in
`order for the entire notice to be illuminated, and that the difference of illumination was large at locations remote from the
`vicinity of the light source.
`Also, in a further aspect relating to a display device, an object of the present invention is to provide a notice board
`device wherein such problems are solved and which is of small thickness and wherein the uniformity of illuminance is
`high.
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`SUMMARY OF THE INVENTION
`
`In order to solve the aforementioned problems, in one aspect thereof, the present invention provides an illumination
`device arranged at the front face of an illuminated object, comprising a light-guide plate of transparent flat plate form
`formed with point-form optical extraction structures on its surface on at a position facing this surface, and a light source
`arranged facing the end face of this light-guide plate. Suitably the light source is a point light source. Also for example
`the optical extraction structures are distributed relatively sparsely in the vicinity of the point light source and progres-
`sively more densely going away from the point light source. Further, a rod-shaped optical diffuser may be provided
`arranged between the end face of the light-guide plate and the point light source. For example a milky white transparent
`body in which optical diffusing material is dispersed may be employed as the rod-shaped diffuser. Also for example a
`transparent body formed with optical extraction shapes may be employed as the rod-shaped optical diffuser. As an
`example, rib-shaped projections may be provided as the optical extraction structures on the optical output face facing
`the illuminated object of the light-guide plate. Further, pillar-shaped projections may be provided as the optical extrac-
`tion structures on the optical output face facing the illuminated object of the light-guide plate.
`Also, as a further aspect, in a liquid crystal display device comprising a liquid crystal display and an illumination
`device arranged at the front face of this liquid crystal display, the illumination device comprises a light-guide plate form-
`ing a transparent flat plate shape and formed with point-form optical extraction structures on its surface or at a position
`facing this surface, and a point light source arranged facing the end face of this |ight—guide plate.
`Further, as another aspect, there is provided a display device arranged at the front face of an illuminated object
`comprising a light-guide plate formed of transparent plate shape and formed at its surface with projections or concavi-
`ties for optical diffusion having an inclined surface of under about 30° with respect to this surface, and a light source
`arranged opposite the end face of this light-guide plate. In this case, a reflecting member can be arranged adjacent the
`other end face of the light-guide plate apart from the end face where the light source is arranged. Also, a reflecting
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`member may be arranged so as to cover this light source and the end face of the light-guide plate where the light source
`is arranged.
`in an illumination device
`Also, as a further aspect of an illumination device according to the present invention,
`wherein optical flux from a light source is transmitted in one direction practically orthogonal to the plate face direction
`and that is provided with a light-guide plate that delivers this optical flux as illumination from a face in this one direction,
`a sheet-form transparent member is arranged facing the face of the light-guide plate on the opposite side to this one
`direction.
`
`As another aspect there is further provided, in an illumination device arranged at the front face of an illuminated
`object, a light-guide plate formed with optical extraction structures at its surface and constituting a transparent sheet-
`shaped member, and a light emitting diode (LE D) arranged integrally with an end face of this light-guide plate. Suitably,
`the optical extraction structures are distributed relatively sparsely in the vicinity of the light emitting diode and progres-
`sively more densely going away from the light emitting diode. Also, pillar-shaped projections could be provided as the
`optical extraction structures on a face facing the illuminated object of the light-guide plate. Concave shapes or convex-
`shaped projections could be provided as the optical extraction structures on a face opposite to the face facing the illu-
`minated object of the light-guide plate. Also as a further aspect, in an illumination device arranged at the front face of
`an illuminated object, there are provided a light-guide plate formed with optical extraction structures at at least one face
`of the transparent flat plate, a light source arranged at a position remote from the same planar position as this light-
`guide plate, and means for converting that convert the direction of the rays from this light source so that they are guided
`into the optical input face of the light-guide plate.
`Yet further, in an aspect of the present invention relating to a display device, as one mode thereof, in a liquid crystal
`display device of the reflective type wherein at least a front light is arranged at the top face and a polarisation separating
`plate is arranged at the bottom face, this front light comprises a |ight—guide plate comprising a flat transparent plate
`formed with a plurality of point-form optical extraction elements in its surface, and a point light source arranged facing
`the end face of this light-guide plate. For example these optical extraction structures may be distributed relatively
`sparsely in the vicinity of the light source and progressively more densely going away from the light source. Also suitably
`a rod-shaped optical diffuser is provided arranged at the end face of the light-guide plate and the point light source is
`arranged at the end of this rod-shaped optical diffuser. Also suitably pillar-shaped projections are provided as the opti-
`cal extraction structures on a face facing the illuminated object of the light-guide plate. Furthermore, concave shapes
`or convex shapes could be provided as optical extraction structures on the face on the opposite side to the face facing
`the illuminated object of the |ight—guide plate.
`In yet a further aspect of a display device according to the present invention, in a notice board device having an
`illumination function, a transparent plate is arranged at the front face of the notice and a function of planar illumination
`of this notice is conferred on this transparent plate. Features relating to further structure and benefits of the present
`invention will be apparent from the detailed description of the accompanying drawings and the following embodiments.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Fig. 1A and Fig. 1B are a diagrammatic cross-sectional view and perspective view illustrating a first embodiment of
`the present invention;
`Fig. 2A and Fig. 2B are diagrams illustrating a problem of the prior art;
`'
`. 3 is a diagrammatic cross-sectional view of a modification of the first embodiment;
`'
`. 4 is a diagrammatic cross-sectional view of a further modification of the first embodiment;
`'
`. 5 is a diagrammatic cross-sectional view of a further modification of the first embodiment;
`'
`. 6 is a diagrammatic cross-sectional view of a further modification of the first embodiment;
`'
`. 7 is a diagrammatic cross-sectional view of a further modification of the first embodiment;
`'
`. 8 is a plan view showing a second embodiment of the present invention;
`'
`. 9A and Fig. 9B are diagrammatic cross-sectional views of a modification of the second embodiment;
`'
`. 10 is a diagrammatic cross-sectional view showing a third embodiment of the present invention;
`'
`. 11 is a diagram showing a further modification;
`Fig. 12A and Fig. 12B are diagrammatic plan views showing a further modification;
`Fig. 13 is a diagram showing yet a further modification;
`Fig. 14A and Fig. 14B are a diagrammatic cross-sectional view and perspective view showing a fourth embodiment
`of the present invention;
`Fig. 15 is a detail diagram of a convex shape constituting a structural element for extraction of light;
`Fig. 16A to 16D are diagrams of further convex shapes;
`Fig. 17 is a diagram of a further convex shape;
`Fig. 18 is a diagrammatic plan view of a further modification of the fourth embodiment;
`Fig. 19 is a diagrammatic cross-sectional view of a further modification of the fourth embodiment;
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`EP 0 878 720 A1
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`Fig. 20 is a diagrammatic cross-sectional view of a further modification of the fourth embodiment;
`
`Fig. 21 is a diagrammatic cross-sectional view of a further modification of the fourth embodiment;
`Fig. 22 is a diagrammatic cross-sectional view of a further modification of the fourth embodiment;
`Fig. 23A and Fig. 23B are a diagrammatic cross-sectional view and perspective view of a fifth embodiment of the
`present invention;
`Fig. 24 is a detail diagram of a concave shape constituting a structural element for an extraction of light;
`Fig. 25A to 25D are diagrams of a further concave shape;
`Fig. 26 is a diagram of a further concave shape;
`Fig. 27 is a diagrammatic cross-sectional view showing a sixth embodiment of the present invention;
`Fig. 28 is a diagrammatic perspective view of a device illustrating an example of application;
`Fig. 29A and Fig. 29B are diagrammatic cross-sectional views showing a seventh embodiment of the present
`invention;
`Fig. 30A and Fig. 30B are a diagrammatic cross-sectional view and perspective view showing a seventh embodi-
`ment;
`Fig. 31 is a diagrammatic cross-sectional view of a modification of the seventh embodiment;
`Fig. 32A and Fig. 32B are diagrammatic cross-sectional views of a modification of the seventh embodiment;
`Fig. 33A and Fig. 33B are diagrammatic perspective views showing an eighth embodiment of the present invention;
`Fig. 34A is a diagrammatic perspective view of a modification of the eighth embodiment;
`Fig. 34B and Fig. 34C are diagrams of a modification of the eighth embodiment;
`Fig. 35 is a diagrammatic cross-sectional view showing a ninth embodiment of the present invention;
`Fig. 36 is a diagrammatic cross-sectional view of a modification of the ninth embodiment;
`Fig. 37A and Fig. 37B are a diagrammatic cross-sectional view and perspective view showing a tenth embodiment
`of the present invention;
`Fig. 38 is a diagrammatic cross-sectional view of a modification of the tenth embodiment;
`Fig. 39 is a diagrammatic cross-sectional view of a further modification of the tenth embodiment;
`Fig. 40 is a diagrammatic cross-sectional view of a further modification of the tenth embodiment;
`Fig. 41 is a diagrammatic plan view of a further modification of the tenth embodiment;
`Fig. 42A and Fig. 42B are a diagrammatic cross-sectional view and perspective view of a further modification of the
`tenth embodiment;
`Fig. 43 is a partial diagrammatic cross-sectional view showing a modification of the tenth embodiment;
`'
`. 44 is a partial diagrammatic cross-sectional view showing a modification of the tenth embodiment;
`'
`. 45 is a diagrammatic cross-sectional view of a further modification of the tenth embodiment;
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`. 46 is a diagrammatic cross-sectional view of a further modification of the tenth embodiment;
`'
`. 47 is a diagrammatic cross-sectional view showing an eleventh embodiment of the present invention;
`'
`. 48 is a perspective view showing an example of application of the eleventh embodiment;
`'
`. 49 is a perspective view showing a further example of application of the eleventh embodiment;
`'
`. 50 is a perspective view showing a further example of application of the eleventh embodiment;
`'
`. 51 is a perspective view showing a further example of application of the eleventh embodiment;
`Fig. 52A and Fig. 52B are a diagrammatic cross-sectional view and perspective view showing a twelfth embodiment
`of the present invention;
`Fig. 53 is a diagrammatic plan view of a modification of the twelfth embodiment;
`Fig. 54 is a diagrammatic cross-sectional view of a further modification of the twelfth embodiment;
`Fig. 55 is a diagrammatic cross-sectional view showing a thirteenth embodiment of the present invention;
`Fig. 56 is a diagrammatic cross-sectional view of a modification of the thirteenth embodiment;
`Fig. 57A and Fig. 57B are a diagrammatic cross-sectional view and perspective view showing a fourteenth embod-
`iment;
`Fig. 58A and Fig. 58B are a diagrammatic cross-sectional view and perspective view showing a fifteenth embodi-
`ment of the present invention;
`Fig. 59 is a diagrammatic cross-sectional view of a sixteenth embodiment of the present invention;
`Fig. 60 is a diagrammatic cross-sectional view showing a modification of the sixteenth embodiment;
`Fig. 61A and Fig. 61B are a diagrammatic cross-sectional view and perspective view showing a seventeenth
`embodiment of the present invention;
`Fig. 62 is a cross-sectional view showing a diagram of a prior art reflective type liquid crystal display device;
`Fig. 63 is a cross-sectional view for purposes of functional description of a polarisation separating plate used in the
`present invention;
`Fig. 64 is a cross-sectional view for purposes of functional description of a further polarisation separating plate
`used in the present invention;
`Fig. 65 is a cross-sectional view showing a diagram of a reflective type liquid crystal display device using a polari-
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`EP 0 878 720 A1
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`sation separating plate used in the present invention;
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`Fig. 66 is a cross-sectional view showing a diagram of a prior art semi-transparent liquid crystal display device;
`Fig. 67 is a diagrammatic cross-sectional view given in explanation of an eighteenth embodiment of the present
`invention;
`Fig. 68 is a diagrammatic cross-sectional view given in explanation of a nineteenth embodiment of the present
`invention;
`Fig. 69 is a diagrammatic cross-sectional view given in explanation of a twentieth embodiment of the present inven-
`tion;
`Fig. 70 is a diagrammatic plan view given in explanation of a twenty-first embodiment of the present invention;
`Fig. 71 is a diagrammatic cross-sectional view given in explanation of a tvventy-second embodiment of the present
`invention;
`Fig. 72 is a diagrammatic plan view given in explanation of a twenty-third embodiment of the present invention;
`Fig. 73A and Fig. 73B are diagrammatic cross-sectional views of a modification of the twenty-third embodiment;
`Fig. 74 is a diagrammatic cross-sectional view of a modification of the twenty-third embodiment;
`Fig. 75 is a diagrammatic cross-sectional view given in explanation of a twenty-fourth embodiment of the present
`invention;
`Fig. 76 is a perspective view showing an example of a display device according to a tvventy-fifth embodiment of the
`present invention;
`Fig. 77 is a perspective view showing a further example of a display device according to a twenty-sixth embodiment
`of the present invention;
`Fig. 78 is a perspective view showing a further example of a display device according to a tvventy-seventh embod-
`iment of the present invention;
`Fig. 79 is a perspective view showing a further example of a display device according to a twenty-eighth embodi-
`ment of the present invention;
`Fig. 80 is a perspective view showing an example of a display device according to a twenty-ninth embodiment of
`the present invention;
`Fig. 81A and Fig. 81 B are a diagrammatic cross-sectional view and a perspective view showing the twenty-ninth
`embodiment of the present invention;
`Fig. 82 is a diagrammatic cross-sectional view relating to a modification of the twenty-ninth embodiment;
`Fig. 83 is a diagrammatic cross-sectional view relating to a further modification of the twenty-ninth embodiment;
`Fig. 84 is a diagrammatic cross-sectional view relating to a further modification of the twenty-ninth embodiment;
`Fig. 85 is a diagrammatic plan view relating to a further modification of the twenty-ninth embodiment;
`Fig. 86 is a diagrammatic cross-sectional view relating to a further modification of the twenty-ninth embodiment;
`Fig. 87 is a perspective view showing an example of a display device employing the twenty-ninth embodiment;
`Fig. 88 is a perspective view showing a further example of a display device employing the twenty-ninth embodi-
`ment;
`Fig. 89 is a perspective view showing a further example of a display device employing the twenty-ninth embodi-
`ment;
`Fig. 90 is a perspective view showing a further example of a display device employing the twenty-ninth embodi-
`ment; and
`Fig. 91A to Fig. 91C are a perspective view, plan view and side view of an illumination device of a handy type
`employing the present invention.
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`DESCRIPTION OF THE PREFERRED EMBODIMENTS
`
`Embodiments of the present invention and modifications thereof are described below with reference to the draw-
`ings.
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`(First embodiment)
`
`A first embodiment of the present invention is described with reference to the drawings. In Fig. 1A one or a plurality
`of point light sources 2 are arranged at the end face of light-guide plate 11. As shown in Fig. 1B, light-guide plate 11 is
`provided with projections 12 on one face of the transparent plate; the faces of projections 12 are in all cases constituted
`by faces practically parallel to optical output face 13 (bottom face 14) and faces practically perpendicular thereto (side
`faces 15). Light-guide plate 11 is formed by transparent material of refractive index about 1.4 or more. After the optical
`flux from a point light source 2 is input from end face 16 as shown by ray 19a or ray 19b, it is subjected to total reflection
`within light-guide plate 11 and is emitted solely from the side faces 15 of projections 12, so the optical output from the
`back face of the illumination device is large, enabling illuminated element 6 to be efficiently illuminated.
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`EP 0 878 720 A1
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`Also, for the transparent material forming light-guide plate 11, there may be employed transparent resin such as
`acrylic resin, polycarbonate resin, or amorphous polyolefin resin etc. or inorganic transparent material such as glass or
`a combination of these; these may be formed by a method such as joining a film or resin layer on to an injection mould-
`ing, thermosetting resin, photosetting resin, etching, transparent resin or flat glass sheet.
`
`As light sources 2, light emitting diodes (LEDs) or electric light bulbs etc. may be employed. In comparison with the
`fluorescent tubes that were conventionally employed, these do not require special equipment such as voltage step-up
`devices and are of lightweight and compact and excellent safety since they do not employ high frequencies or high volt-
`ages. Also, power control is easy and they can easily cope with applications requiring low power consumption. In par-
`ticular the life of LEDs is semi-permanent and, regarding colours, they have recently become available with red, green,
`blue, mixtures of these and white colour. if electric light bulbs are employed, although their life is short, they are cheap
`and can easily be changed.
`With the above construction, by arranging this illumination device at the front face of illuminated body 6, part-time
`illumination can be achieved in which the illuminated body 6 is observed by turning off illumination under bright condi-
`tions when there is sufficient external light, whereas the illuminated body can be observed by switching on illumination
`under dark conditions when external light is insufficient.
`As the illuminated body 6 of an illumination device as described above, printed material such as printed paper or a
`liquid crystal display or the like are suitable.
`However, as shown in Fig. 2A,
`it is found that the lines of intersection of the optical output face 13 of light-guide
`plate 11 and side faces 15 of projections 12 have a minute curved face in manufacture, so some reflected light 19c leaks
`towards the face 17 opposite the optical output face (the observer's side) and this can be observed by the observer as
`bright points. As shown in Fig. 2B, when projections 12 are in the form of ribs and these intersection lines are straight
`lines, the point light source 2, the aforesaid bright points, and the observer are located in the same plane, with the result
`that specific positions on the light-guide plate appear as bright points to the observer and these bright points move with
`movement of the observer's eye. These adversely affect recognisability of illuminated body 6. In contrast, with the for
`example cylindrical shape of projections 12 as in this embodiment, since the bright points do not move in the plane of
`the light-guide plate 1 1, uniform recognisability can be obtained irrespective of the observation position of the observer.
`Regarding the size of projections 12, since the wavelength of visible light is about 380 nm to 700 nm, this should
`be at least about 5 pm in order to avoid diffraction effects and in order to make the size of projections 12 such as not to
`be noticed with the naked eye should be less than about 300 um. Apart from the above, from the point of view of con-
`venience in manufacture, the size of the projections is desirably above about 10 pm and below 100 um. Regarding the
`ratio of the height and width (i.e. the diameter in the case where these are approximately cylindrical) of projections 12,
`this may be below 1
`: 1 since the angle of elevation of a light ray within light-guide plate 11 in the planar direction is less
`than 45°; and in fact satisfactory performance is exhibited up to a ratio of about 1
`: 2, since rays of under 20° represent
`more than 90%.
`
`A modification is shown in Fig. 3. In Fig. 3, a concave shape 12a is provided on the side of the face 17 opposite the
`optical output face of light-guide plate 11. Concave shape 12a can have arbitrary size and shape; it has the function of
`converting optical flux that reaches this concave shape 12a into optical flux having a large angle of elevation with
`respect to light-guide plate 11; it is found that a satisfactory characteristic is obtained by making this approximately a
`spherical surface of central angle under 90°. Optical flux that is fed from point light source 2 into light-guide plate 11 is
`guided within light-guide plate 11 by repeated total reflection but, thanks to the provision of concave shapes 12a in the
`face 17 opposite the optical output face of light-guide plate 11, optical flux arriving at these is converted to optical flux
`having a large angle of inclination with respect to the plane of light-guide plate 11, and can thus be output from optical
`output face 13. By arranging illuminated body 6 on the side of optical output face 13 of light-guide plate 11, this con-
`struction functions as planar illumination. And since regions of the surface other than the concave shapes on the side
`of face 17 opposite the optical output face are parallel with output face 13, these also have the function of vertical ray
`transparency i.e. of transmitting rays in the direction that intersects the flat plate at right angles.
`These concave shapes 12a can be provided in any desired area ratio. However, although the efficiency of illumina-
`tion can be raised by making the area ratio of concave shapes 12a large, recognisability is lowered by decreasing the
`ratio of perpendicularly transmitted rays. In fact to set an area ratio exceeding 50% is not realistic and as part-time illu-
`mination under dark conditions an area ratio of about 10% may suitably be set. Also, if it is desired to increase/decrease
`their density in order to achieve uniformity of illumination brightness as described above, at about 10%, the area ratio
`of the perpendicular transmission section is a range of about 80 ~ 90%, so there is no sensation of unevenness of rec-
`ognisability at different positions. Regarding the size of concave shapes 12a, since the wavelength of visible light is
`about 380 nm to 700 nm, it is necessary that this size should be at least about 5 pm in order that diffractio