US20010022636A1 - Reflective display device - Google Patents

Abstract

A reflective display device includes a panel, a light guide plate and a light source. The panel is provided with a transparent first substrate lying on the side of the external incident light, a second substrate joined to the first substrate with a predetermined gap therebetween and lying on the reflection side, a guest-host liquid crystal layer held in the gap between the substrates, and electrodes provided on each substrate for applying a voltage to the guest-host liquid crystal layer. The light guide plate is composed of a transparent material and is arranged on the outside of the first substrate. The light source is arranged on the end of the light guide plate and generates illumination light as required. The light guide plate normally transmits external light onto the first substrate and emits the external light reflected from the second substrate, and also, as required, guides illumination light onto the first substrate and emits the illumination light reflected from the second substrate.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention[0001]
• The present invention relates to a reflective display device to perform a display by using external light such as natural light, and more specifically, it relates to an illuminating structure of a reflective display device, which is used as an auxiliary when external light is scarce.[0002]
• 2. Description of the Related Art[0003]
• Among current various modes of display devices, mainly adopted are a TN mode or an STN mode in which a nematic liquid crystal having a twisted or super twisted alignment is used. However, these modes require a pair of polarizers for operation, and because of the light absorption thereof, they have a low transmittance, incapable of achieving a bright display screen. In addition to the above modes, a guest-host mode which uses a dichroic dye has been developed. A liquid crystal display device having a guest-host mode takes advantage of the anisotropy of the absorption coefficient of the dichroic dye added to the liquid crystal, in order to perform the display. By using a rod-shaped dichroic dye, the alignment direction of the dye changes as the molecular alignment of the liquid crystal is changed by applying a voltage to the electric field since the molecules of the dye are aligned in parallel to the molecules of the liquid crystal. The dye does or does not develop a color depending on the direction, and therefore by applying a voltage, the coloring mode of the liquid crystal display device can be switched.[0004]
• FIG. 5A and FIG. 5B show a HEILMEIER type guest-host liquid crystal display device. FIG. 5A shows the state in the absence of an applied voltage, while FIG. 5B shows the state in the presence of an applied voltage. This liquid crystal display device includes a p-type dye and a nematic liquid crystal having a positive dielectric anisotropy (Np liquid crystal). The p-type dichroic dye having an absorption axis which is substantially parallel to the molecular axis, strongly absorbs the polarization component Lx which is parallel to the molecular axis, and hardly absorbs the polarization component Ly which is perpendicular to it. In the state shown in FIG. 5A when no voltage is applied, the polarization component Lx included in the incident light is strongly absorbed by the p-type dye, resulting in the coloring of the liquid crystal display device. On the other hand, in the state shown in FIG. 5B when a voltage is applied, the Np liquid crystal having a positive dielectric anisotropy rises in response to the electric field and accordingly the p-type dye is perpendicularly aligned. Therefore, the polarization component Lx is only slightly absorbed, resulting in the liquid crystal display device being substantially colorless. The other polarization component Ly included in the incident light is hardly absorbed by the dichroic dye whether the state of the voltage is being applied or not being applied. Accordingly, in the HEILMEIER type guest-host liquid crystal display device, a polarizer is provided beforehand to remove the other polarization component Ly for improving the contrast.[0005]
• Although the guest-host liquid crystal display device shown in FIG. 5 is a transmissive type, a reflective liquid crystal display device is also known. For example, a reflective guest-host liquid crystal display device, as shown in FIG. 6, has been proposed, in which a polarizer is removed on the side of the incident light, while a quarter-wavelength plate and a reflector are provided on the emission side. In this device, the polarization directions of the two polarizing components Lx and Ly which are orthogonal to each other are rotated by 90 degrees at both incident light and reflected light paths by the quarter-wavelength plate in order to exchange the polarizing components with each other. Therefore, in the off-state (absorption state) shown in FIG. 6A, individual polarizing components Lx and Ly are absorbed either at the incident light path or at the reflected light path. In the on-state (transmission state) shown in FIG. 6B, both polarizing components Lx and Ly are hardly absorbed. Thus, the utilization efficiency of the incident light can be improved.[0006]
• In the transmissive display device shown in FIG. 5, a panel holding a liquid crystal as an electro-optical material is provided between a pair of transparent electrodes, and a light source (backlight) for supplying illumination light is arranged on the rear of the panel. The image is viewed from the front of the panel. A backlight is essential to the transmissive type, and, for example, a cold cathode fluorescent tube or the like is used. Accordingly, from the viewpoint of the display device as a whole, the backlight consumes most of the electric power, which is unsuitable for displays of portable apparatuses. On the other hand, in the reflective type shown in FIG. 6, a reflector is arranged on the rear of the panel. External light such as natural light enters from the front and the image is viewed also from the front of the panel by making use of the reflected light. Differing from the transmissive type, the reflective type does not use a light source for supplying illumination light in the back, resulting in a relatively low rate of electric power consumption, which is suitable for displays of portable apparatuses. However, in the reflective display device, the image cannot be viewed in an environment where external light is scarce, for example, at night, which remains to be a problem to be solved.[0007]
SUMMARY OF THE INVENTION
• Accordingly, it is an object of the present invention to provide a reflective display device provided with an illumination structure which enables the viewing of an image in a dark environment while not spoiling the image quality in a bright environment.[0008]
• A reflective display device, in accordance with the present invention, includes a panel, a light guide plate and a light source as fundamental components. The panel includes a transparent first substrate lying on the side of the external incident light, a second substrate joined to the first substrate with a predetermined gap therebetween and lying on the reflection side, an electro-optical material held in the gap, and an electrode provided on at least one of the first substrate and the second substrate for applying a voltage to the electro-optical material. The light guide plate is composed of a transparent material and arranged on the outside of the first substrate. The light source is arranged on the end of the light guide plate and generates illumination light as required. Notably, the light guide plate normally transmits external light onto the first substrate and emits the light reflected from the second substrate, and also as required guides illumination light onto the first substrate and emits the illumination light reflected from the second substrate.[0009]
• Preferably, the light guide plate includes a planar section divided into bands and an inclined step lying between each band of the planar section. The thickness of the light guide plate decreases stepwise from the end where the light source lies toward the front. The light guide plate reflects the illumination light guided forward at each step so as to guide it onto the first substrate, and emits the illumination light reflected from the second substrate through the planar section. In such a case, the step of the light guide plate inclines from 40 to 50 degrees toward the planar section. Further, the panel may use a guest-host liquid crystal layer, including a liquid crystal as a host to which a dichroic dye is added as a guest, as the electro-optical material. In such a case, the panel includes a reflecting layer lying on the side of the second substrate for scattering and reflecting external light, and a quarter-wavelength layer provided between the guest-host liquid crystal layer and the reflecting layer. Or, the panel may include a polarizing plate provided on the side of the first substrate and may use a liquid crystal layer, which functions as a quarter-wavelength plate in response to the state of an applied voltage, as the electro-optical material. In such a case, a quarter-wavelength plate is provided between the polarizing plate and the liquid crystal layer, and the liquid crystal layer includes a nematic liquid crystal layer having a positive dielectric anisotropy and a twisted alignment, functions as a quarter-wavelength plate in the absence of an applied voltage, and does not function as a quarter-wavelength plate in the presence of an applied voltage.[0010]
• Also preferably, the light guide plate and the panel are joined to each other with a transparent intervening layer therebetween, for suppressing undesirable reflection of illumination light and external light at the interface between the light guide plate and the panel by appropriately setting a refractive index of the intervening layer. The intervening layer is composed of, for example, a transparent resin having adhesion. In such a case, the light guide plate may be provided with a groove on the back surface thereof for preventing the transparent adhesive resin from leaking out when the back surface of the light guide plate and the surface of the panel are joined to each other. Also preferably, the reflective display device includes a collimating means for collimating the illumination light radiating from the light source and leading it perpendicularly onto the end of the light guide plate. In such a case, the light source is, for example, semicylindrically formed and arranged facing the end of the light guide plate, and the collimating means corresponds to a semicylindrical collimator lens arranged between the light source and the light guide plate. Also preferably, a polarizing plate is provided between the light source and the light guide plate for converting unpolarized light radiating from the light source into linearly polarized light, leading it onto the light guide plate and suppressing undesirable scattering of illumination light inside the light guide plate. In such a case, the polarizing plate converts illumination light into linearly polarized light which is parallel to or perpendicular to the light guide plate. On the other hand, the electro-optical material includes a liquid crystal which can be controlled in the alignment direction parallel to or orthogonally to the polarization direction of the illumination light converted into linearly polarized light. Also preferably, the step of the light guide plate includes a curved inclined area for reflecting illumination light diffusively so as to lead it onto the first substrate. Or, each step of the light guide plate may be formed so as to have a different angle of inclination for reflecting illumination light in accordance with the angle of inclination and leading it onto the first substrate at a different angle. Also preferably, the light guide plate includes a trapezoidal section divided into bands and an inclined step lying between each band of the trapezoidal section, and each band of the trapezoidal section includes a curved lens area. The light guide plate reflects the illumination light radiating from the light source at each step so as to guide it onto the first substrate and emits the illumination light reflected from the second substrate through the lens area of each band of the trapezoidal section.[0011]
• In accordance with the present invention, the light guide plate is arranged on the surface of the reflective panel, and the light source is arranged on the end of the light guide plate. In a dark environment, the light source is turned on and the illumination light enters into the panel through the light guide plate for displaying the image In a bright environment, the light source is turned off and external light is directly used through the transparent light guide plate for displaying the image. The light guide plate is basically transparent and thus it will not prevent the viewer from seeing the image even in a bright environment. As described above, in accordance with the present invention, the light source is turned on only when required, thus the electric power consumed in the display as a whole can be largely reduced, which is suitable for displays of portable apparatuses. As an alternative to the structure in accordance with the present invention, a flat-type backlight source may be arranged in the rear of the panel in order to perform an auxiliary illumination in a dark environment. However, in such a case, it is required either to provide an opening onto the reflecting layer included in the panel in order to transmit the illumination light from the back to the front, or to provide a transflector-type structure to the reflecting layer. This will lower the reflection efficiency and sacrifice the brightness of the displayed image in a bright environment. In the present invention, it is possible to provide an auxiliary illumination in a dark environment without sacrificing the display brightness in a bright environment.[0012]
• Further, in accordance with the present invention, various means have been tried in order to improve the utilization efficiency of the illumination light radiating from the light source and to enhance the display quality. For example, an intervening layer for matching is provided between the light guide plate and the first substrate in order to suppress undesirable reflection of illumination light at the interface between the light guide plate and the first substrate. Also, a collimating means such as a collimator is provided between the light source and the end of the light guide plate in order to lead illumination light efficiently onto the light guide plate. Also, a polarizing plate is inserted between the light source and the light guide plate in order to suppress undesirable scattering of illumination light inside the light guide plate. Further, since the step of the light guide plate is formed as a curved inclined area, illumination light is diffusively led onto the first substrate, resulting in an improvement in the visual characteristics. Or, each step is formed so as to have a different angle of inclination and leads illumination light onto the first substrate at a different angle, and thus the visual characteristics can be improved. Furthermore, by providing the planar section (trapezoidal section) divided into bands with a curved lens area on the light guide plate, a microlens is provided. The microlens can suppress the interference fringes caused by the periodical prismatic structure of the light guide plate, enabling a high-quality display.[0013]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic partial sectional view which shows a reflective display device, as a first embodiment of the present invention, in use in a dark environment;[0014]
• FIG. 2 is a schematic partial sectional view which shows the reflective display device as the first embodiment of the present invention, in use in a bright environment;[0015]
• FIG. 3 is a schematic representation showing the optical properties of the light guide plate included in the first embodiment shown in FIG. 1 and FIG. 2;[0016]
• FIG. 4 is a schematic partial sectional view which shows a reflective display device as a second embodiment of the present invention;[0017]
• FIG. 5 is a schematic diagram showing an example of a conventional transmissive display device;[0018]
• FIG. 6 is a schematic diagram showing an example of a conventional reflective display device;[0019]
• FIG. 7 is a sectional view which shows a reflective display device as a third embodiment of the present invention;[0020]
• FIG. 8 is a schematic diagram showing the light guide plate used in the third embodiment;[0021]
• FIG. 9 is a plan view showing a light guide plate used in a variation to the third embodiment;[0022]
• FIG. 10 is a sectional view showing the light guide plate of the same;[0023]
• FIG. 11 is a schematic perspective view which shows a reflective display device provided with the light guide plate shown in FIG. 9 and FIG. 10;[0024]
• FIG. 12 is a schematic representation which shows a method for fabricating the reflective display device shown in FIG. 11;[0025]
• FIG. 13 is a sectional view which shows a reflective display device as a fourth embodiment of the present invention;[0026]
• FIG. 14 is an enlarged partial sectional view which shows in detail the reflective display device as the fourth embodiment of the present invention:[0027]
• FIG. 15 is a perspective view which shows the entire structure of the fourth embodiment;[0028]
• FIG. 16 is a perspective view which shows the shape of a collimator lens used in the fourth embodiment;[0029]
• FIG. 17 is a schematic representation which shows the optical properties of the light guide plate incorporated in the fourth embodiment;[0030]
• FIG. 18 is a partial sectional view which shows a reflective display device as a fifth embodiment of the present invention;[0031]
• FIG. 19 is a partial sectional view of the important part of a reflective display device as a sixth embodiment of the present invention;[0032]
• FIG. 20 is a geometric representation for explaining the sixth embodiment;[0033]
• FIG. 21 is a partial sectional view showing a variation to the sixth embodiment;[0034]
• FIG. 22 is a schematic diagram showing a typical structure of the light guide plate used in the reflective display device in accordance with the present invention;[0035]
• FIG. 23 is a schematic diagram showing the usage and the properties of the light guide plate shown in FIG. 22;[0036]
• FIG. 24 is a sectional view showing a light guide plate as the important part of a reflective display device as a seventh embodiment of the present invention;[0037]
• FIG. 25 is a schematic diagram showing the usage and the properties of the light guide plate shown in FIG. 24;[0038]
• FIG. 26 is a schematic partial sectional view which shows a reflective display device as an eighth embodiment of the present invention;[0039]
• FIG. 27 is a schematic diagram which explains the function of the reflective display device as the eighth embodiment.[0040]
DESCRIPTION OF THE PREFERRED EMBODIMENT
• The preferred embodiments of the present invention will be explained with reference to the attached drawings.[0041]
• FIG. 1 is a schematic partial section view which shows a reflective display device as a first embodiment of the present invention. As shown in the drawing, the reflective display device includes a[0042]panel0, alight guide plate20 and alight source30 as fundamental components. Thepanel0 comprises a transparentfirst substrate1 lying on the side of the external incident light, asecond substrate2 joined to thefirst substrate1 with a predetermined gap therebetween and lying on the reflection side, an electro-optical material held between bothsubstrates1 and2, andelectrodes10 and11 provided on thefirst substrate1 and thesecond substrate2 respectively for applying a voltage to the electro-optical material. Thelight guide plate20 is composed of the injection-molded piece of a transparent material, for example, an acrylic resin, and arranged on the outside of thefirst substrate1. Further, in accordance with the embodiment of the present invention, although thelight guide plate20 and thefirst substrate1 are separately formed, they may be integrally molded. Thelight source30 is arranged on the end of thelight guide plate20 and generates illumination light as required. Thelight source30 is composed of, for example, a cold cathode fluorescent tube, and is a so-called edge light. In order to improve the illumination efficiency of the edge light, a reflectingmirror31 is provided behind the cylindricallight source30. In such a structure, thelight guide plate20 normally transmits external light onto thefirst substrate1 and emits the external light reflected from thesecond substrate2, and also as required guides illumination light onto thefirst substrate1 and emits the illumination light reflected from thesecond substrate2.
• In accordance with the embodiment of the present invention, the[0043]light guide plate20 includes aplanar section22 divided into bands, and aninclined step21 lying between each band of theplanar section22. The thickness of the light guide plate decreases stepwise from the end where thelight source30 lies toward the front. Thelight guide plate20 totally reflects the illumination light directed forward at eachstep21 so as to guide it onto thefirst substrate1, and emits the illumination light reflected from thesecond substrate2 through each band of theplanar section22. Thestep21 of thelight guide plate20 inclines from 40 to 50 degrees toward theplanar section22. In the drawing, the angle of inclination is shown as θ. FIG. 1 shows the reflective display device in use in a dark environment, where the light source constituting the edge light is turned on. The illumination light radiating from thelight source30 illuminates thepanel0 through thelight guide plate20. That is to say, the illumination light advancing horizontally in the light guide plate is totally reflected at thestep21 and enters into thepanel0, while the illumination light reflected from thesecond substrate2 is emitted through theplanar section22 of thelight guide plate20.
• The[0044]panel0 includes a guest-hostliquid crystal layer3, as the electro-optical material, which comprises aliquid crystal4 as a host to which a dichroic dye5 is added as a guest. However, the present invention is not limited to a guest-host liquid crystal layer, and other materials can also be used as the electro-optical material. Thepanel0 includes a reflectinglayer8 and a quarter-wavelength layer9. The reflectinglayer8 lies on the side of thesecond substrate2 for scattering and reflecting external light. The quarter-wavelength layer9 is provided between the guest-hostliquid crystal layer3 and the reflectinglayer8. The structure of thepanel0 will be explained in detail, as follows. The guest-hostliquid crystal layer3 includes a mixture of a nematicliquid crystal4 and a dichroic dye5 and is homogeneously aligned by upper andlower alignment layers6 and7. Also, a reflectinglayer8 is provided on the side of thesecond substrate2 in the gap between thesubstrates1 and2. Further, the quarter-wavelength layer9 is provided between the guest-hostliquid crystal layer3 and the reflectinglayer8.Electrodes10 and11 are formed on the sides of thefirst substrate1 and thesecond substrate2 respectively for applying a voltage to the guest-hostliquid crystal layer3. In accordance with the embodiment of the present invention, theupper electrode10 is formed on the inner surface of thefirst substrate1 and thelower electrode11 is formed on the inner surface of thesecond substrate2.
• The reflecting[0045]layer8 has a corrugated surface and scatters light. Accordingly, its paper-white appearance is suitable for the display background and since it reflects the incident light with a relatively wide angle range, the viewing angle range is enlarged, and thus the display is easily viewed as well as the brightness of the display being increased. In accordance with the embodiment of the present invention, atransparent flattening layer12 is provided between the reflectinglayer8 and the quarter-wavelength layer9 for compensating the corrugation. The quarter-wavelength layer9 is composed of a polymericliquid crystal material13 which is aligned uniaxially along the surface of theflattening layer12. In order to uniaxially align the polymericliquid crystal material13, anunderlying alignment layer14 is provided between the flatteninglayer12 and the quarter-wavelength layer9. The reflectinglayer8 includes aresin layer15 having a corrugation and ametal film16 formed on the surface thereof, composed of, for example, aluminum. Theresin layer15 is a photosensitive resin layer whose corrugation is patterned by means of photolithography.
• The[0046]photosensitive resin layer15 formed on the surface of thesecond substrate2 is composed of, for example, a photo resist, which is applied to the entire surface of the substrate. It is exposed to light through a given mask and, for example, is formed into a cylindrical pattern. Next, by heating to melt, the corrugation is formed stably. On the surface of the corrugation formed as described above, ametal film16, composed of aluminum or the like having a predetermined thickness and a good reflectance, is provided. If the depth of the corrugation is set at several μm, a good light scattering property is obtained and the-reflectinglayer8 will have a white color. On the surface of the reflectinglayer8, theflattening layer12 is provided to compensate the corrugation. It is preferable that theflattening layer12 is composed of a transparent organic substance, for example, an acrylic resin or the like. By providing theflattening layer12, theunderlying alignment layer14 can be stably formed and rubbed. Thereby, the quarter-wavelength layer9 is precisely formed. Thealignment layer7 is formed on the quarter-wavelength layer9. Thealignment layer7 provided on the side of thesecond substrate2 and thealignment layer6 provided on the side of thefirst substrate1 align the guest-hostliquid crystal layer3 homogeneously (horizontally). Alternatively, the guest-hostliquid crystal layer3 may be aligned homeotropically (perpendicularly).
• Next, the operation for performing the black-and-white display by using the reflective guest-host liquid crystal display device will be explained briefly. In the absence of an applied voltage, the nematic[0047]liquid crystal4 is aligned horizontally and the dichroic dye5 is similarly aligned. When the illumination light entering from the upperfirst substrate1 advances to the guest-hostliquid crystal layer3, a component, of the illumination light, having a plane of vibration which is parallel to the major axes of the molecules of the dichroic dye5, is absorbed by the dichroic dye5. Another component, having a plane of vibration which is perpendicular to the major axes of the molecules of the dichroic dye5, passes through the guest-hostliquid crystal layer3, and is circularly polarized by the quarter-wavelength layer9 provided on the surface of the lowersecond substrate2, and then it is reflected from the reflectinglayer8. At this stage, the polarization of the reflected light is reversed, and after passing through the quarter-wavelength layer9 again, the component will have a plane of vibration which is parallel to the major axes of the molecules of the dichroic dye5. Since the component is absorbed by the dichroic dye5, a substantially black display is obtained. On the other hand, in the presence of an applied voltage, the nematicliquid crystal4 is aligned perpendicularly along the direction of the electric field, and the dichroic dye5 is similarly aligned. The illumination light entering from the upperfirst substrate1 passes through the guest-hostliquid crystal layer3 without being absorbed by the dichroic dye5, and is reflected from the reflectinglayer8 without being substantially affected by the quarter-wavelength layer9. The reflected light passes though the quarter-wavelength layer9 again and is emitted without being absorbed by the guest-hostliquid crystal layer3. Accordingly, a white display is obtained.
• FIG. 2 shows the reflective guest-host liquid crystal display device shown in FIG. 1 in use in a bright environment. In a bright environment, because of an ample supply of external light such as natural light, the display is performed by making use of it. Therefore, the[0048]light source30 is turned off. Thus the electric power consumed by the display device as a whole can be reduced. Thelight guide plate20 transmits the light entered from the side of the viewer onto thefirst substrate1 and emits the light reflected from thesecond substrate2 through theplanar section22. Since thelight guide plate20 is basically transparent, it does not hinder the viewer from seeing the display.
• FIG. 3 is a schematic representation showing the optical properties of the[0049]light guide plate20. As shown in FIG. 3A, the illumination light horizontally guided from the end of thelight guide plate20 is totally reflected at substantially a right angle from thestep21 and is guided to the side of thepanel0. By setting the angle of inclination θ of thestep21 appropriately, it is possible to guide the substantially total volume of the illumination light to the side of thepanel0 without leaking light. The angle of inclination θ depends on the refractive index of the transparent material constituting thelight guide plate20 and is generally set in the range from 40 to 50 degrees.
• As shown in FIG. 3B, the illumination light guided to the side of the[0050]panel0 through thestep21 is reflected from thepanel0 and is emitted through theplanar section22 of thelight guide plate20. In such a case, it is preferable to set the angle of inclination θ of thestep21 so that the angle Ψ between the line perpendicular to theplanar section22 of thelight guide plate20 and the reflected light is smaller than the angle of total reflection determined by the refractive index of thelight guide plate20. Thus the illumination efficiency is improved because the substantially total volume of the illumination light is emitted through theplanar section22 of thelight guide plate20 without being totally reflected. Generally, in order to satisfy the above condition, the angle of inclination θ of thestep21 is set at 40 to 50 degrees.
• FIG. 4 is a schematic partial sectional view which shows a reflective display device as a second embodiment of the present invention. The embodiment also has a basically flat structure including a light guide plate deposited on a panel. Notably, the panel is an active matrix type. An[0051]upper substrate101 lying on the side of the incident light in contact with the light guide plate is composed of a transparent material such as glass. On the other hand, alower substrate102 lying on the reflection side is not necessarily composed of a transparent material. A guest-hostliquid crystal layer103 is provided between the pair ofsubstrates101 and102. The guest-hostliquid crystal layer103 contains mainly a nematicliquid crystal104 having a negative dielectric anisotropy and contains, at a given ratio, adichroic dye105. On the inner surface of theupper substrate101, acounter electrode106 and analignment layer107 are provided. Acolor filter150 is also provided on it. Thealignment layer107 is composed of, for example, a polyimide film and vertically aligns the quest-hostliquid crystal layer103. On thelower substrate102, at least, a switching element composed of athin film transistor108, a reflectinglayer109, a quarter-wavelength layer110 and apixel electrode111 are provided. The quarter-wavelength layer110 is formed on thethin film transistor108 and the reflectinglayer109 and is also provided with acontact hole112 which is connected with thethin film transistor108. Thepixel electrode111 is formed on the quarter-wavelength layer by patterning. Therefore, it is possible to sufficiently impress the electric field to the guest-hostliquid crystal layer103 between thepixel electrode111 and thecounter electrode106. Thepixel electrode111 is electrically connected with thethin film transistor108 through thecontact hole112 which passes through the quarter-wavelength layer110.
• The individual components will be explained in detail, as follows. In accordance with the embodiment of the present invention, the quarter-[0052]wavelength layer110 is composed of a polymeric liquid crystal layer which is aligned uniaxially. In order to uniaxially align the polymeric liquid crystal layer, anunderlying alignment layer113 is provided. In order to compensate for the unevenness of thethin film transistor108 and the reflectinglayer109, aflattening layer114 is provided, and the above-mentionedunderlying alignment layer113 is formed on theflattening layer114. The quarter-wavelength layer110 is also formed on the surface of theflattening layer114. In such a case, thepixel electrode111 is connected with thethin film transistor108 through thecontact hole112 passing through the quarter-wavelength layer110 and theflattening layer114. The reflectinglayer109 is fragmented corresponding to theindividual pixel electrodes111. Each fragmented part is connected with the correspondingpixel electrode111 with the same electric potential. Owing to the structure described above, the quarter-wavelength layer110 and theflattening layer114 provided between the reflectinglayer109 and thepixel electrode111 are not impressed with an electric field unnecessarily. As shown in the drawing, the reflectinglayer109 is provided with a scattering reflective surface, which prevents the regular reflection of the incident light, thus improving the image quality. Thealignment layer115 is formed so as to cover the surface of thepixel electrode111 and is in contact with the guest-hostliquid crystal layer103 for controlling the alignment thereof. In accordance with the embodiment of the present invention, thealignment layer115 together with the facingalignment layer107 vertically aligns the guest-hostliquid crystal layer103. Finally, thethin film transistor108 has a bottom-gate structure where agate electrode116, agate insulating film117, and a semiconductorthin film118 are deposited in that order from the bottom. The semiconductorthin film118 is composed of, for example, polycrystalline silicon and the channel area which matches with thegate electrode116 is protected with astopper119 from the top. Thethin film transistor108 having the bottom-gate structure as described above is covered with alayer insulation film120. Thelayer insulation film120 has a pair of contact holes, through which asource electrode121 and adrain electrode122 are electrically connected with thethin film transistor108. Theelectrodes121 and122 are formed by patterning, for example, aluminum. Thedrain electrode122 and the reflectinglayer109 have the same electric potential. Also, thepixel electrode111 is electrically connected with thedrain electrode122 through the above-mentionedcontact hole112. On the other hand, a signal voltage is supplied to thesource electrode121.
• FIG. 7 is a schematic sectional view which shows a reflective display device as a third embodiment of the present invention. This has the same fundamental structure as the first embodiment of the present invention shown in FIG. 1 and the same reference numerals are assigned to the corresponding parts. Notably, the[0053]light guide plate20 and thefirst substrate1 of thepanel0 are joined to each other with atransparent intervening layer40 therebetween. The undesirable reflection of illumination light and external light at the interface between thelight guide plate20 and thefirst substrate1 is suppressed by appropriately setting a refractive index of the interveninglayer40. The interveninglayer40 may be composed of, for example, a transparent resin having adhesion. The transparent resin is applied to the surface of thefirst substrate1 of thepanel0 and thelight guide plate20 is bonded thereto. Since optical matching is required to suppress the undesirable reflection, the refractive index of the resin constituting the interveninglayer40 is selected so as to be substantially the same as the refractive indices of thelight guide plate20 and thefirst substrate1. For example, if thefirst substrate1 is composed of glass, the refractive index of the resin constituting the interveninglayer40 is set at approximately 1.5. Also, in order not to trap air bubbles between thelight guide plate20 and thepanel0 when they are bonded together, the resin preferably has relatively low viscosity and the viscosity is adjusted, for example, to approximately 1,000 cp.
• The display quality and the processibility were evaluated by changing the material for the intervening[0054]layer40. First, for reference, thelight guide plate20 was directly arranged on thepanel0 with air therebetween. In such a case, when the illumination light radiating from thelight source30 enters vertically into the side of thepanel0, it is reflected from the interface between the lower surface of thelight guide plate20 and the air layer as well as from the interface between the air layer and the upper surface of thefirst substrate1. The undesirably reflected light amounts to approximately 10% of the illumination light. Since the intensity of the undesirably reflected light is substantially the same as the amount of light reflected from thesecond substrate2 of thepanel0, the display contrast is extremely lowered. Since the display contrast in this case reached approximately zero, the image shown on thepanel0 was not clearly visible. Next, water was introduced as the interveninglayer40 between thelight guide plate20 and thepanel0. That is, water having a refractive index of 1.33 was used to fill the interface between thelight guide plate20 and thepanel0 by means of capillarity so that they were optically joined together. As a result, the undesirable reflection at the interface between thelight guide plate20 and thepanel0 decreased extremely and thus a contrast which was sufficient enough to view the display was obtained. Further, an ultraviolet curing epoxy resin was used to fill the interface between thelight guide plate20 and thepanel0. The refractive index of the epoxy resin was 1.56 and most of the undesirable surface reflection was eliminated. Thereby, a high level of contrast which was adequate for practical purposes was obtained. However, the relatively high viscosity of the epoxy resin, i.e. approximately 5,000 cp, made it difficult to use it to uniformly fill the interface between thelight guide plate20 and thepanel0. Further, an ultraviolet curing epoxy resin having a viscosity of approximately 1,000 cp was used to fill the interface between thelight guide plate20 and thepanel0. Since the epoxy resin has a relatively low viscosity and a high refractive index, it could be used to uniformly fill the interface between thelight guide plate20 and thepanel0 and it could almost completely suppress the undesirable reflection.
• FIG. 8 shows a specific structure of the[0055]light guide plate20 shown in FIG. 7, where FIG. 8A is a plan view, FIG. 8B is a sectional view and FIG. 8C is an enlarged sectional view. The layeredlight guide plate20 is joined to thepanel0 at alower surface28 of thelight guide plate20. At this stage, if a bonding resin adheres to anend25, anupper surface26, or aside27, the optical properties will be damaged. Therefore, it is preferable that theend25, theupper surface26 and theside27 of thelight guide plate20 are protected with a tape or the like beforehand when thelight guide plate20 and thepanel0 are bonded together with an ultraviolet curing resin. By removing the tape after thelight guide plate20 and thepanel0 have been bonded together through the radiation of ultraviolet rays, it is possible to prevent the bonding resin from unnecessarily adhering. After bonding, thelight guide plate20 and thepanel0 are integrated.
• When the light guide plate is provided on the front surface of the panel, if an air layer intervenes between the light guide plate and the panel, nearly 10% of the incident light is reflected because a refractive index between the air and the light guide plate disagrees with that between the air and the panel. Since the reflected light does not participate in the electro-optical switching of the panel, it significantly decreases the contrast of the reflective display device. Therefore, in the third embodiment described above, in order to prevent the interfacial reflection, the light guide plate and the panel are bonded with a transparent resin which has a refractive index close to those of them. There is, however, a possibility that an excess adhesive may leak out of the gap between the light guide plate and the panel when they are bonded to each other, and if it should stick to other components, the reflective display device will have a poor appearance. The structure for preventing the adhesive from leaking out will be described as follows. FIG. 9 is a plan view of an improved light guide plate and FIG. 10 is a sectional view of the same. The[0056]light guide plate20 is fabricated by cutting an acrylic plate which is 90 mm by 120 mm in size and has a thickness of 3.0 mm with a diamond cutter having an inclination of 135 degrees. Thus, steps21 having an inclination angle of 45 degrees are formed with a distance of 200 μm on the surface of thelight guide plate20. Aplanar section22 is formed between the adjacent steps21. Theplanar section22 inclines slightly and thelight guide plate20 as a whole has a given thickness. Prior to the cutting fabrication, twogrooves29 are formed on the back surface of the acrylic plate. Eachgroove29 is parallel to thestep21, and one is arranged on the side of the light source and another is arranged on the side opposite the light source. Thegroove29 is formed, for example, 1 mm away from the end of thelight guide plate20, and has a width of, for example, 1 mm, and a depth of, for example, 0.2 mm.
• FIG. 11 schematically shows a reflective display device fabricated by bonding the[0057]light guide plate20 shown in FIG. 9 and FIG. 10 to apanel0. An adhesive40ahaving a refractive index of 1.50 is used to bond the back surface of thelight guide plate20 to the surface of thepanel0. As described above, twogrooves29 are formed on the back surface of thelight guide plate20, beforehand. Alight source30 is arranged on the end of thelight guide plate20 and thelight source30 is partially covered with a reflectingmirror31. As thelight source30, a semicylindrical cold cathode fluorescent tube may be used. Astep21 having an inclination of 45 degrees and aplanar section22 are formed on the surface of thelight guide plate20. A diffusingarea20zis also formed on the periphery close to thelight source30. The illumination light radiating from thelight source30 is totally reflected from thestep21 formed on the surface of thelight guide plate20 and guided onto thepanel0 lying below thelight guide plate20. Accordingly, even in a dark environment, the image on thepanel0 can be displayed by the illumination of thelight source30. The diffusingarea20zis formed for diffusing and absorbing the oblique components which have a relatively high incident angle in the illumination light radiating from thelight source30 so that the intensity of the illumination light to thepanel0 is uniformed. This embodiment is characterized by forming agroove29 on the back surface of thelight guide plate20 for preventing the leakage of the adhesive40a(a transparent resin having adhesion) by thegroove29 when bonding the back surface of thelight guide plate20 and the surface of thepanel0 to each other.
• FIG. 12 is a schematic representation which shows a method of the fabrication in order to bond the[0058]light guide plate20 and thepanel0 to each other. As described above, twogrooves29 are formed on the back surface of thelight guide plate20, beforehand. Next, an adhesive40ais applied by printing or the like to at least one of the back surface of thelight guide plate20 and the surface ofpanel0. Then, by pressurizing by apressing roller90 while thelight guide plate20 and thepanel0 are superposed, thelight guide plate20 and thepanel0 are bonded to each other. A heating treatment is performed to cure the adhesive40aas required. Thus, by using thelight guide plate20 provided with thegroove29 on the back surface, the leakage of the adhesive40acan be eliminated when thelight guide plate20 is bonded to the surface of thepanel0, enabling the prevention of the decline of the production yield in the bonding step.
• FIG. 13 is a schematic sectional view which shows a reflective display device as a fourth embodiment of the present invention. This has the same fundamental structure as the second embodiment of the present invention shown in FIG. 4 and the same reference numerals are assigned to the corresponding parts for facilitating understanding. Notably, the embodiment is provided with a collimating means for collimating the illumination light radiating from the[0059]light source30 and leading it perpendicularly onto the end of thelight guide plate20. In the present embodiment, the collimating means corresponds to acollimator lens50. Thelight source30 is composed of, for example, a semicylindrical fluorescent tube and is arranged facing theend25 of thelight guide plate20. Thecollimator lens50 is also semicylindrical and is arranged between thelight source30 and thelight guide plate20. Thecollimator lens50 is stored together with thelight source30 in acover30a. As a collimating means, a parabolic reflector arranged at the rear of thelight source30 may be used, instead of thecollimator lens50.
• FIG. 14 is a partial sectional view which shows an enlarged pixel of the reflective display device shown in FIG.[0060]13. Thelight guide plate20 is provided on the outer surface of anupper substrate101. A guest-host liquid crystal layer composed of a nematicliquid crystal104 including adichroic dye105 is held between theupper substrate101 and alower substrate102. The guest-host liquid crystal layer is driven in response to the electrical field generated between acounter electrode106 formed on theupper substrate101 and apixel electrode111 formed on thelower substrate102. On thelower substrate102, a quarter-wavelength layer114 for converting polarization and a reflectinglayer109 for scattering are provided. The reflectinglayer109 includes ametal film109acomposed of, for example, aluminum, which is formed on acorrugated resin layer109b. Aresin layer109cis thinly applied in order to adjust the corrugation of theresin layer109b. Thepixel electrode111 is electrically connected to themetal film109athrough anintermediate electrode112a. Themetal film109ais fragmented corresponding to thepixel electrode111 and is electrically connected to adrain electrode122 of athin film transistor108. Thethin film transistor108 has a double gate structure and is provided with a pair ofgate electrodes116. Also, a supplementary capacitor Cs is connected to thethin film transistor108.Gate insulating films117aand117blying between thegate electrode116 and the semiconductorthin film118 have a two-layer structure. Also,layer insulation films120aand120bhave a two-layer structure.
• FIG. 15 is a schematic perspective view which shows the entire structure of the fourth embodiment. The[0061]light guide plate20 is provided on thepanel100. Thecover30aprovided is connected to theend25 of thelight guide plate20. A light source such as a fluorescent tube and a collimator lens are stored in thecover30a. As shown in FIG. 16, thecollimator lens50 is semicylindrical, i.e., the shape of a cylinder cut vertically to its ends. The fluorescent tube is arranged facing and parallel to the curved surface of the collimator lens. The flat surface of thecollimator lens50 comes into contact with theend25 of thelight guide plate20.
• FIG. 17 is a schematic representation of the[0062]light guide plate20, showing the optical properties. As shown in the drawing, thelight guide plate20 has a larger thickness at the side which comes into contact with thecollimator lens50 and the thickness decreases toward the front, being a so-called wedge-shape. A minute striped groove is formed on the inclined upper surface of thelight guide plate20 and corresponds to astep21. The illumination light radiating from thelight source30 is collimated with thecollimator lens50 and is totally reflected from eachstep21 so as to efficiently enter into a panel (not shown in the drawing). The light reflected from the panel is emitted toward the side of the viewer mainly through theplanar section22. The angle of inclination of thestep21 is set at 42 degrees. When thelight guide plate20 is composed of glass, the refractive index is 1.5. The collimated illumination light is totally reflected from thestep21 and enters into the panel. When air is intervened at the interface between thelight guide plate20 and the panel, the incident angle of illumination light toward the panel is 4.5 degrees. Or, when thelight guide plate20 is composed of a transparent resin material having a refractive index of 1.4, the angle of total reflection is 45 degrees. If the angle of inclination of thestep21 is equally set at 45 degrees, the illumination light collimated with thecollimator lens50 is totally reflected from thestep21 and enters into the panel substantially perpendicularly. Thus, the dichroic ratio of the guest-host liquid crystal layer can be effectively reflected in the display contrast. The light guide plate having the structure described above can be fabricated inexpensively if it is processed with a resin material by using, for example, a stamper. Also, if the alignment pitch of thestep21 is designed in accordance with the pixel alignment pitch in the side of the panel, the moire appearing between them can be minimized.
• FIG. 18 is a schematic partial sectional view which shows a reflective display device as a fifth embodiment of the present invention. Basically, this embodiment is the same as the first embodiment shown in FIG. 1, and the same reference numerals are assigned to the corresponding parts for facilitating understanding. Notably, the embodiment includes a[0063]polarizing plate60 provided between thelight source30 and thelight guide plate20 for converting the unpolarized illumination light which is radiating from thelight source30 into linearly polarized light and leading it onto theend25 of thelight guide plate20. The structure described above enables suppression of the undesirable scattering of illumination light inside thelight guide plate20, resulting in the improvement of the display contrast. Preferably, thepolarizing plate60 converts illumination light into linearly polarized light which is parallel to or perpendicular to the light guide plate. On the other hand, the electro-optical material held in apanel0 comprises a guest-hostliquid crystal layer3 including a liquid crystal. The liquid crystal is controlled in the alignment direction parallel to or orthogonally to the polarization direction of the illumination light converted into linearly polarized light, which can enhance the dichroic ratio of the guest-hostliquid crystal layer3, resulting in the improvement of the display contrast. When thestep21 is formed on thelight guide plate20 by cutting operations or the like, a residual distortion generally occurs inside thelight guide plate20. Thereby, uniaxial anisotropy occurs inside thelight guide plate20, creating an area which causes double refraction. If illumination light enters into the double-refraction area, it is scattered. Since the scattered light is not related with the optical switching of thepanel0, it lowers the display contrast of thepanel0. Therefore, thepolarizing plate60 is provided between thelight source30 and thelight guide plate20 in order to improve the contrast. In view of the residual distortion of thelight guide plate20, the scattered light reaches minimum when the linear polarization direction of illumination light is parallel to or perpendicular to thelight guide plate20. Also, by controlling the alignment direction of the liquid crystal parallel to or orthogonally to the linearly polarized light, the switching effect of the liquid crystal can be maximized, and thus the display contrast can be improved.
• FIG. 19 shows a reflective display device as a sixth embodiment of the present invention. FIG. 19A shows the shape of the light guide plate which is the most important part of the embodiment, and FIG. 19B shows the shape of another light guide plate in comparison with the embodiment. As shown in FIG. 19A, the[0064]step21 in accordance with the embodiment includes a curved inclined area and reflects illumination light diffusively so as to lead it onto the first substrate of the panel (not shown in the drawing). In the example shown in the drawing, eachstep21 has a convex face and the angle of inclination ranges from 40 to 50 degrees. The height of the step is 6 μm and the alignment pitch between thesteps21 is 118 μm. However, these dimensions are just one example. In the present embodiment, thestep21 has a curvature and the angle of incidence of illumination light toward the panel is changed continuously in the range of ±5 degrees. That is, the angle between the tangent line of the convex face and the surface of theplanar section22 ranges from 40 to 50 degrees. Thus, the scattered illumination light enters into the panel and the frontal reflection is suppressed. Therefore, the glare observed when the image is viewed from the front can be controlled.
• In the example to be compared, shown in FIG. 19B, the step has a flat surface having an inclination of 45 degrees. The illumination light radiating from the light source advances inside the[0065]light guide plate20 parallel to the bottom face and is totally reflected at thestep21 having an inclination of 45 degrees so as to perpendicularly enter into the panel which is arranged directly below. In such a case, if the image is viewed from the front of the panel, the parallel illumination light perpendicularly entering and the light reflected from the panel interfere with each other, resulting in glare in the display.
• FIG. 20 is a schematic view of a cutout of the[0066]curved step21. The curved surface of thestep21 lies on a circle with radius R=48.6 μm. By cutting out a circular arc at 31.25 μm from the origin both in the X and Y axes, a curved surface having an angle of inclination varying in the range from 40 to 50 degrees is obtained. In order to form the step having a circular arc like this, for example, a wedged surface of the light guide plate may be cut into a stripe with a diamond cutter. Or, after cutting a master disc with a diamond cutter, it may be used as a stamper to massproduce the light guide plate.
• FIG. 21 shows a variation to the sixth embodiment. In this example, steps[0067]21a,21b,21cand21dare formed with different angles of inclination and reflect illumination light according to the angles of inclination and lead it onto the side of the panel with different angles. In such a structure, the same effect as the sixth embodiment can be obtained.
• As described above, the reflective display device does not consume much electric power and is expected to be used as a display for information terminals. However, differing from a transmissive display device which is provided with a backlight, a reflective display device does not allow an image to be viewed in a dark environment. In order to solve this problem, in the present invention, a light guide plate is used. FIG. 22 shows a typical structure of the light guide plate in accordance with the present invention. As has been frequently explained, the[0068]light guide plate20 is arranged on the surface of the glass substrate in the front side of the reflective panel. Thelight guide plate20 includes, for example, thestep21 having an area with an inclination angle of 45 degrees and theplanar section22 which is parallel to the glass substrate of the panel, and corresponds to a prism sheet having a periodical structure.
• FIG. 23 is a schematic diagram showing the usage of the prism sheet shown in FIG. 22. A light source composed of, for example, a cold cathode fluorescent tube is arranged near the[0069]end25 of thelight guide plate20. The illumination light radiating from the cold cathode fluorescent tube horizontally enters through theend25 and is almost totally reflected perpendicularly downward at thestep21 having an inclination of 45 degrees. The reflected light can illuminate thereflective panel0 from the front side. In a bright environment, the screen is viewed by using external light, while in a dark environment, the screen can be viewed by illuminating thepanel0 with the cold cathode fluorescent tube turned on. By using the cold cathode fluorescent tube and the light guide plate in this way, the reflective display device which is usable in any environment while maintaining a low consumption of electric power can be obtained. However, when thelight guide plate20 shown in FIG. 18 is provided on the front surface of thereflective panel0, interference fringes may occur owing to the periodical structure of the light guide plate (prism sheet) depending on the circumstance, making it difficult to view the screen. When the illumination light radiating from the light source is reflected perpendicularly downward, it is diffracted owing to the periodical structure of thelight guide plate20, generating first-order light, second-order light, and so on, in addition to zero-order light. The illumination light reflected from thesecond substrate2 passes through thelight guide plate20 again, however, it is diffracted again and the zero-order light and the first-order light, etc. are generated. The zero-order light and the first-order light caused by diffraction which has occurred twice interfere with each other and bright and dark fringes overlapping on the screen may be viewed. This may slightly spoil the good view of the display. In particular, when the screen is viewed from the front side of thepanel0 obliquely opposite to the light source, the interference fringes become distinct. That is, if the eyes of the viewer slant toward the opposite side to the inclined area of thestep21, the interference fringes become distinct. As the slant of the eyes of the viewer becomes larger, the interference fringes become more distinct.
• FIG. 24 shows a seventh embodiment of the present invention, presenting the structure of the light guide plate for improving the defect described above. As shown in the drawing, the[0070]light guide plate20 includes atrapezoidal section22mdivided into bands and aninclined step21 lying between each band of the trapezoidal section. Thetrapezoidal section22mshown in FIG. 24 corresponds to theplanar section22 shown in FIG. 22. In the present embodiment, each trapezoidal section includes a curved lens area instead of a flat area. Thelight guide plate20 reflects the illumination light radiating from the light source at eachstep21 so as to guide it onto the first substrate and also emits the illumination light reflected from the second substrate through the lens area of eachtrapezoidal section22m. The surface of thelight guide plate20 is, for example, 90 mm×120 mm in size. Theend25 of thelight guide plate20 facing the light source has a thickness of, for example, H=3.2 mm. The thickness decreases stepwise moving away from the light source and the tip has a thickness of, for example, 0.2 mm. Thelight guide plate20 is obtained by processing, for example, a transparent acrylic plate. Thestep21 can be formed by mechanically processing the acrylic plate with a diamond cutter having an inclination of 135 degrees. The light guide plate processed like this includes a base of the acrylic plate, thestep21 having an inclination of45 degrees, and thetrapezoidal section22mlying between each step. The arrangement distance L between theadjacent steps21 is, for example, 120 μm. Notably, when thetrapezoidal section22mis processed, the acrylic plate is cut with a diamond cutter being concaved to a radius R=1 mm. By using such a cutter, the surface of thetrapezoidal section22mis processed into a curved lens area. In the end, the surface structure of thelight guide plate20 includes a microprism corresponding to eachstep21 and a microlens corresponding to eachtrapezoidal section22m. By using a light guide plate having such a structure, it is possible to perform the front illumination without causing interference fringes to the reflective panel.
• FIG. 25 is a schematic diagram showing the usage and the properties of the light guide plate shown in FIG. 24. The[0071]light guide plate20 is fixed on the front surface of thereflective panel0 through the interveninglayer40 composed of a transparent adhesive or the like. As described above, in thelight guide plate20, thestep21 having an inclination of45 degrees is arranged with a distance of, for example, 120 μm. Thetrapezoidal section22mis provided between the adjacent steps21. In the drawing, for facilitating understanding, the properties of thetrapezoidal section22mare schematically shown using a microlens ML. A light source (not shown in the drawing) composed of a cold cathode fluorescent tube or the like is provided adjacently to theend25 of thelight guide plate20. The illumination light radiating from the cold cathode fluorescent tube is reflected perpendicularly downward at eachstep21 and illuminates thepanel0. The illumination light is reflected from thesecond substrate2 and passes through thelight guide plate20 to reach the viewer. As described above, in a dark environment, thepanel0 is viewed by making use of the illumination light of the cold cathode fluorescent tube. Further, the adhesive used for the interveninglayer40 may be selected from, for example, a transparent resin material having a refractive index of 1.50 in order to improve the optical coupling of thelight guide plate20 and thepanel0. Also, a deflector may be used in order to improve the illumination efficiency of the cold cathode fluorescent tube.
• Notably, the microlens ML condenses the parallel diffracted light, such as the zero-order light and the first-order light emitting from the[0072]trapezoidal section22mperiodically arranged, into the focus of the microlens ML. Accordingly, the viewer who may be present at substantially an infinite position is not directly affected by the diffracted light. By using the light guide plate provided with the microlens, the influence of the interference fringes can be eliminated. In the structure of the light guide plate shown in FIG. 18, theplanar section22 does not include a collimating means, therefore, the parallel diffracted light emitting from the panel interferes at the infinite point, resulting in bright and dark fringes. By contrast, in the light guide plate shown in FIG. 20 and FIG. 21, because of the properties of the microlens ML, bright and dark fringes are formed at a short distance right above thepanel0 by several millimeters. Accordingly, the bright and dark fringes are not at all observed by the viewer who is present at an appropriate distance and a good quality display is obtained. As described above, in accordance with the present embodiment, thelight guide plate20 provided with the microlens in addition to the microprism can cancel the interference fringes caused by the periodical prism structure, and a good quality display is obtained even in the case of the front illumination.
• FIG. 26 is a schematic partial sectional view which shows a reflective display device as an eighth embodiment of the present invention. The same reference numerals are assigned to the parts corresponding to those of the first embodiment shown in FIG. 1 for facilitating understanding. In this embodiment a Twist Nematic-Electrically Controlled Birefringence (TN-ECB) mode liquid crystal panel is used as the[0073]panel0, while in the first embodiment a guest-host mode liquid crystal panel is used. As shown in the drawing, the reflective display device includes alight guide plate20 and apanel0. Astep21 and aplanar section22 are formed on the surface of thelight guide plate20. The back surface of thelight guide plate20 is deposited on the surface of thepanel0. Apolarizing plate70 and a quarter-wavelength plate80 are provided on the surface of thepanel0. Thepanel0 includes afirst substrate1 composed of transparent glass or the like lying on the side of external incident light joined to asecond substrate2 lying on the reflection side with a predetermined gap therebetween. A nematicliquid crystal layer3ais held in the gap between both thesubstrates1 and2. Theliquid crystal molecules4 are twistedly aligned by the upper andlower alignment layers6 and7.Electrodes10 and11 are provided on the inner surfaces of thesubstrates1 and2, respectively, for applying a voltage to the nematicliquid crystal layer3aof each pixel. In this embodiment, theelectrode10 provided on the side of thefirst substrate1 is patterned into a stripe, and theelectrode11 provided on the side of thesecond substrate2 is also formed into a stripe. Bothelectrodes10 and11 are arranged orthogonally to each other and pixels are delimited by the crossing parts, which is a so-called “passive matrix” type. Thepolarizing plate70 and the quarter-wavelength plate80 are arranged on the side of thefirst substrate1 of thepanel0. The reflective liquid crystal display device having such a structure is a TN-ECB type and has a so-called “normally white” mode. That is, in the absence of an applied voltage, the nematicliquid crystal layer3afunctions as a quarter-wavelength plate by maintaining a twisted alignment and performs a white display by transmitting external light in cooperation with thepolarizing plate70 and the quarter-wavelength plate80. In the presence of an applied voltage, the nematicliquid crystal layer3adoes not function as a quarter-wavelength plate by shifting to a vertical alignment and performs a black display by intercepting external light in cooperation with thepolarizing plate70 and the quarter-wavelength plate80.
• In reference to FIG. 26, each component will be described in detail. As mentioned above, the[0074]polarizing plate70 is provided on the surface of thefirst substrate1 of thepanel0. The quarter-wavelength plate80 is provided between thepolarizing plate70 and thefirst substrate1. The quarter-wavelength plate80 is composed of, for example, a uniaxially stretched polymeric film, and a phase difference by a quarter-wavelength arises between an ordinary ray and an abnormal ray. The optical axis (uniaxial anisotropic axis) of the quarter-wavelength plate80 is arranged at an angle of45 degrees toward the polarization axis (transmission axis) of thepolarizing plate70. The external light is converted into a linearly polarized light by passing through thepolarizing plate70. The linearly polarized light is converted into a circularly polarized light by passing through the quarter-wavelength plate80. It is again converted into a linearly polarized light by passing through the quarter-wavelength plate. In such a case, the polarization direction rotates by 90 degrees from the original polarization direction. As described above, the quarter-wavelength plate in combination with the polarizing plate can rotate the polarization direction, which is used for displaying.
• The[0075]panel0 uses the nematicliquid crystal layer3a, as the electro-optical material, which is basically horizontally aligned and has a positive dielectric anisotropy. The nematicliquid crystal layer3afunctions as a quarter-wavelength plate by appropriately setting its thickness. In this embodiment, the refractive index anisotropy Δn of the nematicliquid crystal layer3ais approximately 0.7 and the thickness of the nematicliquid crystal layer3ais approximately 3 μm. Therefore, the retardation Δn·d of the nematicliquid crystal layer3ais from 0.2 to 0.25 μm. By twistedly aligning the nematicliquid crystal molecules4 as shown in the drawing, the above-mentioned retardation value will reach approximately 0.15 μm (150 nm). The value is approximately one fourth of the central wavelength of the external light (approximately 600 nm), thus enabling the nematicliquid crystal layer3ato optically function as a quarter-wavelength plate. A predetermined twisted alignment can be obtained by sandwiching the nematicliquid crystal layer3abetween the upper andlower alignment layers6 and7. Theliquid crystal molecules4 align along the rubbing direction of thealignment layer6 on the side of thefirst substrate1 and theliquid crystal molecules4 align along the rubbing direction of thealignment layer7 on the side of thesecond substrate2. By shifting the rubbing direction of thealignment layer6 from that of thealignment layer7 by 60 to 70 degrees, a predetermined twisted alignment can be obtained.
• A reflecting[0076]layer8 is formed below theelectrode11 on the side of thesecond substrate2. The reflectinglayer8 has a corrugated surface and scatters light. Accordingly, its paper-white appearance is suitable for the display background and since it reflects the incident light with a relatively wide angle range, the viewing angle range is enlarged, and thus the display is easily viewed as well as the brightness of the display being increased. Atransparent flattening layer12 is provided between the reflectinglayer8 and theelectrode11 for compensating the corrugation. The reflectinglayer8 includes aresin layer15 having a corrugation and ametal film16 formed on the surface thereof, composed of, for example, aluminum. Theresin layer15 is a photosensitive resin layer whose corrugation is patterned by means of photolithography. Thephotosensitive resin layer15 is composed of, for example, a photo resist and applied to the entire surface of the substrate. It is exposed to light through a given mask and, for example, is formed into a cylindrical pattern. Next, by heating to melt, the corrugation is formed stably. On the surface of the corrugation formed as described above, ametal film16, composed of aluminum or the like having a predetermined thickness and a good reflectance, is provided. If the depth of the corrugation is set at several μm, a good light scattering property is obtained and the reflectinglayer8 will have a white color. On the surface of the reflectinglayer8, theflattening layer12 is provided to compensate the corrugation. It is preferable that theflattening layer12 is composed of a transparent organic substance, for example, an acrylic resin or the like. By intervening theflattening layer12, theelectrode11 and thealignment layer7 are formed stably.
• With reference to FIG. 27, the performance of the reflective display device shown in FIG. 26 will be described in detail. In the drawing, (OFF) shows the state in the absence of an applied voltage and (ON) shows the state in the presence of an applied voltage. As shown in (OFF), the reflective display device includes the[0077]polarizing plate70, the quarter-wavelength plate80, the nematicliquid crystal layer3aand the reflectinglayer8 deposited in that order from the side of the viewer. The polarization axis (transmission axis) of thepolarizing plate70 is shown by70P. Theoptical axis80S of the quarter-wavelength plate80 and thetransmission axis70P form an angle of 45 degrees. Also, thealignment direction3R of theliquid crystal molecules4 on the side of the first substrate is parallel to the polarization axis (transmission axis)70P of thepolarizing plate70.
• The[0078]incident light201 is converted into a linearlypolarized light202 by passing through thepolarizing plate70. Its polarization direction is parallel to thetransmission axis70P, and hereinafter it is referred to as a “parallel linearly polarized light”. The parallel linearlypolarized light202 is converted into a circularlypolarized light203 by passing through the quarter-wavelength plate80. The circularlypolarized light203 is converted into a linearly polarized light by passing through the nematicliquid crystal layer3awhich functions as a quarter-wavelength plate. However, the polarization direction of the linearly polarized light is rotated by 90 degrees and is orthogonal to the parallel linearlypolarized light202. Hereinafter, it is referred to as an “orthogonal linearly polarized light”. The orthogonal linearlypolarized light203 is converted into a circularlypolarized light204 because it again passes through the nematicliquid crystal layer3awhich functions as a quarter-wavelength plate after being reflected from the reflectinglayer8. The circularlypolarized light204 is converted into a parallel linearlypolarized light205 the same as before because it yet again passes through the quarter-wavelength plate80. The parallel linearlypolarized light205 eventually becomes an emitting light206 after passing through thepolarizing plate70 to reach the viewer, and thus a white display is obtained.
• In the state shown in (ON), in the presence of an applied voltage, the[0079]liquid crystal molecules4 have a vertical alignment instead of a twisted alignment and do not function as a quarter-wavelength plate. Theexternal light201 passing through thepolarizing plate70 is converted into a parallel linearlypolarized light202. The parallel linearlypolarized light202 is converted into a circularlypolarized light203 by passing through the quarter-wavelength layer80. The circularly polarized light203 passes through the nematicliquid crystal layer3aas a circularly polarized light, is reflected by the reflectinglayer8, and reaches the quarter-wavelength layer80 as a circularly polarized light204a. The quarter-wavelength layer80 converts the circularly polarized light204ainto an orthogonal linearly polarized light205a. Since the orthogonal linearly polarized light205acannot pass through thepolarizing plate70, a black display is obtained.
• As described above, in accordance with the present invention, a light guide plate is arranged on a reflective panel, and a light source for performing an auxiliary illumination is arranged on the end of the light guide plate. The light guide plate normally transmits external light onto the panel and emits the external light reflected from the panel, and also, as required, guides illumination light onto the panel and emits the illumination light reflected from the panel. In a dark environment, although the panel is a reflective type, the image can be viewed by turning the light source on. On the other hand, in a bright environment which has an abundant amount of external light, electric power can be saved by turning the light source off. Also, in accordance with the present invention, the light guide plate and the panel are joined to each other with a transparent intervening layer therebetween in order to suppress undesirable reflection of illumination light and external light at the interface between the light guide plate and the panel by appropriately setting a refractive index of the intervening layer. Since matching of the refractive index between the light guide plate and the panel is obtained, in an environment which has an abundant amount of external light, for example, in the daytime, external light is efficiently led into the panel, and also when illumination light is required, for example, at night, the undesirable reflection can be almost completely suppressed. By joining the light guide plate and the panel with an adhesive, they can be integrated. In particular, by providing a groove on the back surface of the light guide plate, the adhesive resin can be prevented from leaking out when the back surface of the light guide plate and the surface of the panel are joined to each other. Also, in accordance with the present invention, a collimating means like a collimator lens is employed in order to collimate the illumination light radiating from the light source and lead it perpendicularly onto the end of the light guide plate, and thus the utilization efficiency of the illumination light can be greatly improved. Also, in accordance with the present invention, by providing a polarizing plate between the light source and the light guide plate, the undesirable scattering of the illumination light inside the light guide plate can be suppressed, and thus the display contrast can be improved. In addition, by leading linearly polarized light into the panel, the optical switching properties of the panel become more efficient, resulting in the improvement of the display contrast. Also, in accordance with the present invention, the step of the light guide plate includes a curved inclined area for reflecting illumination light diffusively and leading it into the side of the panel. Thus, the intensity of the frontal reflected light can be lowered and the glare of the display screen can be controlled. Also, in accordance with the present invention, the light guide plate includes a trapezoidal section divided into bands and an inclined step lying between each trapezoidal section, and also each trapezoidal section includes a curved lens area. Thus, the interference fringes caused by the periodical structure of the light guide plate can be controlled, and the quality of the display screen can be improved.[0080]

Claims (18)

What is claimed is:

1. A reflective display device comprising:

a panel comprising a transparent first substrate lying on the side of external incident light, a second substrate joined to said first substrate with a predetermined gap therebetween and lying on the reflection side, an electro-optical material held in said gap, and an electrode provided on at least one of said first substrate and said second substrate for applying a voltage to said electro-optical material;

a light guide plate arranged on the outside of said first substrate, said light guide plate normally transmitting the external light onto said first substrate and emitting the external light reflected from said second substrate, while, as required, guiding illumination light onto said first substrate and emitting the illumination light reflected from said second substrate; and

a light source arranged on the end of said light guide plate.

2. A reflective display device according to

claim 1

, wherein said light guide plate and said panel are joined to each other with a transparent intervening layer therebetween and a refractive index of said intervening layer is appropriately set so as to suppress undesirable reflection of the illumination light and the external light at the interface between said light guide plate and said panel.

3. A reflective display device according to

claim 1

, wherein said intervening layer comprises a transparent resin having adhesion.

4. A reflective display device according to

claim 3

, wherein said light guide plate is provided with a groove on the back surface thereof for preventing said resin from leaking out when the back surface of said light guide plate and the surface of said panel are joined to each other.

5. A reflective display device according to

claim 1

, wherein said reflective display device further comprises a collimating means for collimating the illumination light radiating from said light source and leading it perpendicularly onto the end of said light guide plate.

6. A reflective display device according to

claim 5

, wherein said light source is semicylindrically formed and arranged facing the end of said light guide plate and said collimating means corresponds to a semicylindrical collimator lens arranged between said light source and said light guide plate.

7. A reflective display device according to

claim 1

, wherein a polarizing plate is provided between said light source and said light guide plate for converting the unpolarized illumination light radiating from said light source into linearly polarized light, leading it onto said light guide plate, and suppressing undesirable scattering of the illumination light inside said light guide plate.

8. A reflective display device according to

claim 7

, wherein said polarizing plate converts illumination light into linearly polarized light which is parallel to or perpendicular to said light guide plate.

9. A reflective display device according to

claim 7

, wherein said electro-optical material comprises a liquid crystal which can be controlled in the alignment direction parallel to or orthogonally to the polarization direction of said illumination light converted into linearly polarized light.

10. A reflective display device according to

claim 1

, wherein said light guide plate comprises a planar section divided into bands and an inclined step lying between each band of said planar section and said light guide plate reflects the illumination light guided forward at each step so as to guide it onto said first substrate and also emits the illumination light reflected from said second substrate through said planar section.

11. A reflective display device according to

claim 10

, wherein said step of said light guide plate inclines in the range from 40 to 50 degrees toward said planar section.

12. A reflective display device according to

claim 10

, wherein said step comprises a curved inclined area for reflecting illumination light diffusively so as to lead it onto said first substrate.

13. A reflective display device according to

claim 10

, wherein each said step is formed so as to have a different angle of inclination for reflecting illumination light in accordance with the angle of inclination and leading it onto said first substrate at a different angle.

14. A reflective display device according to

claim 1

, wherein said light guide plate comprises a trapezoidal section divided into bands and an inclined step lying between each band of said trapezoidal section, each band of said trapezoidal section comprising a curved lens area, and said light guide plate reflects the illumination light radiating from said light source at each step so as to guide it onto said first substrate and emits the illumination light reflected from said second substrate through said lens area of each band of said trapezoidal section.

15. A reflective display device according to

claim 1

, wherein said panel comprises a guest-host liquid crystal layer as said electro-optical material, comprising a liquid crystal as a host to which a dichroic dye is added as a guest.

16. A reflective display device according to

claim 15

, wherein said panel comprises a reflecting layer lying on the side of said second substrate for scattering and reflecting external light, and a quarter-wavelength layer provided between said guest-host liquid crystal layer and said reflecting layer.

17. A reflective display device according to

claim 1

, wherein said panel comprises a polarizing plate provided on the side of said first substrate and a liquid crystal layer as said electro-optical material, said liquid crystal layer functioning as a quarter-wavelength plate in response to the state of an applied voltage.

18. A reflective display device according to

claim 17

, wherein a quarter-wavelength plate is provided between said polarizing plate and said liquid crystal layer, and said liquid crystal layer comprises a nematic liquid crystal layer having a positive dielectric anisotropy and a twisted alignment, functions as a quarter-wavelength plate in the absence of an applied voltage, and does not function as a quarter-wavelength plate in the presence of an applied voltage.

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