WO2004015489A1

WO2004015489A1 - Display device with light guiding substrate

Info

Publication number: WO2004015489A1
Application number: PCT/IB2003/003127
Authority: WO
Inventors: Dirk K. G. De Boer; Dirk J. Broer; Hugo J. Cornelissen; Gerardus P. Karman; Armanda C. Nieuwkerk; Emiel Peeters; Hubertina P. M. Huck; Giovanni Nisato
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-08-07
Filing date: 2003-07-11
Publication date: 2004-02-19
Also published as: AU2003247028A1

Abstract

This invention relates to a display device (1) comprising a display panel, with a first substrate being one of a front (2) or back substrate (3) and a layer of non-emissive electro-optical material (4) being positioned on said substrate, said display device further comprising an illumination system (10) being arranged to illuminate said display panel (1) through an edge thereof, and a light guide system for guiding the light emanating from said illumination system (10) into said display panel. According to the invention, said light guide system is constituted by said first substrate.

Description

DISPLAY DEVICE WITH LIGHT GUIDING SUBSTRATE

This invention relates to a display device comprising a display panel, with a first substrate being one of a front or back substrate and a layer of non-emissive electro- optical material being positioned on said substrate, said display device further comprising an illumination system being arranged to illuminate said display panel through an edge thereof, and a light guide system for guiding the light emanating from said illumination system into said display panel.

Flat displays, utilizing various technologies, are becoming increasingly popular. For many applications, such as mobile applications (cellular telephones or PDAs), flat displays are virtually the only reasonable choice, while they offer a space efficient alternative for other applications, such as computer monitors and television sets.

Most flat displays, such as liquid crystal displays or electrophoretic displays, generally comprises one or two substrates, being made of plastics or glass. The substrates are provided with an electrode structure, and may also comprise a transistor structure for active matrix driving. It is also possible to connect the electrodes to one substrate, so-called in-plane switching. Between the substrates (or on the single substrate, as the case may be), an electro- optically active layer is arranged, which layer is responsible for the optical display effect. Generally, the displays further comprises a suitable optical structure, in order to make the displayed content visible. The optical structure is for example used to direct light from a front light, a back light or ambient light in the desired directions in order to generate a visible picture.

Recent development has resulted in flexible flat display devices, as a complement to less flexible varieties. These displays are essentially built in the same way as the above-described flat displays, with the exemption that the materials used have flexible properties, so that the achieved display may be bent or flexed. In order to provide thin displays, displays have been developed in which a light source for illuminating the display is arranged at one side of the display panel, whereby a light guide is used to transport the light to the appropriate part of the display. Such a light guide may for example be constituted by a slab of plastics or glass, covering the entire display area. One example of such a light guide is disclosed in WO 99/22268. However, such prior art displays have a tendency to become bulky, and comprise a plurality or components. An improved display device is therefore desired.

Hence, an object of the present invention is to provide a display device, overcoming the above drawbacks with the prior art and has an edge illumination source in which the light is distributed within the display device in an improved way.

The above and other objects are at least in part achieved by a display device by way of introduction, being characterized in that said light guide system is constituted by said first substrate. Thereby, a compact display structure may be achieved, taking full advantage of the components already in the display device. Moreover, the inventive display structure has a good mechanical robustness, and also exhibits a potentially higher brightness than prior art displays, due to less scattering in the display device.

Suitably, the display device further comprises a leakage prevention layer, being positioned between said light guide system and the non-emissive electro-optical material; said leakage prevention layer being of a material having a lower refractive index than said light guide systems. By including a low index layer between the light guide and the non-emissive layer, total internal reflection may be achieved within the light guide. Thereby, the light may be efficiently guided to all places where it may be coupled out of the light guide, and hence unnecessary spreading and absorption of light is avoided, and the illumination source may be used in a more efficient manner. It may be derived that optimal leakage prevention of light from the light guide

>n ²+l, where nt and n₂ are the refractive indices of the light guide and the leakage prevention layer, respectively. Suitably, said non-emissive electro-optical material is one of a liquid crystal material, an electrophoretic material or an electrochromic material, but also other display materials may be feasible. Moreover, the display device suitably comprises means for coupling out the light from said light guide towards the layer of non-emissive electro-optical material. According to one embodiment, said out-coupling means comprises a pattern of microgrooves or indentations. According to another embodiment, diffractive elements are arranged in said light guide. The out-coupling means may also be provided as a rough surface or other light scattering elements. Alternatively, said out-coupling means comprises a first foil being provided with a plurality of protrusions, and a second foil being a flat foil having air gaps. Said foils may be laminated together. Said air gaps provide proper light guiding, while the protrusions serve both as spacers and as out-coupling means. As a further alternative, said out-coupling means may be arranged to provide polarization-selective out-coupling from said substrate. Thereby, only light having a desired polarization will be coupled out of the light guide, while light having unwanted polarization will be kept within the light guide. The display device may further be a curved or a flexible, reflective display.

Finally, said leakage prevention layer is of a material having a refractive index being less than about 1.4, such as air or an fluorinated acrylate polymer or a meso-porous silica layer, having a lower refraction index than most commonly used substrate materials.

This invention will hereinafter be described in closer detail by means of presently preferred embodiments thereof, with reference to accompanying drawings.

Fig. la is a schematic cross section of a liquid crystal display with a back light, according to prior art. Fig. lb is a schematic cross section of a liquid crystal display in accordance with a first embodiment of this invention, in which a back substrate serves as a light guide for the display.

Fig. 2a is a schematic cross section of a liquid crystal display with a front light, according to prior art. Fig. 2b is a schematic cross section of a liquid crystal display in accordance with a second embodiment of this invention, in which a front substrate serves as a light guide for the display.

Fig. 3 is a schematic cross section of a liquid crystal display in accordance with a third embodiment of this invention, in which color separation is achieved by means of diffraction optics.

Fig. 4 is a schematic cross section of a liquid crystal display devices in accordance with a fourth embodiment of this invention, in which a back substrate serves as a light guide for the display, and the back substrate, or a laminate attached thereto, exhibits an out-coupling means being polarization selective. Fig. 5 is a schematic cross section of a liquid crystal display devices in accordance with a fifth embodiment of this invention, in which a front substrate serves as a light guide for the display, and the front substrate, or a laminate attached thereto, exhibits an out-coupling means being polarization selective. Fig. 6 is a schematic cross section of a typical design for a πexioie αispiay substrate, constituting a flexible light guide.

Fig. 7a and 7b is a schematic cross section of a non-curved and curved flexible display, respectively. Fig. 8 is a schematic cross section of a flexible display device in accordance with a sixth embodiment of this invention, in which a front substrate serves as a light guide for the display.

Fig. 9a-c is a schematic cross section of a flexible display devices in accordance with a seventh embodiment of this invention, in which a suitable laminate of a foil with protrusions and a flat foil with air gaps is used, where the air gaps provide proper light guiding and the protrusions serve both as spacers and as out-coupling means.

Fig. 10 is a cross section of an eight embodiment of this invention, wherein the display comprises a switchable light scattering layer, and light may be coupled in through one or more edges of the front and/or back substrate.

A display device 1, in accordance with this invention, generally comprises a display panel and an illumination system 10, whereby an image is displayed by the display panel, and the illumination system is used to visualize the image for a human eye. An example of a prior art display device is disclosed in Fig. 1 a or 2a and a corresponding display device in accordance with the invention is disclosed in Fig. lb or 2b.

A display panel essentially comprises a front substrate 2 and a back substrate 3 and a non-emissive electro-optical material layer 4, being sandwiched between said front and back substrates 2, 3. On said front and back substrates 2,3, electrode structures 5, 6 are arranged, so that a desired electrical field may be applied over said electro-optical material layer 4. When a single substrate is used, the electrodes are attached to the single substrate with an in-cell switching pattern The electro-optical material layer may for example be a liquid crystal layer. Said electrode structures may comprise a transistor structure in order to enable active matrix driving on per se known manner. Furthermore, in the case of most liquid crystal display devices, the display panel comprises a polariser, such as a front polariser 7 or an in-cell polariser 8, in order to control the polarization of the light from said illumination system 10.

The illumination system 10 may for example comprise a light source and a reflector (not shown) for directing light into the display panel, and is in the present case being arranged in one end of the display panel. In accordance with the prior art, πgnt may oe directed into the display by means of a separate light guide, either in proximity with the back substrate 3, as is shown in Fig. la, or in proximity with the front substrate 2, as is shown in Fig. 2a. The separate light guide is used to couple in light to the display and is on one side (the one facing away from the electro-optical layer 4) provided with a microstructure or the like in order achieve out-coupling of light in a desired direction.

A first embodiment of this invention will hereinafter be described with reference to Fig. lb, disclosing a display device in which the back substrate 3 is used for incoupling of light. The main components of the display are the same as described above, with the exception that the light guide is constituted by one of the substrates. In accordance with the invention, the substrate used for incoupling of light is constituted by a high refractive index light guide element, such as an element of polycarbonate, having a refractive index n=1.585. On said back substrate 3, on the side facing the electro-optical layer 4, a layer 9 having a low refractive index is arranged, such as a layer having a refractive index n=l .4 or less. The illumination system 10 is arranged so that collimated light is coupled in through an end surface of the high refractive index light guide element. In the embodiment shown in Fig. lb, out-coupling from the light guide 3 takes place using a microstructure 14 in the light guide, as is described above, but it is also possible to use diffractive or holographic structures. For a good light transport in the light guide, the refractive index of the light guide should be larger than that of the material at the side of the light guide where out-coupling takes place. This may be achieved, for instance, by using the above layer 9 having a low refractive index. It is also possible to use a high-index light guide, such as a light guide of glass having a high refractive index, e.g. lead glass (n«1.8), having a standard electro-optical material (n=i.5) on top of it. In the latter case it is possible to make the out-coupling structure as a layer of for example a polymeric material, being attached to the substrate. By using a substrate as a light guide in order to spread the light over the display panel, the light function is integrated in the display, providing a compact solution with few components. Due to fact that the layer 9 has a lower refractive index than the substrate, total internal reflection may occur within the substrate, resulting in leakage prevention. In the case of most liquid crystal displays, further optical components such as polarisers 8 and or 7 (and often advantageously retarders) are needed for a proper function. In the above described configuration, such polarisers (and retarders) maybe realized by means of in-cell components.

A second alternative embodiment of this invention is disclosed in Fig. 2b, and corresponds with the description above, regarding the display device shown in Fig. lb, with the exception that it utilizes the front substrate 2 as a light guide as is disclosed in Fig. 2b. Consequently, the low index layer 9 is arranged between the front substrate and the electro- optically active layer 4.

An third embodiment of this invention is disclosed in Fig. 3. This display device facilitates color separation by means of a diffractive structure 14, instead of color filters, which technique is suitably used with the embodiments described above. In this embodiment, a diffractive optic element 14 is incorporated in the high refraction index substrate 3, on the side of the substrate facing the non-emissive electro-optical material layer 4. This diffractive optic element 14 results in that different colors are coupled out into different directions. Different color components are directed through different pixels that are now characterized by the absence of commonly used light-absorbing color filters. In this embodiment, a large gain in optical efficiency is achieved, as no light is absorbed by such color filters. Moreover, the display device also comprises a diffuser 11, in order to re-mix the various colors. This embodiment further comprises a spacer layer 13, arranged between the diffractive optics layer 14 and the electro-optically active material in order to angularly separate the colors, and moreover it comprises a black mask, positioned right between the spacer layer 13 and the electro-optically active material 4 in order to achieve wavelength selection. An advantage of this embodiment is that the outlining is easy if compared with a backlight having a separate diffractive element. Also, no lenses are required as was the case in the prior art, for example as described in WO 00/50953.

A fourth embodiment of this invention is disclosed in Fig. 4 and makes use of a polarization-selective light-guide structure. In this case, the back substrate 3 is used as a light guide. In the light guide (back substrate 3), outcoupling means 14 are arranged to couple out light from said light guide, said outcoupled light having a desired polarization. Said outcoupling means 14 may for instance be made of an optically anisotropically scattering material or composite. The outcoupling means may alternatively be made in the form of a micro-optical structure, formed in or from such an optically anisotropic medium. The outcoupling means may further be formed directly in said back light guide 3 (see left part of Fig. 4), or may be formed in a separate foil or layer, laminated on said substrate (see right part of Fig. 4). The advantage of this embodiment is that the in-cell polariser, as disclosed in Fig. lb and described above, may be omitted, and hence the display structure may be further simplified.

A fifth embodiment of this invention, as is disclosed in Fig. 5, also makes use of a polarization selective out-coupling structure, but in this case the front substrate 2 is used as a light guide. Otherwise, the structure is similar to the one disclosed in J ig. 4. inis embodiment does not directly allow removal of the in-cell polariser, as was the case in the above forth embodiment, but it has the advantage that the efficiency of the display illumination system maybe increased. Moreover, if the polarization selective out-coupling structure has a suitably high polarization selectivity, the in-cell polariser 8 may be replaced by an external front polariser 7.

The above description is mainly aimed towards traditional displays. However, the invention is equally applicable to other types of displays, such as flexible displays.

Here follows a brief description of a one example of flexible display according with prior art, namely a flexible, bistable, reflective display, based on a cholesteric texture liquid crystal (CTLC) cell. Similar parts are given the same reference numbers as for the embodiments described above. Such a flexible display comprises a front and a back substrate 2, 3, preferably being polymer-based, transparent, temperature resistant, flexible and thin (<250 μxrx). Moreover, the substrates are provided with diffusion barriers against gasses, such as oxygen and water vapour, which ensures a long life time for the display. Between said substrates 2,3 a layer of non-emissive electro-optical material 4 is arranged, in this case a cholesteric liquid crystal having Bragg-reflecting and scattering textures. Examples of prior art flexible displays are disclosed in Fig. 7a (non-curved) and Fig. 7b (curved). Displays based on such textures are intrinsically reflective, and their performance is depending on the amount of ambient light. However, the viewing performance may be enhanced by applying a flexible front light illumination system. Typically, the illumination system comprises a light source, being coupled to a sheet light guide, being applied over the entire surface of the display.

However, in accordance with the invention, the sheet light guide function may be integrated with the front substrate of the display. The light may be coupled out from the substrate, towards the non-emissive electro-optical material layer 4, by means of an indentation or groove out-coupling pattern provided on said substrate. It is also possible to use a diffractive optics layer or a holographic layer in order to provide said out-coupling, in the same way as described above. When designing a flexible substrate, that may function as a light guide, three main issues must be regarded; the substrate must be flexible to the same extent as the rest of the display, efficient coupling of light into the liquid crystal layer 4 must be achieved, and the diffusion barrier properties must be left intact. One embodiment of such a substrate in accordance with the invention is disclosed in Fig. 6. This substrate comprises of a base layer 16, comprising for example of a polymer such as polycarbonate or a laminated thin glass material. On the side of the base layer 16 facing away from the non-emissive electro-optical layer 4, a diffusion barrier layer 15 is arranged, such as a layer on CH SiO. On the opposite side of the base layer 16, an electrode 5, for example of ITO, is arranged. Moreover, in accordance with the invention a low index layer 9 is arranged between the electrode 5 and the base layer 16, said low index layer 9 having a lower refractive index than the base layer 16, the low index layer 9 for example consisting of a fluorinated acrylate polymer or a meso-porous silica material. The total thickness of the substrate preferably does not exceed 250 μxxi.

A sixth embodiment of this invention is shown in Fig. 8, disclosing a flexible substrate utilizing a substrate as described above. This display essentially comprises a front substrate 2, a back substrate 3 and a non-emissive electro-optical layer 4, such as CTLC, being sandwiched between said substrates 2,3. The front substrate 2 is of the basic type disclosed above with reference to Fig. 6, and further comprises a plurality of microgrooves or indentations, applied directly in the flexible substrate 2. The size of said microgrooves or indentations should not exceed the thickness of the base layer 16. An illumination system 10 is further arranged at the edge of the front substrate, and is arranged to couple light into said front substrate, while the out-coupling means, in this case consisting of said microgrooves or indentations, is arranged to couple light out of the front substrate 2 and into the non-emissive electro-optical layer 4. As described above, the substrate 2 also comprises a low index layer 9, having a lower refractive index that said base layer 16. This layer is arranged to prevent leakage of light from the base layer 16 to the non-emissive electro-optical layer 4, by arranging so that total internal reflection may occur.

Moreover, the design of the front light may be chosen and tuned in order to adjust the front light to a certain substrate. Yet another embodiment of this invention will now be described with reference to Fig. 9a-c. In this case, the front substrate 2 is arranged to function as a light guide, in the same way as in the embodiment disclosed in Fig. 8. However, in this case, the substrate does not exhibit the microgrooves or indentations described above, but instead, a laminated substrate 2 comprising a foil with protrusions 19 and a flat foil with air gaps is used. (Fig. 9a discloses the display before the light guide has been laminated, while Fig. 9b and 9c discloses the display in a flat and curved state, respectively.) Hence, total internal reflection will occur within said substrate, having a higher refractive index than air. This technology will couple light into the non-emissive electro-optical layer 4 in the positions of the protrusions. This has the advantages that there is no need for grooves to be made in the substrate nor does a separate low index layer need to be applied. However, an additional substrate is needed in order to confine the display cell, and hence this embodiment is larger than the previous embodiments.

Yet an embodiment of this invention will now be described with reference to Fig. 10. In this embodiment, light from an illumination system is coupled in to the respective light guide through at least one side of at least one of the substrates. Moreover, the electro- optical layer 4 comprises a switchable light-scattering material. Such effects in electro-optical material are known for the man skilled in the art, and for example cholesteric texture liquid crystal material (CTLC) may be used, and in the example below CTLC is used. A pixel in a CTLC display may have two states. In one state, the pixel is in the so-called focal conic state, and light passes through the pixel unaffected. The second state is referred to as the planar state, in which the pixel consists of a plurality of micro domains, that exhibit (Bragg) reflection or scattering of light within a specific wavelength range, having an angle of incidence within a specific range. Hence, the light that is coupled into the front substrate, the back substrate or both substrates, from one or more edges, is arranged to simply propagate through the pixels being in a focal conic (transparent) state while it is scattered by the pixels being in the planar state.

In all of the above embodiments light is to be coupled into the light guide/substrate from a light source. Commonly, a point light source is to be used, and hence it has to be transferred to a line source, e.g. by using a line-shaped light guide, in order to improve the transmission of light into the light guide.

It shall be noted, that although the above description mainly focuses on liquid crystal displays, the invention is equally applicable to other kinds of transmissive, reflective or transflective non-emissive electro-optical displays, such as electrophoretic or electrochromic display devices.

It shall also be noted that the present invention may also be used in a so-called stratified liquid crystal display, comprising a single substrate.

Claims

CLAIMS:

1. A display device comprising a display panel, with a first substrate being one of a front or back substrate and a layer of non-emissive electro-optical material being positioned on said substrate, said display device further comprising an illumination system being arranged to illuminate said display panel through an edge thereof, and a light guide system for guiding the light emanating from said illumination system into said display panel characterized in that said light guide system is constituted by said first substrate.

2. A display device as in claim 1, wherein the display device further comprises a leakage prevention layer, being positioned between said light guide system and the non- emissive electro-optical material, said leakage prevention layer being of a material having a lower refractive index than said light guide systems.

3. A display device as in any one of the claims 1 or 2, wherein said non-emissive electro-optical material is one of a liquid crystal material, an electrophoretic material or an electrochromic material.

4. A display device as in any one of the claims 1-3, wherein the display panel further comprises means for out-coupling the light from said light guide towards the layer of non-emissive electro-optical material.

5. A display device as in claim 4, wherein said light guide system comprises a first foil a second foil being separated by means of alternating protruding elements and air gaps.

6. A display device as in claim 4, wherein said out-coupling means comprises a pattern of microgrooves, indentations or diffractive elements being arranged in said light guide.

7. A display device as in claim 4, wherein said out-coupling means is arranged as a diffractive element being arranged to separate different colors of the infalling light, whereby the display device further comprises a diffuser element, positioned on a front side of the electro-optically active layer, in order re-mix the separated colors before the light reaches a viewer.

8. A display device as in claim 4, wherein said out-coupling means is arranged to provide polarization-selective out-coupling from said substrate.

9. A display device as in any one of the preceding claims, wherein said display device is a flexible or curved display.

10. A display device as in any one of the preceding claims, wherein said leakage prevention layer is of a material having a refractive index being less than about 1.4.

11. A display device as in any one of the preceding claims, wherein the device is provided with an in-cell polariser.

12. A display device as in any one of the preceding claims, wherein said electro- optically active layer is constituted by a scattering electro-optical layer, such as a cholesteric texture liquid layer (CTLC), and illumination of the substrates is arranged to occur from at least one edge into at least one substrate.